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Article

Spatial Patterns and Environmental Drivers of Leaf Litter Nutrients in Nitraria tangutorum and Nitraria sphaerocarpa in the Desert Region of Northwestern China

by
Jiyuan Liu
1,2,3,†,
Cheng Wang
1,2,3,4,†,
Ye Tao
1,2,3,*,
Yuanyuan Zhang
1,2,3,
Jing Zhang
1,2,3,
Xiaobing Zhou
1,2,3,
Duoqi Zhou
4 and
Yuanming Zhang
1,2,3,*
1
State Key Laboratory of Ecological Safety and Sustainable Development in Arid Lands, Xinjiang Institute of Ecology and Geography, Chinese Academy of Sciences, Urumqi 830011, China
2
Xinjiang Key Laboratory of Biodiversity Conservation and Application in Arid Lands, Xinjiang Institute of Ecology and Geography, Chinese Academy of Sciences, Urumqi 830011, China
3
Xinjiang Field Scientific Observation Research Station of Tianshan Wild Fruit Forest Ecosystem, Xinjiang Institute of Ecology and Geography, Chinese Academy of Sciences, Xinyuan 844900, China
4
College of Life Sciences, Anqing Normal University, Anqing 246133, China
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Sustainability 2025, 17(18), 8405; https://doi.org/10.3390/su17188405
Submission received: 12 August 2025 / Revised: 12 September 2025 / Accepted: 14 September 2025 / Published: 19 September 2025
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Abstract

Litter nutrient stoichiometry and its drivers are important for understanding nutrient cycling in desert ecosystems, plant adaptation strategies, and the sustainability of ecosystem functions. However, little is known about the spatial variation in litter nutrient stoichiometry and its environmental drivers in desert shrubs. This study focused on two Nitraria species (N. tangutorum Bobrov and N. sphaerocarpa Maxim) in Northwestern China, analyzing leaf litter N, P, and K stoichiometry, their spatial variation, and environmental drivers. Nutrient concentrations and stoichiometric ratios did not differ significantly between the two species. The average N contents in the litters of N. tangutorum and N. sphaerocarpa were 11.363 mg g−1 and 11.295 mg g−1, respectively. The P contents were 0.591 mg g−1 and 0.611 mg g−1, whereas the K contents were 17.482 mg g−1 and 16.255 mg g−1, respectively. With the changes in geographic and climatic factors, the same nutrient elements of the two Nitraria species showed inconsistent variation patterns. Both species showed low P concentration, indicating high P resorption and possible P limitation, reflecting nutrient vulnerability in desert ecosystems according to the scaling exponents among elements. In litter, the residual nutrient contents ranked as K > P > N, suggesting strong N resorption but low K resorption, especially for N. sphaerocarpa. N was mainly influenced by latitude, P by soil properties, and K by mean annual temperature. Moreover, litter stoichiometric ratios of N. tangutorum were relatively stable, whereas those of N. sphaerocarpa were more sensitive to environmental variables. In conclusion, the two Nitraria shrubs exhibited differential nutrient use strategies under nutrient restriction, providing insights into nutrient cycling and supporting sustainable management of desert ecosystems.

Graphical Abstract

1. Introduction

As a key link between plants and soils, litters play a central role in the material cycle and energy flow of ecosystems [1] and thus are closely related to the maintenance of ecosystem sustainability. It is estimated that nearly 90% of 100 billion tons of terrestrial plant production each year goes into the dead organic matter reservoir, forming a complex litter-based food web that regulates carbon and nutrient cycling and supports long-term ecosystem stability [2]. With the increase of CO2 concentration in the atmosphere, the amount of biomass and litter will increase, and the litter decomposition rate will directly affect the rate of nutrient replenishment in the ecosystem, which will have an important impact on species diversity and productivity [3]. The nutrient content in litter plays a key role in the entire plant–litter–soil continuum. Litter input significantly increases soil nitrogen (N) pools, including total nitrogen (TN), dissolved organic nitrogen (DON), ammonium (NH4+), nitrate (NO3−), and microbial biomass nitrogen (MBN), whereas litter removal reduces them by 10–42%, potentially impairing soil fertility and ecosystem resilience [4]. Phosphorus (P) is a key element that makes up cell membranes, DNA, RNA, and ATP, and the return of P from litters is essential for maintaining ecosystem services, structure, and function [5]. The N and P concentrations in leaf litter can also be used as an indicator of plant resorption capacity, corresponding to the degree of nutrient reduction in the plant’s fallen leaves and reflecting plant strategies to adapt to nutrient limitation [6]. As an important macro element, potassium (K) plays a crucial role in litter cycling. In the life cycle of plants, K is not only involved in photosynthesis and water regulation but also has a significant impact on plant stress resistance, growth, and survival under harsh desert conditions [7,8]. In addition, the stoichiometric ratios of nutrients in litters can better reflect the coupling relationship among multi-elements in the ecosystem [9,10]. As such, the concentrations of N, P, and K in leaf litter and their ratios can determine the nutrient decay rate, the return rate to soil, and the subsequent availability of other plants and soil organisms, as well as potential impact on N leaching or nitrous oxide release [11,12].
In recent years, the stoichiometric patterns of litters and the environmental impacts on large spatial scales have gradually attracted more and more attention [1,13]. Similarly to green leaves, the nutrients in litter are also influenced by various environmental factors. Two global studies have indicated that litter N concentration and N:P increase with rising mean annual temperature (MAT, °C) and mean annual precipitation (MAP), whereas P shows the opposite trend [14,15]. However, Kang et al. [16] found that litter P is not significantly correlated with MAT on a global scale. In addition, Xie et al. [17] report that among 12 mineral elements globally, litter P content tends to increase with latitude, whereas litter N and K contents decrease. Studies have shown that soil nutrient concentration directly affects plant nutrient concentration [18,19,20]. For instance, He et al. [18] have found that litter N concentration decreases linearly with increasing total soil N. For Cunninghamia lanceolata, the litter P variation is mainly affected by soil chemical variables. In Eastern China, the same pattern of leaf nutrient uptake capacity and efficiency of woody plants is found, and the study also points out that litter N content decreases linearly with the increase in soil N:P, whereas litter P content increases with the increase in soil P and decreases with the increase in soil N:P [15]. To date, numerous studies on litter nutrients and their influencing factors mostly focus on N and P in forest ecosystems, whereas desert ecosystems are largely neglected [21].
Soil nutrients are scarce in desert areas, and the nutrient use strategies of desert plants differ largely from those in forest or grassland ecosystems; however, the spatial and environmental variations in litter nutrients of desert plants are still not very clear. The desert region in Northwestern China is an important part of the arid deserts in central Asia. The unique climate and habitat types have shaped the distinctive nutrient uptake and allocation strategies of desert plants [22]. The obvious climatic gradient and geographic pattern exist from southeast to northwest in the desert region of China, which provides an important opportunity for the study of large-scale litter stoichiometry [23]. The genus Nitraria belongs to shrub species of Zygophyllaceae, which is characterized by saline–alkali tolerance and drought resistance. Nitraria species are important constructive species in desert ecosystems, as well as excellent woody species for windbreak and sand fixation, soil and water conservation, and afforestation in saline–alkali lands. Importantly, the island-like sand mounds formed beneath Nitraria shrubs not only stabilize the accumulation of litter but also exhibit a “fertile island” effect through long-term nutrient accumulation, thereby serving as important carbon and nutrient sinks in desert ecosystems [24]. The abovementioned feature strongly indicates that the genus is of unique ecological significance and value to carry out research on the nutrient stoichiometry of Nitraria litters.
However, at a large spatial scale, the variation patterns and influencing factors of leaf litter nutrients for different Nitraria species remain unclear, especially in the context of fragile desert ecosystems. In the present study, two species of Nitraria (N. tangutorum and N. sphaerocarpa) in the desert region of Northwestern China were taken as the research object. We aimed to achieve three objectives: (1) to reveal litter stoichiometric characteristics, spatial variation patterns, and interspecific differences in the two species; (2) to clarify the associations among different nutrient elements and test whether the interspecific differences exist; and (3) to explore the effects of geographic, climatic, and soil factors on litter stoichiometry to document the main influencing factors. According to the stoichiometry of green leaves, we hypothesize that litter stoichiometry should also conform to certain hypotheses or theories, such as the N:P Threshold Hypothesis (which suggests that ecological processes shift when the N:P ratio exceeds critical thresholds), the Power Law of nutrient scaling (which describes how nutrient concentrations scale nonlinearly across species or organs), and the Convergent Evolution Hypothesis (which proposes that plants from different lineages evolve towards similar stoichiometric traits under similar environmental pressures). In addition, although the two species belong to the same genus, their litter nutrients might have different patterns with environmental gradients, and their main influencing factors should also be different. The results of this study will provide new insights into the ecological adaptation and nutrient cycling of desert shrubs and also contribute data for modeling litter nutrient return in arid regions under climate change, with implications for desert ecosystem sustainability, restoration, and human well-being.

2. Methods

2.1. Study Area

Located in the mid-latitudes of Eurasia, the Northwest Desert Region of China is an important part of the arid region of central Asia and one of the most sensitive regions in the world to global climate change. The overall temperature in the region has shown an upward trend (0.32 °C 10 yr−1) over the past half century, significantly higher than the national average (0.25–0.29 °C 10 yr−1) and the global average (0.13 °C 10 yr−1) [25]. The total area of the arid region in Northwest China is about 2.88 × 106 km2, accounting for about 30% of China’s land area. The region is mainly a temperate continental climate and can be divided into extreme arid (<0.03), arid (0.03 to 0.2), semi-arid (0.2 to 0.5), dry and semi-humid (0.5 to 0.65), and humid (>0.65) areas according to the annual average of the arid index (AI; precipitation/potential evapotranspiration); so the arid gradient is significant. The mean annual temperature (MAT, °C) in this region is about 4 °C, and the mean annual precipitation (MAP) in most survey areas is below 400 mm, showing a decreasing trend from southeast to northwest. The evaporation is violent, with a mean annual potential evapotranspiration (PET) of 800–3200 mm. Most plants in this area are xerophytic and halophytic shrubs, such as Calligonum, Nitraria, and Tamarix, and there is great spatial heterogeneity in vegetation coverage. The soil is poor in nutrients (such as organic matter, N, P, and K), high in pH, and represents serious salinization in some areas.

2.2. Experimental Design and Sample Collection

In November 2022, a survey was conducted at 29 sampling sites in the desert region of Northwestern China, which included 16 sites for N. tangutorum and 13 sites for N. sphaerocarpa (Figure 1). For each species, a plot with a size of 30 m × 30 m was established, and 6 plants exhibiting optimal growth and uniform size were selected for tagging. A litter collector, constructed from 1 mm mesh nylon netting, was positioned under each sampled plant, close to the ground, and secured with nails driven into the surrounding soil. Litter was collected from under the Nitraria shrubs to ensure that a sufficient quantity of current-year leaf litter (no less than 3 g) was obtained. A mixed soil sample with a depth of 0–10 cm was collected from each plot using a five-point method and then packed into a plastic bag to transport to the laboratory. Furthermore, the geographic coordinates, including longitude (Lon), latitude (Lat), and altitude, were recorded for each sampling plot.

2.3. Litter Sample Determination

The litter samples collected in the field were dried at 70 °C in an oven, powdered using a vibratory disk mill (MM400, Retsch GmbH Inc., Haan, Germany), and subsequently stored in zip bags. The total nitrogen (N) concentration of the samples (mg g−1) was determined using Kjeldahl digestion. The total phosphorus (P) concentration (mg g−1) was measured colorimetrically using the ammonium molybdate method with an Auto Analyzer 3 (SEAL, Haan, Germany). Total potassium (K) was assessed through perchloric acid-concentrated sulfuric acid digestion followed by flame photometry. The results for N, P, and K were expressed in mass concentrations (mg g−1), and the stoichiometric ratios (i.e., N:P, N:K, and P:K) were also calculated.

2.4. Environmental Data Collection

Soil Properties: Soil samples were air-dried and sieved through a 2 mm sieve prior to determination. Before measurement, the powder must be disaggregated and filtered. The total nitrogen (TN) concentration of the samples (mg g−1) was determined using Kjeldahl digestion, while the total phosphorus (TP) concentration (mg g−1) was analyzed following perchloric acid–sulfuric acid digestion of soil samples and quantified using the ammonium molybdate colorimetric method with the AA3 Auto Analyzer (SEAL, Germany). The total potassium (TK) concentration was assessed through perchloric acid-concentrated sulfuric acid digestion followed by flame photometry. Soil available N (AN, mg kg−1) was assessed using the alkali hydrolyzation diffusion method, and available P (AP, mg kg−1) using the sodium hydrogen carbonate solution–Mo–Sb anti-spectrophotometric method. Soil pH and electrical conductivity (EC, μS cm−1) were measured using a PHS-3C digital pH meter and a conductivity meter (SevenExcellence-S470, Columbus, USA) in suspensions with soil-to-water mass ratios of 1:5 (for EC) and 1:2.5 (for pH). All the methods were referenced from Bao [26].
Climate Factors: The MAT, MAP, PET, solar radiation (Sard), Aridity index (AI), and 10 cm soil moisture content (SWC) environmental data were sourced from the National Tibetan Plateau Scientific Data Center, with detailed datasets in Table S1. Given that the studied genus, Nitraria, is a perennial shrub group with a natural lifespan exceeding 30 years, we utilized R 4.4.1 software and ArcGIS geographic information system to comprehensively process and analyze the collected environmental data. This involved calculating daily and monthly average datasets as multi-year averages from 2000 to 2020. Based on the latitude and longitude information recorded during the survey, we extracted annual average environmental data for the sampling points from 2000 to 2020, which were used as climate and soil indicators in this study.
Geographic Factors: The geographic factors that were measured during field surveys included longitude (Lon), latitude (Lat), and altitude.

2.5. Data Analysis

Before conducting data analysis, it is essential to perform a K–S normality test. An independent sample t-test was employed to assess the differences in litter stoichiometric traits of two of the Nitraria species. The scaling relationships among N, P, and K were represented by a power function: Y = β Xα, where Y and X denoted the contents of any two elements; β was a constant, and α was the scaling exponent. Typically, the power function undergoes logarithmic transformation prior to fitting. When α = 1, it signified an isometric relationship, whereas α ≠ 1 indicated an allometric relationship (with α > 1 representing a hyperallometric relationship and α < 1 representing a hypoallometric relationship). Data analysis was performed using the “SMATR” package in R 4.4.1.
To ascertain the variation patterns of litter nutrient traits of the two Nitraria species with geographic factors (Lon and Lat) and climatic factors (MAT, MAP, and PET), both linear and nonlinear fittings were used.
To initially evaluate the relative importance of various environmental factors on the nutrient traits (concentrations and ratios) of plants, a variable importance analysis was performed based on the %IncMSE (percentage increase in mean squared error) derived from the random forest model. Furthermore, to explore the complex effects of multiple influencing factors on plant nutrients, a partial least squares (PLS) model was utilized to assess the overall impact of environmental factors on the nutrient content and ratios of N, P, and K in plants.

3. Results

3.1. Litter N-P-K Stoichiometry and the Scaling Relationships of Two Nitraria Species

There were no significant interspecific differences in the concentrations or stoichiometric ratios of N, P, and K in the litters among the two species (Figure 2). The average N contents in the litters of N. tangutorum and N. sphaerocarpa were 11.363 mg g−1 and 11.295 mg g−1, respectively. The P contents were 0.591 mg g−1 and 0.611 mg g−1, whereas the K contents were 17.482 mg g−1 and 16.255 mg g−1, respectively. The corresponding N:P ratios were 20.281 and 19.574, and the N:K ratios were 0.909 and 0.898, with both species exhibiting the same P:K ratio of 0.049.
N. tangutorum litters demonstrated significant allometric relationships between N and P (α = 0.778) and between N and K (α = 0.353) (Figure 3). By contrast, N. sphaerocarpa exhibited significant allometric relationships between N and K (α = 0.714) and between P and K (α = 0.475). Notably, the N–K scaling exponent of N. sphaerocarpa was significantly higher than that of N. tangutorum. In all significant allometric relationships above-mentioned, the scaling exponents were significantly less than 1, indicating that the relative nutrient retention rate followed the order of K > P > N.

3.2. Geographic and Climatic Patterns of Litter Stoichiometry of Two Nitraria Species

Litter nutrient contents of N, P, and K of the two species exhibited significant variation patterns in relation to climatic and geographic factors (Figure 4, Figure 5 and Figure 6). With the exception of the N content of N. tangutorum and the K content of N. sphaerocarpa, which did not demonstrate significant variation patterns with MAP, as well as the K content of N. tangutorum, which was not significantly changed with PET, all other nutrients displayed significant responses to environmental factors. For detail, litter N, P, and K contents of two species showed an overall decreasing trend with increasing Lon; except for the N content of N. sphaerocarpa, which exhibited a linear decrease, the other nutrients demonstrated a unimodal trend. Additionally, with the increase in Lat, differences in the variation patterns among all nutrients were observed; specifically, litter K content of N. tangutorum showed a U-shaped trend, whereas other nutrients exhibited a humped trend. Furthermore, litter N, P, and K contents displayed a significant increasing trend with rising MAT and PET; among which, litter N content of the two species showed a significant linear increase with increasing MAT and PET; the K content of N. sphaerocarpa also exhibited the same trend with increasing PET, whereas other nutrients generally exhibited a U-shaped pattern with PET. The litter P content of N. sphaerocarpa displayed a linear decrease with increasing MAP, whereas the other nutrients of the two species exhibited a hump-shaped change trend.
Litter nutrient stoichiometric ratios (N:P, N:K, and P:K) of the two species exhibited significant variations driven by geographic and climatic factors (Figures S1–S3). The stoichiometric ratio of N. tangutorum displayed relatively minor fluctuations overall, with N:P showing a slight increase with Lon and a significant U-shaped trend with Lat, and no marked association with PET. By contrast, the stoichiometric ratios of N. sphaerocarpa were more responsive to geographic and climatic factors, with N:P decreasing significantly with Lon. N:P exhibited a unimodal change with MAT and MAP. In terms of N:K, the two species displayed a similar trend, i.e., demonstrating a unimodal change with rising Lon and MAT. However, their responses to changes in Lat and MAP differed; the N:K of N. tangutorum exhibited a significant unimodal trend with increasing Lat, whereas N. sphaerocarpa showed a U-shaped trend with increasing Lat. For MAP, N:K of N. tangutorum indicated a U-shaped trend with rising MAP, whereas N. sphaerocarpa demonstrated a significant unimodal change. In terms of P:K, the overall fluctuation was relatively small. The two species exhibited differences in responses to changes in Lat and MAP. Specifically, N. tangutorum displayed a unimodal change as Lat increased, whereas N. sphaerocarpa represented a U-shaped trend. Both species presented a U-shaped trend with increasing MAP. Furthermore, with the increase in PET, only N. sphaerocarpa showed a significant downward trend in P:K. These results indicated that the stoichiometric ratio of N. tangutorum was more stable, whereas N. sphaerocarpa was more sensitive to environmental factors, reflecting the differences in nutrient allocation and adaptation strategies between the two species.

3.3. Effects of Geographic, Climatic, and Soil Variables on Litter Stoichiometry of Two Nitraria Species

The PLS model analysis indicated obvious differences in the explanatory power for litter N, P, and K stoichiometry of the two species (Figure 7 and Figure 8). Specifically, the model’s explanatory power for N. tangutorum (19%) was markedly lower than that for N. sphaerocarpa (69%). Geographic factors had the greatest impact on litter N content of N. tangutorum (the factor loading coefficient was −0.382; hereafter), followed by climate factors (−0.309). Similarly, the litter N content of N. sphaerocarpa was also primarily influenced by geographic factors (0.821) and climate factors (−0.457). The model’s explanatory powers for N. tangutorum and N. sphaerocarpa were 36% and 31%, respectively. The environmental influences of N. tangutorum on P were ranked as follows: soil (0.541) > geography (0.469) > climate (0.211). By contrast, the litter P content of N. sphaerocarpa was primarily influenced by soil (−0.830) and climate factors (−0.442), whereas the geographic factors exhibited the least impact (0.123). The model’s explanatory power for litter K content of N. tangutorum (59%) was higher than that of N. sphaerocarpa (13%). Climate factors (−0.723) and soil factors (−0.662) showed a markedly greater impact on litter K content of N. tangutorum, whereas geographic factors had a small influence (0.171). For N. sphaerocarpa, climate factors had the greatest impact on litter K content (0.497), followed by geographic factors (0.340), whereas soil factors showed the least influence. Overall, litter N and K contents of N. tangutorum were markedly influenced by geographic and climate factors, whereas N. sphaerocarpa exhibited high sensitivity to soil factors, particularly for P and K.
The PLS model analysis revealed notable differences in the explanatory power for litter N, P, and K stoichiometric ratios of the two species (Figures S4 and S5). In terms of N:P, the model explanatory power of N. sphaerocarpa (47%) was higher than that of N. tangutorum (36%). Litter N:P ratios of N. tangutorum and N. sphaerocarpa were both mainly affected by geographic factors and climate, but the influence strength was inconsistent. The model’s explanatory power for litter N:K of N. tangutorum (27%) was slightly higher than that of N. sphaerocarpa (22%). For N. tangutorum, litter N:K was predominantly influenced by climate factors (−0.571), followed by soil factors (−0.469), while geographic factors had relatively little effect. For N. sphaerocarpa, the factor loading coefficients of soil factors, geographic factors, and climatic factors on litter N:K were −0.421, 0.362, and 0.339, respectively. In terms of litter P:K, the model explanatory power of N. tangutorum (26%) was also slightly higher than that of N. sphaerocarpa (22%). Litter P:K of N. tangutorum was primarily influenced by climatic factors (−0.569), whereas soil and geographic factors had relatively little effect. Litter P:K of N. sphaerocarpa was mainly affected by soil factors (−0.616), while the effects of geographic and climatic factors were relatively weak. Overall, geographic and climatic factors exerted a more substantial effect on litter N:P, whereas the N:K and P:K ratios were more constrained by soil factors.

3.4. Drivers of Litter N, P, and K Variations in Two Nitraria Species

The results of the random forest analysis revealed that the driving factors influencing litter N, P, and K concentrations varied significantly among species and elements (Figure 9). For litter N of N. tangutorum, Lat emerged as the most significant factor (accounting for approximately 20% of the total variance; hereafter), followed by MAT, TP, Lon, and TN (≈15% for each), whereas factors such as pH, PET, and MAP exhibited low impacts. For litter N of N. sphaerocarpa, Lat remained the most influential factor, followed by PET and Sard. Although MAT and Lon also significantly influenced litter N of N. sphaerocarpa, their effects were considerably lower than those of N. tangutorum. Notable differences were identified in the importance of various environmental factors to the litter P of the two species. For N. tangutorum, pH was identified as the most critical factor influencing P (>15%), followed by Lon, TN, AK, SWC, and Lat (>10% for each), whereas factors such as MAP and AN had low impact. TK emerged as the most important influencing factor to litter P of N. sphaerocarpa (>15%), followed by PET, EC, and MAT (≈10% for each). In the context of litter K, N. tangutorum was predominantly influenced by Lat and MAT (≈20% and 15%, respectively). Furthermore, the relative importance of TK and MAP was also high, whereas other soil factors such as pH and SWC had a relatively small impact. TK was the key factor affecting litter K of N. sphaerocarpa (>15%), followed by pH, Sard, and MAT, whereas other environmental factors such as Lat and SWC had lower impacts.
There were also differences in the importance of environmental factors to litter stoichiometric ratios among the two species (Figure S6). The relative importance of the environmental factors affecting the litter N:P of the two species was in the range of 7–13%, but the N:P of N. tangutorum was primarily influenced by Lon, whereas that of N. sphaerocarpa was mainly affected by TN. Litter N:K of N. tangutorum was more affected by MAP (≈22%), followed by AI (>15%), whereas N:K of N. sphaerocarpa was mainly affected by SWC (≈33%). In terms of litter P:K, AK was the key factor influencing N. tangutorum (≈24%), followed by Lon (>15%), while all other factors were of lower relative importance. Litter P:K of N. sphaerocarpa was mainly affected by TN (≈28%), followed by SWC (>15%), while the other factors had a relatively low effect.

4. Discussion

4.1. Stoichiometric Characteristics of Litter N, P, and K of Two Nitraria Species and Their Spatial Scale Effects

During the decomposition of litters, mass loss is accompanied by the migration and release of nutrients, which are critical components of nutrient cycling within ecosystems [27]. Fallen leaves serve as a significant source of mineralizable nutrients in microbial and plant metabolic processes in terrestrial ecosystems, playing a vital role in the nutrient cycle between plants and soil [28]. The migration of litter elements during decomposition directly influences soil nutrient availability, cation exchange capacity, and soil pH, among other factors [11].
In this study, no significant differences were observed in litter nutrient contents of the two Nitraria species, likely due to their coexistence in the same desert ecosystem. This environmental similarity has fostered the development of consistent adaptive mechanisms over an extended evolutionary period. Several studies have reached analogous conclusions [29,30], indicating that similar environmental or selective pressures across different species can result in the independent evolution of comparable adaptive traits or functions, which supports the “Convergent Evolution Hypothesis”. Furthermore, litter N content of the two species was measured at 11.363 mg g−1 and 11.295 mg g−1, respectively, both exceeding the global average litter N content in woody plants (10.9 mg g−1). By contrast, litter P content of the two species was recorded at 0.591 mg g−1 and 0.611 mg g−1, respectively, which is lower than the global average litter P content in woody plants (0.85 mg g−1) [31]. This suggests that the litter N content of the two Nitraria species in our study area is relatively high, while the P content is comparatively low. It can be inferred that Nitraria may be more limited by P, necessitating greater resorption of P, resulting in a reduction in litter P content. Zhao et al. [32] indicates that NRE and PRE serve as a vital nutrient conservation mechanism and play a crucial role in sustaining plant growth in barren ecosystems. Through analysis on NRE and PRE in global plants, He et al. [33] suggest that plants adjust their nutrient resorption strategies based on nutrient availability to adapt to varying environmental pressures. According to the green leaf N:P threshold hypothesis [31,34,35], litters of the two Nitraria species exhibited N:P ratios exceeding 16, significantly higher than the global average litter N:P of woody plants (12.8) [16]. This observation indicates that plants are primarily limited by P, as a greater amount of N remains after resorption. Furthermore, litter N:K ratios of the two Nitraria species were less than 2.1, while the K:P exceeded 3.4. Olde Venterink et al. [36] indicate that plants with N:K ratios greater than 2.1 and K:P ratios less than 3.4 are limited by K or N + K. This further implies that the litter retains a considerable amount of K, suggesting that plants do not require to resorb a large amount of K, i.e., K may not be a primary limiting element in the present study. As such, compared to P, the two Nitraria species were less limited by N and K.
A significant power function relationship generally exists among nutrient elements of plants. In this study, litter N–P of N. tangutorum exhibited a notable allometric relationship [37,38], with an N–P scaling exponent of 0.788. This exponent aligns with the finding of Zhao et al. [39], who report an allometric relationship between leaf N and P in woody plants, with a scaling exponent of 0.78. In this respect alone, the allometric relationship of litter nutrients shows the same pattern as that of green leaves. Litter N–K scaling exponents of the two Nitraria species were both less than 1; however, the exponent of N. tangutorum (0.353) was markedly lower than that of N. sphaerocarpa (0.714). The hypoallometric relationship indicated that the change rate of litter N of the two species was substantially lower than that of K, suggesting a stronger N return rate. The different litter N–K scaling exponents of the two species suggested that N. tangutorum demonstrated a higher N return capacity, while N. sphaerocarpa exhibited a more conservative route to K reuse, although it still prioritized N return. N, as a key component of proteins and nucleic acids in plants, plays a crucial role in all stages of plant growth and development, as well as in plant production, photosynthesis, and litter decomposition [40,41,42]. By contrast, K is essential for regulating plant water balance and is significantly linked to plant stress resistance [7]. We preliminarily deduced that compared to N. sphaerocarpa, the strategy of preferentially returning N to meet the protein synthesis and metabolic needs of N. tangutorum, meanwhile reducing energy consumption through relatively low K return, may make it more suitable for extremely resource-poor environments.
In summary, through the analyses on litter N, P, and K stoichiometry and the allometric relationship, the different strategies of the two Nitraria species to adapt to various nutrient limitations in resource-scarce environments were revealed. N. tangutorum demonstrated a stronger N return capacity and resource conservativeness, whereas N. sphaerocarpa exhibited a higher K utilization conservativeness. Nevertheless, the types of nutrient limitation identified through stoichiometric ratios did not fully align with the findings derived from stoichiometric scaling relationships. However, it could still confirm that K residues were the highest and resorption rates were the lowest. These findings further substantiate the diverse resource adaptation strategies that plants in arid regions have developed over long-term evolutionary processes, offering new insights and theoretical foundations for nutrient cycling and plant resource management in desert ecosystems.

4.2. Variations in Litter Stoichiometry of Two Nitraria Species Along with Environmental Factors

The climatic factors that vary along biogeographic gradients significantly influence plant taxonomic groups, functional traits, and nutrient stoichiometry [43]. Geographic patterns of stoichiometry can reflect plant adaptation to environments, as plant growth conditions are jointly regulated by multiple factors. Notably, at large spatial scales, plant stoichiometric characteristics often represent significant geographic and climatic patterns [44]. In this study, geographic and climatic factors were found to play a crucial role in controlling litter nutrient stoichiometry of two Nitraria species. Litter nutrients predominantly exhibited nonlinear relationships with the influencing factors, suggesting that litter nutrient contents of the two Nitraria species were affected by multiple variables. Specifically, litter N, P, and K of the two species demonstrated a marked decreasing trend with increasing Lon. This indicated that as Lon increased, the resorption of various nutrients by Nitraria also increased. This phenomenon might be attributed to the fact that in our study area, as Lon increased, temperature and precipitation conditions became more favorable, thereby enhancing the resorption capacity of plants. Zhang et al. [45] propose a similar conclusion on the nutrient absorption efficiency of different plant functional types in arid and semi-arid regions of Northwest China. They indicate that as environmental conditions improve, plants enhance their ability to resorb nutrients from leaves to support future growth and cope with potential fluctuations. Furthermore, we found that with the increase in MAT and PET, the residual N, P, and K nutrients in litters exhibited an overall upward trend. This might be attributed to high-temperature and drought conditions leading to a decrease in the nutrient resorption efficiency of plants, resulting in an increase in nutrient stock within the litters; this phenomenon could be partially explained by the “Temperature-Plant Physiology Hypothesis” [46]. This finding is consistent with the conclusion conducted by Luo et al. [47] on nutrient absorption in typical grasslands, which indicates that both severe and chronic droughts reduce the nutrient resorption efficiency of plants. In addition, litter nutrients of the two Nitraria species showed an overall downward trend with the increase in MAP; this aligned with the conclusions of several previous studies that suggest more favorable moisture conditions in nutrient-poor soil environments often lead to higher nutrient resorption rates in plants [48,49].
In large-scale studies, latitudinal pattern is regarded as a driving factor of climatic variation. Latitude (Lat), as a key geographic factor [31], interacts with various environmental factors to influence plant litter nutrient patterns. In this study, litter N content of the two species increased with the increase in Lat, which was consistent with the conclusion of the plant leaf N content globally [31]. This indicated that litter nutrient variation in Nitraria plants with Lat paralleled that of green leaves. Litter stoichiometric ratios (N:P, N:K, and P:K) of the two species exhibited significant differences in response to geographic and climatic factors. Among which, N. tangutorum remained relatively stable with changes in climatic and geographic factors, potentially leading to a more uniform nutrient release during decomposition. By contrast, N. sphaerocarpa was more sensitive to those factors (i.e., environment-dependent), resulting in the spatiotemporal heterogeneity in soil nutrient release. As Lat increased, litter N:P ratios of both species exhibited a significant upward trend. In Chinese forests [15], it is observed that litter N:P decreases with increasing Lat, which contradicts the conclusion of the present study. The primary reasons for these discrepancies may include that (1) this study focused on desert shrubs rather than forest trees, and (2) there were marked differences in climatic and soil change patterns with Lat between desert and forest ecosystems. This suggests that litter stoichiometric patterns across different ecosystem types have regional features in their responses to geographic and climatic factors. Furthermore, a comparison of litter stoichiometric ratios of the two species in relation to environmental changes revealed that the stoichiometric ratios of N. tangutorum were generally stable, demonstrating low sensitivity to geographic and climatic factors, whereas that of N. sphaerocarpa exhibited significant changes, indicating a higher sensitivity to environmental influences. This finding suggests that the two species of the same genus have more or less divergent adaptations.
In summary, this study revealed the variation patterns of litter nutrient stoichiometry of two Nitraria species across geographic and climatic gradients, and there was a joint regulatory effect of various environmental factors on litter nutrient dynamics. The relationships between litter N, P, and K stoichiometry and geographic and climatic factors (such as MAT, PET, and MAP) reflected the adaptive strategies of the desert shrub Nitraria in response to changes in resource availability and environmental suitability [50,51]. It is particularly noteworthy that litter stoichiometric patterns and their responses to larger-scale environmental changes should receive broader attention [10]. The findings contribute to the understanding of the response mechanisms of Nitraria to alterations in regional environments and provide a scientific basis for further exploration of the nutrient cycling sensitivity to global change.

4.3. Impacts of Environmental Factors on Litter Stoichiometry of Two Nitraria Species

Nutrients derived from litter are essential for numerous functions in terrestrial ecosystems, particularly in biogeochemical cycles, and nutrients are regulated by multiple factors on a broad scale [52]. Notably, climate and soil are frequently regarded as the primary driving forces of litter decomposition rate and nutrient release [53].
This study identified significant differences in driving factors influencing litter stoichiometric characteristics of N, P, and K of two Nitraria species. The PLS model indicated that geographic factors significantly impacted litter N content of both Nitraria species, and geographic factors played a crucial role in climate change and soil property regulation. Based on the random forest model, we deduced that Lat was the primary driving factor influencing litter N nutrients of the two Nitraria species. Lat not only indirectly affected litter N contents of Nitraria through the regulation of climatic conditions and soil properties but also had a direct impact on the N contents itself. These findings aligned with previous studies on plant green leaf nutrients, which conclude that hydrothermal conditions significantly affect the stoichiometric characteristics of plants along the geographical gradient (longitude, latitude, and altitude) [54], such as the significant relationship between plant leaf N content and latitude in terrestrial ecosystems in China [55]. In this study, soil property was found to play a dominant role in influencing litter P contents of the two Nitraria species. However, the random forest model indicated that soil pH was the most significant influencing factor for N. tangutorum, while TK was the key driving factor for N. sphaerocarpa. This indicated that there were significant differences in the responses of different Nitraria species to soil properties. The PLS and random forest analyses revealed that litter K contents of the two species were both primarily regulated by MAT. Under high-temperature stress, plants often influence the nutrient dynamics of vegetation by altering their physiological responses and nutrient cycling processes [56]. As an essential element for plant growth and development, K participates in various critical functions during plant growth and aids plants in resisting abiotic stress conditions, including high temperatures, drought, and salinity [57]. Therefore, with the increase in MAT, both Nitraria species may enhance the accumulation of K in their aboveground parts to withstand stress. However, due to the reduced efficiency of nutrient resorption caused by high temperature and drought, litter K contents in both two Nitraria species increase with the increase in MAT [47].
In this present study, geographic factors exerted the most significant influence on litter N:P ratios of the two Nitraria species. This phenomenon may be attributed to the fact that geographic gradients often align with variations in climate and soil conditions; both Nitraria species strategically optimize their nutrient allocation in response to specific environmental contexts. Plants typically adapt to the variability in geographic and climatic conditions through distinct nutrient allocation strategies across large-scale geographic patterns [58]. Furthermore, our findings indicated that climatic factors exhibit differential effects on litter N:K and P:K ratios of the two Nitraria species. Notably, climatic factors have a more pronounced impact on litter nutrients of N. tangutorum, suggesting a heightened dependence on hydrothermal conditions. By contrast, N. sphaerocarpa demonstrates relatively balanced adaptability to environmental changes concerning those two stoichiometric ratios.
The primary driving mechanisms and differentiated patterns of litter stoichiometric characteristics of N, P, and K of two Nitraria species have been elucidated. Based on the analysis mentioned above, it is evident that litter N of Nitraria is predominantly influenced by geographic factors, P is primarily governed by soil factors, whereas K is chiefly affected by climatic factors. This differentiated driving mechanism suggests that the accumulation and release of various nutrients in litters are regulated by a complex interplay of multiple factors, reflecting the nutrient allocation strategies of plants in adapting to desert environments. These findings categorize litter N, P, and K nutrients of Nitraria species as being predominantly influenced by geographic, soil, and climatic factors, respectively, thereby offering new insights into the nutrient cycling mechanisms within the northwest desert ecosystem in China.

5. Conclusions

This study focuses on two species of Nitraria in the northwestern desert region in China, systematically investigating the spatial variation patterns and driving mechanisms of N, P, and K contents, as well as their stoichiometric ratios in litters. The research found no significant differences in the six nutrient indicators between the two plants, which supports the hypothesis of evolutionary convergence. Not all N, P, and K contents in the two plants exhibited an allometric relationship; however, cases with a scaling exponent of less than 1 indicated that the nutrient retention rates in litters followed the order of K > P > N. In comparison, N. tangutorum tended to prioritize the recycling of N nutrients, while N. sphaerocarpa was more conservative in K utilization. With changes in geographic and climatic factors, the six nutrient traits of the two plants exhibit inconsistently varying patterns. Overall, the dynamics of N were primarily driven by geographic factors, while P was mainly regulated by soil conditions, with N. tangutorum being significantly influenced by soil pH and N. sphaerocarpa being primarily regulated by TK. Litter K was primarily driven by climatic factors, especially mean annual temperature (MAT). Litter stoichiometric ratios of N. tangutorum were relatively stable, showing lower sensitivity to geographic and climatic factors, while those of N. sphaerocarpa were more responsive to environmental factors. This study investigates and clarifies the environmental mechanisms driving the variation in litter nutrient stoichiometry between two dominant desert species. It not only deepens the understanding of nutrient cycling and environmental adaptability of desert shrubs but also provides theoretical support for vegetation management and ecosystem restoration in desert areas, offering data for assessing the potential impacts of global climate change on desert ecosystem functions and their sustainability, with further implications for human well-being in arid regions.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/su17188405/s1, Table S1: Source of all environmental data in this article. Figure S1. Relationship between litter N:P and geoclimatic factors of two Nitraria species. Figure S2. Relationship between litter N:K and geoclimatic factors of two Nitraria species. Figure S3. Relationship between litter P:K and geoclimatic factors of two Nitraria species. Figure S4. Multivariate partial least squares regression analysis of litter N:P, N:K, and P:K concentrations of Nitraria tangutorum. Figure S5. Multivariate partial least squares regression analysis of litter N:P, N:K, and P:K concentrations of Nitraria sphaeocarpa. Figure S6. Random forest analysis of stoichiometric ratios of nutrients and environmental factors in litter of two Nitraria species.

Author Contributions

Investigation, Y.Z. (Yuanyuan Zhang); Writing—original draft, J.L. and C.W.; Writing—review and editing, Y.T. and Y.Z. (Yuanming Zhang); Supervision, J.Z., X.Z., and D.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This work was financially supported by the “Western Light” talent cultivation program of the Chinese Academy of Sciences (no. 2022-XBQNXZ-006), the National Natural Science Foundation of China (no. 42171070), the Youth Top Talents Project of the “Tianshan Talent” Training Plan of Xinjiang Uygur Autonomous Region (no. 2022TSYCCX0011), and the Leading Talents in Sci-Technological Innovation Project of the “Tianshan Talent” Training Plan of Xinjiang Uygur Autonomous Region (no. 2022TSYCLJ0058).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Acknowledgments

We are grateful to Xinyue Jin, Mengting Wang, and Chenquan Gu from Xin Jiang Institute of Ecology and Geography, CAS, for their kind help in fieldwork.

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

References

  1. Zhao, Y.Y.; Li, Z.T.; Xu, T.; Luo, A.R. Leaf litter decomposition characteristics and controlling factors across two contrasting forest types. J. Plant Ecol. 2022, 15, 1285–1301. [Google Scholar] [CrossRef]
  2. Miao, R.H.; Ma, J.; Liu, Y.Z.; Liu, Y.C.; Yang, Z.L.; Guo, M.X. Variability of aboveground litter inputs alters soil carbon and nitrogen in a coniferous–broadleaf mixed forest of central China. Forests 2019, 10, 188. [Google Scholar] [CrossRef]
  3. Qin, L.H.; Liu, Q.J.; Sun, Z.; Xu, Z.Z.; Siqing, B.L. Leaf litter decomposition rate of main tree species in broad-leaved Korean pine forest and its relationship with leaf traits. Acta Ecol. Sin. 2022, 42, 5894–5905. [Google Scholar] [CrossRef]
  4. Jia, B.R. Litter decomposition and its underlying mechanisms. Chin. J. Plant Ecol. 2019, 43, 648–657. [Google Scholar] [CrossRef]
  5. Liang, J.L.; Liu, J.; Jia, P.; Yang, T.T.; Zeng, Q.W.; Zhang, S.C.; Liao, B.; Shu, W.S.; Li, J.T. Novel phosphate-solubilizing bacteria enhance soil phosphorus cycling following ecological restoration of land degraded by mining. ISME J. 2020, 14, 1600–1613. [Google Scholar] [CrossRef]
  6. Killingbeck, K.T. Nutrients in senesced leaves: Keys to the search for potential resorption and resorption proficiency. Ecology 1996, 77, 1716–1727. [Google Scholar] [CrossRef]
  7. Johnson, R.; Vishwakarma, K.; Hossen, M.S.; Kumar, V.; Shackira, A.M.; Puthur, J.T.; Abdi, G.; Sarra, M.; Hasanuzzaman, M. Potassium in plants: Growth regulation, signaling, and environmental stress tolerance. Plant Physiol. Bioch. 2022, 172, 56–69. [Google Scholar] [CrossRef] [PubMed]
  8. Rawat, J.; Pandey, N.K.; Saxena, J. Role of potassium in plant photosynthesis, transport, growth and yield. In Role of Potassium in Abiotic Stress; Springer: Singapore, 2022; pp. 1–14. [Google Scholar]
  9. Güsewell, S.; Verhoeven, J.T.A. Litter N:P ratios indicate whether N or P limits the decomposability of graminoid leaf litter. Plant Soil 2006, 287, 131–143. [Google Scholar] [CrossRef]
  10. Ge, J.L.; Xie, Z.Q. Leaf litter carbon, nitrogen, and phosphorus stoichiometric patterns as related to climatic factors and leaf habits across Chinese broad-leaved tree species. Plant Ecol. 2017, 218, 1063–1076. [Google Scholar] [CrossRef]
  11. Berg, B.; McClaugherty, C. Plant Litter. Decomposition, Humus Formation, Carbon Sequestration; Springer Nature: Cham, Switzerland, 2014. [Google Scholar]
  12. Sariyildiz, T.; Anderson, J.M. Interactions between litter quality, decomposition and soil fertility: A laboratory study. Soil Biol. Biochem. 2003, 35, 391–399. [Google Scholar] [CrossRef]
  13. Yang, Y.; Liu, B.R.; An, S.S. Ecological stoichiometry in leaves, roots, litters and soil among different plant communities in a desertified region of Northern China. Catena 2018, 166, 328–338. [Google Scholar] [CrossRef]
  14. Yuan, Z.Y. Global trends in senesced-leaf nitrogen and phosphorus. Glob. Ecol. Biogeogr. 2009, 18, 532–542. [Google Scholar] [CrossRef]
  15. Tang, S.S.; Yang, W.Q.; Wang, H.P.; Li, X. Stoichiometri characteristics and controlling factors of N and P in forest leaf toichiometri characteristics and controlling factors of N and P in forest leaf llitter of China. Chin. J. Appl. Environ. Biol. 2015, 21, 316–322. [Google Scholar]
  16. Kang, H.Z.; Xin, Z.; Berg, B.; Burgess, P.J.; Liu, Q.L.; Liu, Z.C.; Li, Z.H.; Liu, C.J. Global pattern of leaf litter nitrogen and phosphorus in woody plants. Ann. For. Sci. 2010, 67, 811. [Google Scholar] [CrossRef]
  17. Xie, Y.; Cao, Y.; Xie, Y. Global-scale latitudinal patterns of twelve mineral elements in leaf litter. Catena 2022, 208, 105743. [Google Scholar] [CrossRef]
  18. He, X.J.; Hou, E.Q.; Liu, Y.; Wen, D.Z. Altitudinal patterns and controls of plant and soil nutrient concentrations and stoichiometry in subtropical China. Sci. Rep. 2016, 6, 24261. [Google Scholar] [CrossRef]
  19. Shah, J.; Liu, W.F.; Ullah, S.; Duan, H.L.; Shen, F.F.; Liao, Y.C.; Huang, G.M.; Wu, J.P. Linkages among leaf nutrient concentration, resorption efficiency, litter decomposition and their stoichiometry to canopy nitrogen addition and understory removal in subtropical plantation. Ecol. Process. 2024, 13, 27. [Google Scholar] [CrossRef]
  20. Tang, L.Y.; Han, W.X.; Chen, Y.H.; Fang, J.Y. Resorption proficiency and efficiency of leaf nutrients in woody plants in eastern China. J. Plant Ecol. 2013, 6, 408–417. [Google Scholar] [CrossRef]
  21. Zou, Z.G.; Zhang, M.X.; Huang, X.Y.; Zhang, X.Y.; Li, S.B.; Zhou, L.L. Carbon, nitrogen and phosphorus release and their stoichiometric ratio change during decomposition of Cunninghamia lanceolata leaf litter at different temperatures. For. Res. 2024, 37, 52–62. [Google Scholar]
  22. Zhang, Y.; Xie, J.B.; Li, Y. Effects of increasing root carbon investment on the mortality and resprouting of Haloxylon ammodendron seedlings under drought. Plant Biol. 2016, 19, 191–200. [Google Scholar] [CrossRef]
  23. Chen, Y.M.; Shan, L.S.; Ma, J.; Wang, H.Y.; Xie, T.T.; Yang, J.; Li, M. Carbon, nitrogen and phosphorus stoichiometry in leaves and fine roots of desert plants in arid region of Northwest China. Acta Ecol. Sin. 2024, 44, 3648–3659. [Google Scholar]
  24. Wei, Y.J.; Guo, J.; Dang, X.H.; Xie, Y.H.; Wang, J.; Li, X.L.; Wu, H.M. Morphological characteristics and influencing mechanisms of Nitraria tangutorum nebkhas at different sandy land types in desert-oasis ecotone of Jilantai. J. Nanjing For. Univ. (Nat. Sci. Ed.) 2023, 47, 172–180. [Google Scholar]
  25. Zhang, Q.; Yang, J.H.; Wang, W.; Ma, P.L.; Lu, G.Y.; Liu, X.Y.; Yu, H.P.; Fang, F. Climatic Warming and Humidification in the Arid Region of Northwest China: Multi-Scale Characteristics and Impacts on Ecological Vegetation. J. Meteorol. Res. 2021, 35, 113–127. [Google Scholar] [CrossRef]
  26. Bao, S.D. Soil and Agricultural Chemistry Analysis, 3rd ed.; China Agriculture Press: Beijing, China, 2000. [Google Scholar]
  27. Sterner, R.W.; Elser, J.J. Ecological Stoichiometry: The Biology of Elements from Molecules to the Biosphere; Princeton University Press: Princeton, NJ, USA, 2002. [Google Scholar]
  28. Silveira, M.; Reddy, K.; Comerford, N. Litter decomposition and soluble carbon, nitrogen, and phosphorus release in a forest ecosystem. Open J. Soil. Sci. 2011, 1, 86–96. [Google Scholar] [CrossRef]
  29. Heyduk, K.; Moreno-Villena, J.J.; Gilman, I.S.; Christin, P.A.; Edwards, E.J. The genetics of convergent evolution: Insights from plant photosynthesis. Nat. Rev. Genet. 2019, 20, 485–493. [Google Scholar] [CrossRef]
  30. Sackton, T.; Clark, N. Convergent evolution in the genomics era: New insights and directions. Philos. Trans. R. Soc. Lond. Proc. R. Soc. B 2019, 374, 20190102. [Google Scholar] [CrossRef]
  31. Reich, P.B.; Oleksyn, J. Global patterns of plant leaf N and P in relation to temperature and latitude. Proc. Natl. Acad. Sci. USA 2004, 101, 11001–11006. [Google Scholar] [CrossRef]
  32. Zhao, Q.G.; Zhang, Y.H.; Wang, Y.B.; Han, G.D. Different responses of foliar nutrient resorption efficiency in two dominant species to grazing in the desert steppe. Sci. Rep. 2024, 14, 4090. [Google Scholar] [CrossRef]
  33. He, M.S.; Yan, Z.B.; Cui, X.Q.; Gong, Y.M.; Li, K.H.; Han, W.X. Scaling the leaf nutrient resorption efficiency: Nitrogen vs. phosphorus in global plants. Sci. Total Environ. 2020, 729, 138920. [Google Scholar] [CrossRef]
  34. Koerselman, W.; Meuleman, A.F.M. The vegetation N:P ratio: A new tool to detect the nature of nutrient limitation. J. Appl. Ecol. 1996, 33, 1441–1450. [Google Scholar] [CrossRef]
  35. Güsewell, S. N: P ratios in terrestrial plants: Variation and functional significance: Tansley review. New Phytol. 2004, 164, 243–266. [Google Scholar]
  36. Olde Venterink, H.; Wassen, M.J.; Verkroost, A.W.M.; Ruiter, P.C.D. Species richness–productivity patterns differ between N-, P-, and K-limited wetlands. Ecology 2003, 84, 2191–2199. [Google Scholar]
  37. Niklas, K.; Owens, T.; Reich, P.; Cobb, E. Nitrogen/phosphorus leaf stoichiometry and the scaling of plant growth. Ecol. Lett. 2005, 8, 636–642. [Google Scholar] [CrossRef]
  38. Reich, P.B.; Oleksyn, J.; Wright, I.J.; Niklas, K.J.; Hedin, L.; Elser, J. Evidence of a general 2/3-power law of scaling leaf nitrogen to phosphorus among major plant groups and biomes. Proc. R. Soc. B 2010, 277, 877–883. [Google Scholar]
  39. Zhao, N.; Yu, G.R.; He, N.P.; Xia, F.C.; Wang, Q.F.; Wang, R.L.; Xu, Z.W.; Jia, Y.L. Invariant allometric scaling of nitrogen and phosphorus in leaves, stems, and fine roots of woody plants along an altitudinal gradient. J. Plant Res. 2016, 129, 647–657. [Google Scholar] [CrossRef] [PubMed]
  40. Wang, Q.; Li, S.S.; Li, J.R.; Huang, D. The Utilization and Roles of Nitrogen in Plants. Forests 2024, 15, 1191. [Google Scholar] [CrossRef]
  41. Klausmeier, C.A.; Litchman, E.; Daufresne, T.; Levin, S. Optimal nitrogen-to-phosphorus stoichiometry of phytoplankton. Nature 2004, 429, 171–174. [Google Scholar] [CrossRef] [PubMed]
  42. Chen, Y.L.; Chen, L.Y.; Peng, Y.F.; Ding, J.Z.; Li, F.; Yang, G.B.; Kou, D.; Liu, L.; Fang, K.; Zhang, B.B.; et al. Linking microbial C:N:P stoichiometry to microbial community and abiotic factors along a 3500-km grassland transect on the Tibetan Plateau. Glob. Ecol. Biogeogr. 2016, 25, 1416–1427. [Google Scholar] [CrossRef]
  43. Du, B.; Ji, H.; Peng, C.; Liu, X.J.; Liu, C.J. Altitudinal patterns of leaf stoichiometry and nutrient resorption in Quercus variabilis in the Baotianman Mountains, China. Plant Soil 2017, 413, 193–202. [Google Scholar] [CrossRef]
  44. Hu, Y.K.; Zhang, Y.L.; Liu, G.F.; Pan, X.; Yang, X.J.; Li, W.B.; Dai, W.H.; Tang, S.L.; Xiao, T.; Chen, L.Y.; et al. Intraspecific N and P stoichiometry of Phragmites australis: Geographic patterns and variation among climatic regions. Sci. Rep. 2017, 7, 43018. [Google Scholar] [CrossRef]
  45. Zhang, J.K.; Yu, W.Y.; Wang, Y.; Huang, Z.Y.; Liu, G.F. Effects of climate, soil, and leaf traits on nutrient resorption efficiency and proficiency of different plant functional types across arid and semiarid regions of northwest China. BMC Plant Biol. 2024, 24, 1093. [Google Scholar] [CrossRef]
  46. Tian, D.; Yan, Z.B.; Fang, J.Y. Review on characteristics and main hypotheses of plant ecological stoichiometry. Chin. J. Plant Ecol. 2021, 45, 682–713. [Google Scholar] [CrossRef]
  47. Luo, Y.H.; Shen, Y.; Elrys, A.S.; Du, L.; Mahmood, M.; Zhang, J.T.; Ren, H.Y.; Zhang, J.B.; Li, N.; Tian, R.; et al. Drought and nitrogen deposition regulate plant nutrient resorption in a typical steppe. Agric. Ecosyst. Environ. 2024, 374, 109160. [Google Scholar] [CrossRef]
  48. Lü, X.T.; Han, X.G. Nutrient resorption responses to water and nitrogen amendment in semi-arid grassland of Inner Mongolia, China. Plant Soil 2010, 327, 481–491. [Google Scholar]
  49. Ren, H.; Kang, J.; Yuan, Z.; Xu, Z.W. Responses of nutrient resorption to warming and nitrogen fertilization in contrasting wet and dry years in a desert grassland. Plant Soil 2018, 432, 65–73. [Google Scholar] [CrossRef]
  50. Sardans, J.; Rivas-Ubach, A.; Penuelas, J. The elemental stoichiometry of aquatic and terrestrial ecosystems and its relationships with organismic lifestyle and ecosystem structure and function: A review and perspectives. Biogeochemistry 2012, 111, 1–39. [Google Scholar]
  51. Gotelli, N.; Mouser, P.; Hudman, S.; Morales, S.; Ross, D.; Ellison, A. Geographic variation in nutrient availability, stoichiometry, and metal concentrations of plants and pore-water in ombrotrophic bogs in New England, USA. Wetlands 2008, 28, 827–840. [Google Scholar] [CrossRef]
  52. Ge, J.L.; Wang, Y.; Xu, W.T.; Xie, Z.Q. Latitudinal patterns and climatic drivers of leaf litter multiple nutrients in chinese broad-leaved tree species: Does leaf habit matter? Ecosystems 2017, 20, 1124–1136. [Google Scholar]
  53. Parton, W.J.; Silver, W.L.; Burke, I.C.; Grassens, L.; Harmon, M.E.; Currie, W.S.; King, J.Y.; Adair, E.; Brandt, L.; Hart, S.; et al. Global-scale similarities in nitrogen release patterns during long-term decomposition. Science 2007, 315, 361–364. [Google Scholar]
  54. Gong, X.S.; Xu, Z.Y.; Lu, W.; Tian, Y.Q.; Liu, Y.H.; Wang, Z.X.; Dai, C.; Zhao, J.H.; Li, Z.Q. Spatial patterns of leaf carbon, nitrogen, and phosphorus stoichiometry of aquatic macrophytes in the arid zone of northwestern China. Front. Plant Sci. 2018, 9, 1398. [Google Scholar] [CrossRef] [PubMed]
  55. Feng, W.L.; Yang, J.L.; Xu, L.G.; Zhang, G.L. The spatial variations and driving factors of C, N, P stoichiometric characteristics of plant and soil in the terrestrial ecosystem. Sci. Total Environ. 2024, 951, 175543. [Google Scholar] [CrossRef]
  56. Xu, H.W.; Wang, M.G.; You, C.M.; Tan, B.; Xu, L.; Li, H.; Zhang, L.; Wang, L.X.; Liu, S.N.; Hou, G.R.; et al. Warming effects on C:N:P stoichiometry and nutrient limitation in terrestrial ecosystems. Soil Till. Res. 2024, 235, 105896. [Google Scholar] [CrossRef]
  57. Li, X.W.; You, X.L.; Wang, Y. Research progress of HAK/KUP/KT potassium transporter family in plant response to salt stress. Plant Sci. J. 2019, 37, 101–108. [Google Scholar]
  58. Luo, W.T.; Li, M.H.; Sardans, J.; Lü, X.T.; Wang, C.; Peñuelas, J.; Wang, Z.W.; Han, X.G.; Jiang, Y. Carbon and nitrogen allocation shifts in plants and soils along aridity and fertility gradients in grasslands of China. Ecol. Evol. 2017, 7, 6927–6934. [Google Scholar] [CrossRef] [PubMed]
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Figure 1. Sampling sites, summer individual plants, and leaf litter samples of two Nitraria species in Northwestern China. NT: Nitraria tangutorum; NS: Nitraria sphaeocarpa; AI: aridity index.
Figure 1. Sampling sites, summer individual plants, and leaf litter samples of two Nitraria species in Northwestern China. NT: Nitraria tangutorum; NS: Nitraria sphaeocarpa; AI: aridity index.
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Figure 2. Litter N, P, and K concentrations and stoichiometric ratios of two Nitraria species. NT: Nitraria tangutorum; NS: Nitraria sphaeocarpa. ns: not significant.
Figure 2. Litter N, P, and K concentrations and stoichiometric ratios of two Nitraria species. NT: Nitraria tangutorum; NS: Nitraria sphaeocarpa. ns: not significant.
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Figure 3. Scaling relationships among litter N, P, and K concentrations of two Nitraria species. NT: Nitraria tangutorum; NS: Nitraria sphaeocarpa; α: scaling exponents.
Figure 3. Scaling relationships among litter N, P, and K concentrations of two Nitraria species. NT: Nitraria tangutorum; NS: Nitraria sphaeocarpa; α: scaling exponents.
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Figure 4. Relationship between litter N concentrations and geographic and climatic factors of two Nitraria species. NT: Nitraria tangutorum; NS: Nitraria sphaeocarpa; Lon: longitude; Lat: latitude; MAT: mean annual temperature; MAP: mean annual precipitation; PET: mean annual potential.
Figure 4. Relationship between litter N concentrations and geographic and climatic factors of two Nitraria species. NT: Nitraria tangutorum; NS: Nitraria sphaeocarpa; Lon: longitude; Lat: latitude; MAT: mean annual temperature; MAP: mean annual precipitation; PET: mean annual potential.
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Figure 5. Relationship between litter P concentrations and geographic and climatic factors of two Nitraria species. NT: Nitraria tangutorum; NS: Nitraria sphaeocarpa; Lon: longitude; Lat: latitude; MAT: mean annual temperature; MAP: mean annual precipitation; PET: mean annual potential.
Figure 5. Relationship between litter P concentrations and geographic and climatic factors of two Nitraria species. NT: Nitraria tangutorum; NS: Nitraria sphaeocarpa; Lon: longitude; Lat: latitude; MAT: mean annual temperature; MAP: mean annual precipitation; PET: mean annual potential.
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Figure 6. Relationship between litter K concentrations and geographic and climatic factors of two Nitraria species. NT: Nitraria tangutorum; NS: Nitraria sphaeocarpa; Lon: longitude; Lat: latitude; MAT: mean annual temperature; MAP: mean annual precipitation; PET: mean annual potential.
Figure 6. Relationship between litter K concentrations and geographic and climatic factors of two Nitraria species. NT: Nitraria tangutorum; NS: Nitraria sphaeocarpa; Lon: longitude; Lat: latitude; MAT: mean annual temperature; MAP: mean annual precipitation; PET: mean annual potential.
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Figure 7. Multivariate partial least squares regression analysis of litter N, P, and K concentrations of Nitraria tangutorum. Geography: (Lon, Lat); Climate: (MAT, MAP, PET, AI, Sard); Soil: (TN, TP, TK, AN, AP, AK, pH, EC); *: p < 0.05, **: p < 0.01, ***: p < 0.001.
Figure 7. Multivariate partial least squares regression analysis of litter N, P, and K concentrations of Nitraria tangutorum. Geography: (Lon, Lat); Climate: (MAT, MAP, PET, AI, Sard); Soil: (TN, TP, TK, AN, AP, AK, pH, EC); *: p < 0.05, **: p < 0.01, ***: p < 0.001.
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Figure 8. Multivariate partial least squares regression analysis of litter N, P, and K concentrations of Nitraria sphaeocarpa. Geography: (Lon, Lat); Climate: (MAT, MAP, PET, AI, Sard); Soil: (TN, TP, TK, AN, AP, AK, pH, EC); *: p < 0.05, **: p < 0.01, ***: p < 0.001.
Figure 8. Multivariate partial least squares regression analysis of litter N, P, and K concentrations of Nitraria sphaeocarpa. Geography: (Lon, Lat); Climate: (MAT, MAP, PET, AI, Sard); Soil: (TN, TP, TK, AN, AP, AK, pH, EC); *: p < 0.05, **: p < 0.01, ***: p < 0.001.
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Figure 9. Random forest analysis of nutrient content and environmental factors in litters of two Nitraria species. NT: Nitraria tangutorum; NS: Nitraria sphaeocarpa; Lon: longitude; Lat: latitude; MAT: mean annual temperature; MAP: mean annual precipitation; PET: mean annual potential; Sard: solar radiation; AI: Aridity index; SWC: 10 cm soil moisture content; EC: electrical conductivity; TN: total nitrogen; TP: total phosphorus; TK: total potassium; AN: soil available N; AP: soil available P; AK: soil available K.
Figure 9. Random forest analysis of nutrient content and environmental factors in litters of two Nitraria species. NT: Nitraria tangutorum; NS: Nitraria sphaeocarpa; Lon: longitude; Lat: latitude; MAT: mean annual temperature; MAP: mean annual precipitation; PET: mean annual potential; Sard: solar radiation; AI: Aridity index; SWC: 10 cm soil moisture content; EC: electrical conductivity; TN: total nitrogen; TP: total phosphorus; TK: total potassium; AN: soil available N; AP: soil available P; AK: soil available K.
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MDPI and ACS Style

Liu, J.; Wang, C.; Tao, Y.; Zhang, Y.; Zhang, J.; Zhou, X.; Zhou, D.; Zhang, Y. Spatial Patterns and Environmental Drivers of Leaf Litter Nutrients in Nitraria tangutorum and Nitraria sphaerocarpa in the Desert Region of Northwestern China. Sustainability 2025, 17, 8405. https://doi.org/10.3390/su17188405

AMA Style

Liu J, Wang C, Tao Y, Zhang Y, Zhang J, Zhou X, Zhou D, Zhang Y. Spatial Patterns and Environmental Drivers of Leaf Litter Nutrients in Nitraria tangutorum and Nitraria sphaerocarpa in the Desert Region of Northwestern China. Sustainability. 2025; 17(18):8405. https://doi.org/10.3390/su17188405

Chicago/Turabian Style

Liu, Jiyuan, Cheng Wang, Ye Tao, Yuanyuan Zhang, Jing Zhang, Xiaobing Zhou, Duoqi Zhou, and Yuanming Zhang. 2025. "Spatial Patterns and Environmental Drivers of Leaf Litter Nutrients in Nitraria tangutorum and Nitraria sphaerocarpa in the Desert Region of Northwestern China" Sustainability 17, no. 18: 8405. https://doi.org/10.3390/su17188405

APA Style

Liu, J., Wang, C., Tao, Y., Zhang, Y., Zhang, J., Zhou, X., Zhou, D., & Zhang, Y. (2025). Spatial Patterns and Environmental Drivers of Leaf Litter Nutrients in Nitraria tangutorum and Nitraria sphaerocarpa in the Desert Region of Northwestern China. Sustainability, 17(18), 8405. https://doi.org/10.3390/su17188405

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