Dynamics of Plant Litter Sodium Storage in a Subtropical Forest Headwater Stream

Abstract

Headwater streams serve as a crucial link between forest and downstream aquatic ecosystems and also act as crucial agents in carbon (C) and nutrient storage and flux. These aquatic systems play a pivotal role in regulating biogeochemical cycles. Plant litter is an important contributor of nutrients to headwater streams, having significant impacts on downstream ecosystems. However, current research predominantly focuses on the dynamics of plant litter C and nutrients such as nitrogen and phosphorus, and we know little about those of nutrients such as sodium (Na). In this study, we conducted a comprehensive evaluation of the annual dynamics of plant litter Na storage within a subtropical headwater stream. This study took place over a period of one year, from March 2021 to February 2022. Our results showed that (1) the average annual concentration and storage of litter Na was 538.6 mg/kg and 2957.6 mg/m², respectively, and litter Na storage exhibited a declining trend from stream source to mouth, while demonstrating significantly higher values during the rainy season compared to the dry season; (2) plant litter type had significant impacts on Na concentration and storage, with leaf, twig, and fine woody debris accounting for the majority of litter Na storage; and (3) hydrological (precipitation, discharge) and physicochemical (water temperature, flow velocity, pH, dissolved oxygen, alkalinity) factors jointly affected Na storage patterns. Overall, the results of this study clearly reveal the dynamic characteristics of Na storage in plant litter in a subtropical forest headwater stream, which contributes to a more comprehensive understanding of the role of headwater streams in nutrient cycling and the dynamic changes of nutrients along with hydrological processes. This research will enhance our predictive understanding of nutrient cycling at the watershed scale.

1. Introduction

Forest headwater streams act as a link between forests and downstream aquatic ecosystems, which are crucial to maintaining the functioning of the forest ecosystem as a whole. Organic matters derived from riparian plant litter and soils enters these headwater stream systems through, for example, surface runoff and throughfall [1,2]. These organic matters become the principal sources of C and nutrients for heterotrophic organisms because of the low primary productivity in headwater streams [3]. Among which, plant litter may account for the majority of the inputted organic matters [4]. Consequently, a significant body of research has focused on investigating the storage and decomposition processes of plant litter in headwater streams, especially their regulatory effects on the C and nutrient cycles within forest ecosystems [5,6]. However, investigations into headwater stream litter have largely centered on decomposition and the associated release of C and major nutrients, such as nitrogen and phosphorus [7,8]. In contrast, our knowledge of the dynamics of other nutrients associated with plant litter, such as Na, is very limited.

As an essential nutrient for plant growth, Na is of vital importance to the physiological functions of plants [9]. Also, Na can replace the function of potassium (K) to a certain extent, regulate osmotic pressure of cell [10], participate in photosynthetic pathway [11], and participate in the formation of chlorophyll [12]. In forest headwater streams, Na is mainly derived from plant litter. Given this, it follows that various factors that have an impact on the input, storage, export, and decomposition of plant litter would also play a crucial role in driving the dynamics of Na storage within headwater streams [13]. In forest ecosystems, the riparian forest type constitutes a crucial element that impacts stream litter Na input. This influence is twofold: first, it directly governs the quantity, quality (specifically the initial Na concentration), and species composition of litterfall [14,15]; second, it indirectly regulates litter decomposition and the associated release of Na [16]. Additionally, various physical and chemical characteristics of headwater streams play a vital role in influencing stream litter decomposition and element release. Stream characteristics, such as discharge, flow velocity, water temperature, and pH, are all interconnected with these processes either directly or indirectly [17,18,19]. In addition, precipitation amount and intensity—individually or in combination with riparian forest type and stream characteristics—would also affect plant litter Na storage in headwater streams as these factors can impact the discharge, flow velocity, and thus the export of plant litter [20,21]. However, previous studies have not systematically quantified how riparian forest type, plant litter type, precipitation patterns, and stream characteristics influence plant litter Na storage in headwater streams.

In this study, we monitored the dynamics of plant litter Na concentration and storage in a subtropic forest headwater stream between March 2021 to February 2022. A field survey was carried out at the end of each month, and riparian forest type, stream physicochemical characteristics, and air free precipitation regimes were surveyed or continuously monitored. The primary objectives of this study were (1) to analyze the temporal dynamics of Na storage and concentration in plant litter and (2) to assess the multivariate factors—including riparian vegetation composition, precipitation regimes, and stream physicochemical properties—affecting Na storage in headwater stream ecosystems. We hypothesized that (1) litter Na storage would show significant spatiotemporal heterogeneities, with higher storages observed during dry than during rainy seasons because of low output and decomposition rates and at the source reaches of streams where there is greater input and slower output of plant litter compared to downstream areas; and (2) plant litter Na storage would be critically modulated by multivariate controls, encompassing litter type, riparian forest type, rainfall regime, and stream physicochemical properties.

2. Methods and Materials

2.1. Study Area

A headwater stream ecosystem within the Shaxi Basin—a forested montane catchment situated in the upper Minjiang River watershed (Sanming City, Fujian Province, southeastern China)—was selected as the study area. This site exemplifies representative subtropical headwater systems characterized by steep topography and high forest cover. The headwater stream was located at the National Field Scientific Observation and Research Station of Forest Ecosystem in Sanming, Fujian Province (26°19′ N, 117°36′ E). This area falls within a subtropical monsoon climate zone, characterized by long-term (30 year) average annual temperature of 19.3 °C and an average annual precipitation of 1610 mm [5]. The majority of precipitation occurs from March to August, which is referred to as the rainy season. The terrain of the studied regions is dominated by low mountains and hills, with a mean elevation of approximately 300 m a.s.l. The soils of the region are mainly sandy red soil and yellow soil, with a thickness > 80 cm and pH between 4.5 and 4.9. The riparian zone is dominated by subtropical evergreen broad-leaved forests, with Castanopsis carlesii (Cas. carlessii) as the primary tree species. In some regions, mixed stands of Cunninghamia lanceolata and Cas. carlessii also exist.

2.2. Experimental Design

We conducted sampling and classification along the river section from the source (S1) to the mouth (S17) of the headwater stream based on riparian forest composition (broadleaved vs. mixed forests), tributary confluence presence (with or without tributaries), and channel characteristics of streams (Figure 1). Within each reach, three replicate quadrats (1 m × bank width) were established perpendicular to the flow direction. We used bank width instead of 1 m for the quadrats because bank width fluctuated considerably across different sampling sections, and the distribution of plant litter showed significant differences between the middle and the sides of the stream bank. Plant litter within each quadrat was then sampled at the end of each month. Precipitation was continuously monitored using a Tumbler Rain Sensor (SL3-1, Shanghai, China), and the ArcGIS 10.2 was used to obtain gradient of the studied headwater stream. Tape measures were used at each sampling reach to measure stream physical characteristics, including streambank width, effective channel width, and water depth. During each sampling campaign, flow velocity measurements were acquired using a portable current meter (LD-SVR, Lanende, Shandong, China). Additionally, stream water temperature, pH, dissolved oxygen (DO), and electrical conductivity (EC) were measured using a multi-parameter water quality meter (YSI Professional Plus, Xylem Inc., Yellow Springs, OH, USA).

2.3. Laboratory Analysis

Initially, samples were rinsed with distilled water to remove surface contaminants and manually sorted into distinct morphological components: leaf, twig, fine woody debris, reproductive parts, and bark. Following sorting, samples were air-dried at ambient temperature. Subsequently, air-dried material was transferred to oven-dried at 65 °C until constant mass was achieved. Dried samples were then precisely weighed, pulverized using a mechanical grinder, and sieved through a 60-mesh screen (0.3 mm aperture). Litter Na storage was determined using an inductively coupled plasma mass spectrometer (ICP-MS, ULTIMA-2, 72052224, JA France, Paris) [22].

2.4. Statistical Analysis

All the data statistical analyses were conducted in R version 4.3.1 (R Core Team 2023). Linear mixed-effects models (LMM) were employed to (1) compare the differences in Na storage and concentration within the same organ litter type from forested headwater streams across different forest types; (2) assess differences among different organ types within the same month or the same stream reach; and (3) examine differences between forest types specifically at confluences and riparian zones. To investigate the influence of environmental factors on Na storage, the linear mixed-model were used to calculate the estimate value (calculating the positive or negative effects of the influencing) and p-value (indicating statistical significance) for each factor. The linear mixed-model includes fixed effects and random effects. In this study, each variable was regarded as a fixed effect, and different river sections were regarded as random effects to explore the influence of environmental factors on Na storage.

3. Results

3.1. Dynamics of Stream Litter Na Concentration and Storage

The concentration of Na ranged from 457.9 to 256.1 mg/kg across different sampling reaches, showing an overall increase pattern from stream source to mouth (Figure 2a). In contrast, Na storage showed a decrease pattern from stream source to mouth, ranging from 2.1 to 13 mg/m², and the largest storage was found in the source sampling reach (S1) (Figure 2b). From March 2021 to February 2022, both Na concentration and storage showed a decreasing trend, with the highest values found in March and April of 2022, respectively (Figure 2c,d). Litter type showed significant impacts on both the concentration and storage of litter Na. Within each sampling reach, Na concentration was higher in leaf litter, reproductive parts, and bark, while Na storage was largest in leaf litter, followed by FWD, twig, and other litter types (Figure 3b). Similar results were found when assessing the effects of litter type within each sampling month (Figure 4b).

3.2. Factors Affecting Stream Litter Na Storage

Riparian forest type and the presence of a tributary did not significantly affect the concentration or storage of litter Na, neither in total nor for each type of plant litter (Figure 5). However, this indicates that litter Na were numerically higher in broadleaf riparian forests compared to mixed forests, a trend observed across all litter types. Additionally, the presence of a tributary was associated with numerically higher Na storage in both the total litter and in litter leaf specifically. However, Na storage significantly varied between the dry and rainy seasons regardless less of litter type and can be four times higher in rainy season compared with that in dry season (Figure 6). Rainfall amount, stream water temperature, discharge, and flow velocity generally showed significantly positive impacts on Na storage of total plant litter and/or a specific type of litter, while alkalinity showed consistently negative impacts on litter Na storage regardless of litter type (

Table 1. Impacts of rainfall amount and stream characteristics on the storages of plant litter Na as a whole or within different litter types as assessed using linear mixed-effect models. Estimated slope and the associated p-value were reported, and bold indicates significant effects. Red indicates significant impacts.

Influence Factors	Leaf		Twig		FWD		RP		Bark		Total
	Estimate	p	Estimate	p	Estimate	p	Estimate	p	Estimate	p	Estimate	p
Rainfall amount	0.001	<0.05	0.001	<0.001	0.001	<0.001	0.001	0.144	0.000	0.202	0.034	<0.001
Water temperature (°C)	0.003	0.057	0.009	<0.001	0.001	<0.001	−0.011	0.600	0.008	<0.01	0.159	<0.01
Discharge (L/s)	0.028	0.474	0.075	<0.001	0.142	<0.001	−0.014	0.333	−0.028	<0.05	1.657	<0.05
Flow velocity (m/s)	−0.006	0.163	0.362	<0.001	0.380	<0.001	0.432	0.117	0.847	<0.01	0.424	<0.001
Active channel width (cm)	0.054	0.678	−0.242	0.455	−0.152	0.251	−0.819	0.433	−0.563	0.467	2.258	0.753
Water depth (cm)	0.081	0.665	−0.096	0.190	−0.180	<0.01	−0.515	0.502	−0.955	0.757	4.292	0.726
Stream gradient (°)	0.007	0.439	−0.040	0.731	−0.027	0.523	−0.129	0.187	−0.102	0.338	0.311	<0.05
Water pH	0.011	<0.001	−0.075	0.392	−0.047	0.676	−0.254	<0.01	−0.178	0.545	0.644	<0.05
Dissolved oxygen (mg/L)	0.116	0.507	−0.577	<0.001	−0.388	<0.01	−1.663	<0.05	−1.265	0.616	4.416	<0.05
Electrical conductivity (μm/m)	−0.025	0.012	0.345	0.264	0.234	0.150	1.164	0.365	0.791	0.807	−2.730	0.417
Alkalinity (mg/L)	−0.020	<0.001	−0.020	<0.001	−0.010	<0.01	−0.071	<0.001	−0.044	<0.05	−0.202	<0.001

). Stream water pH showed positive impacts on the Na storage of leaf litter and total litter, but negative influence on that of reproductive parts. Similarly, DO had positive effects on total litter Na storage but negative impacts on the Na storage of twig, FWD, and reproductive parts.

4. Discussion

Results demonstrate that Na storage in plant litter has significant spatiotemporal heterogeneity, with a temporal peak occurring in April (early wet season) and a spatial peak concentrated in headwater stream reaches. Statistical analyses revealed no significant effects of riparian vegetation type or tributary confluence presence on Na storage. However, litter type exerted a decisive influence, and leaf litter consistently exhibited significantly higher Na storage than other litter fractions across both temporal and spatial scales. From the perspective of the total input of litter, leaf litter plays a dominant role in the overall input of plant litter to streams [23], contributing the main litter matrix to the Na storage in streams. Moreover, from the chemical composition analysis, the Na concentration of leaf litter is significantly higher than that of woody litter [13], making leaf litter a key driver of Na storage in streams and playing a core role in shaping the temporal and spatial patterns. The reasons for the highest plant litter Na storage during the rainy season are as follows. On the one hand, April is a critical period for the input of litter in subtropical forests, with a large amount of litter flowing into the ecosystem at this time [20]. Moreover, the water depth during the rainy season is significantly higher than that in the dry season, providing more storage space for the litter and promoting the accumulation of sodium [2]. Furthermore, the characteristic low-flow velocity and low-discharge hydrologic conditions in headwaters substantially diminished hydraulic transport competence, causing a large amount of riparian plant litter to accumulate on the streambed [24,25]. In addition, the narrow stream channels, high canopy cover, and low water temperatures in headwater reaches collectively constrain microbial decomposition rates, thereby further prolonging the retention time of plant litter (and its associated sodium) within the headwater streams. Notably, the absence of riparian vegetation effects likely arises from the dominance of Cas. carlesii in both forest types, leading to homogeneous nutrient inputs [26]. For the phenomenon where the influence of tributary is not significant, the root cause lies in the physical and chemical characteristics of the headwater stream itself, such as its unique substrate composition and acid-base buffering system, which play a crucial role in regulating the retention capacity of litter and the decomposition rate. These strong local factors largely mask the potential impact brought by the input of tributaries [27].

Analysis based on linear mixed models revealed that stream physicochemical characteristics are key determinants of litter Na storage. Precipitation, water temperature, and flow velocity exerted significant positive effects on litter Na storage. Stream discharge positively influenced Na storage in leaf litter and fine woody debris. Conversely, discharge exhibited a negative effect on Na storage in bark. Stream pH positively affected Na storage in leaf litter, whereas it had a negative effect on Na storage in reproductive parts. DO negatively impacted Na storage in twig litter, fine woody debris, and reproductive parts. Alkalinity consistently exhibited a negative influence on Na storage across all litter categories. Rainfall and temperature are the main climatic factors affecting the interannual variation of plant litter production in forest ecosystems [28]. An increase in rainfall can slow down the decomposition of litter [29], and rainwater washes the ground and brings more litter to stream, so litter storage increases [2,30]. Previous studies have shown that an increase in water temperature will speed up the decomposition of litter and thus reduce the storage of elements in litter [31,32]. However, the observed positive correlation between elevated water temperature and increased Na storage in litter may be attributed to the ability of microbes participating in the decomposition process to absorb and accumulate Na from adjacent environment, and litter Na storage can increase when the accumulating rate is larger than the release rate along with litter decomposition. Stream discharge and flow velocity represent two pivotal hydrological factors governing litter export dynamics in streams [33], thereby exerting indirect impacts on Na storage within plant litter. Since the bark entering the stream is mostly small and light, it is easily moved by buoyancy. When the stream discharge increases, the flow velocity also rises. Under the action of water flow, the bark is easily washed away and is difficult to accumulate on the riverbed, thus reducing the accumulation of bark. However, the increase in flow velocity can erode and destroy the colonization of microorganisms on the litter, which can cause the microorganisms attached to the litter to be washed away from the litter, inhibiting the decomposition process of the litter. Stream pH primarily influences litter decomposition by regulating microbial activity and modifying the stability of structural constituents. Notably, pH exerts significant inhibitory effects on leaf litter decomposition [34], consequently increasing the Na storage in the litter leaf. In contrast, the reproductive parts have a higher concentration of easily decomposable substances and a lower concentration of difficult-to-decompose substances such as lignin [35]. Elevated pH accelerates their structural breakdown and elemental release while concurrently suppressing certain microbial functional groups, ultimately reducing Na storage. Similarly, sufficient DO supplies oxygen for aerobic microorganisms, promoting their growth and accelerating litter decomposition, thereby facilitating Na release from litter. The alkalinity of the stream can also affect the Na storage in litter by influencing the decomposition process of litter.

Our results showed that the headwater stream plant litter has a certain Na storage and exhibits significant temporal and spatial heterogeneity. Leaf litter consistently exhibited the higher Na storage capacity among litter components. Notably, neither the riparian forest type nor the presence of tributaries exerted a significant influence on overall plant litter Na storage. However, the stream environmental factors have a certain influence on the distribution of Na storage in the plant litter. These findings collectively indicate that Na dynamics in headwater stream litter are primarily governed by local habitat conditions and the quality of the plant litter. However, this study also has certain limitations. Biological factors are important determinants of litter decomposition, but this study did not conduct research on them. This restricts our study on the influencing factors of litter decomposition. Future research should further explore the mechanism of the role of biological factors in the variation of litter Na content.

An appropriate amount of Na in leaf litter plays a positive role in forest ecosystems. It participates in energy metabolism and maintains the normal operation of the ecosystem. However, when the Na storage in leaf litter exceeds a certain level, a series of negative effects will occur. Excessive Na will affect the structure and function of microbial communities, thereby interfering with the decomposition process of leaf litter and changing the nutrient cycle in streams, ultimately causing damage to the health of stream ecosystems [36,37]. However, human interference has altered the natural regulatory mechanism of Na. Urbanization has led to a change in land use, thereby increasing pollutant emissions and causing the Na concentration in water bodies near human settlements to rise [38]. Na enrichment alters physicochemical water properties, adversely affecting the development, survival, and mortality of aquatic biota, thereby disrupting ecological equilibrium [39,40]. Future research should strengthen the monitoring and assessment of the Na cycle under the influence of human activities in order to better understand the circulation mechanism of Na in forest and water ecosystems and its impact on ecosystem functions. This will provide more comprehensive scientific theoretical support and practical guidance for the protection and management of forest ecosystems to address the challenges faced in the context of global climate change and to promote their sustainable development.

5. Conclusions

In summary, the temporal and spatial distribution of Na storage in plant litter is influenced by the interaction of multiple ecological processes and environmental factors. The dominant input characteristics of leaf litter and the unique ecological hydrological environment of the headwater stream jointly contribute to the significant temporal and spatial heterogeneity. The weakening of the influence of the riverbank forest type and tributary streams highlights the crucial role of local factors in the nutrient cycle, providing a key perspective and empirical basis for deepening our understanding of the nutrient dynamics of forest ecosystems. At the same time, it also provides basic data for understanding the migration dynamics of trace elements in the headwater streams of subtropical forests. Furthermore, these results offer crucial insights for understanding the pivotal roles of headwater streams in nutrient storage, elemental (especially trace element) cycling, and associated hydrological processes.