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Article

Thermal Modulation of Leaf Nitrogen Forms in Chinese Fir Under Soil-Warming Conditions

by
Xing Chen
1,2,
Lijuan Zhu
1,
Zhijie Yang
3,
Caixia Shen
1,
Yin Li
2,
Zexuan Tang
4 and
Yankun Zhu
1,*
1
National Park Research Center, Sanming University, Sanming 365003, China
2
Fujian Provincial Key Laboratory of Resources and Environmental Monitoring and Sustainable Management and Utilization, Sanming University, Sanming 365004, China
3
Sanming Forest Ecosystem National Observation and Research Station, Sanming 365002, China
4
Rutgers Biomedical and Health Sciences, Rutgers University, Newark, NJ 07103, USA
*
Author to whom correspondence should be addressed.
Forests 2025, 16(6), 942; https://doi.org/10.3390/f16060942
Submission received: 22 April 2025 / Revised: 30 May 2025 / Accepted: 3 June 2025 / Published: 4 June 2025
(This article belongs to the Section Forest Health)
Browse Figures
Versions Notes

Abstract

:
While soil warming has been demonstrated to significantly alter the processes of the nitrogen cycle in forest ecosystems, how leaf-available nitrogen, representing the primary forms of nitrogen absorbed by plants, responds to such thermal alterations remains insufficiently understood. In the present study, a control (CK) group and a soil-warming treatment (W) were set up. The nitrogen contents of nitrate (NO3−-N), ammonium (NH4+-N), and amino acids (AA-N) in previous- and current-year leaves from the upper and lower canopy of Chinese fir were measured under both CK and W conditions. By comparing the differences in available nitrogen distribution across different canopy layers or leaf ages, we aimed to illustrate the effects of soil warming on the allocation of available nitrogen in leaves. It was shown that soil warming can alter the distribution of available nitrogen in Chinese fir leaves, and its impact on leaf AA-N was significantly greater than its impact on inorganic nitrogen. Additionally, the allocation of available nitrogen in Chinese fir under soil warming was also influenced by leaf position and leaf age. Soil warming altered the distribution patterns of available nitrogen in leaves of Chinese fir across different canopy layers or leaf ages, which provides a scientific basis for coniferous tree species to adapt to the thermal environment by regulating available nitrogen allocation.

1. Introduction

Soil warming driven by global climate change profoundly influences the nitrogen cycle in forest ecosystems [1,2]. This phenomenon changes plant strategies for nitrogen uptake, transportation, and allocation, ultimately affecting plant photosynthesis [3,4,5]. Ammonium (NH4+-N), nitrate (NO3−-N), and amino acids (AA-N) are the primary forms of nitrogen directly assimilated by plants, and their absorption and distribution are highly sensitive to soil temperature [6,7,8,9,10,11]. A nationwide study in China revealed that soil surface temperature increased 31% more than air temperature from 1962 to 2011, with warming extending to 3.2 m depth, highlighting the critical role of soil thermal dynamics in regulating belowground nitrogen processes [12]. Elevated soil temperatures can reduce soil microbial activity, retard organic matter decomposition, and diminish nutrient availability, particularly for nitrogen [13,14]. After root absorption, a small portion of NO3− and NH4+ is assimilated into amino acids. At the same time, the majority is transported to leaves through transpiration and utilized for photosynthetic protein synthesis to maintain the normal operation of the photosynthetic system [15,16,17]. Previous research has indicated that rising global temperatures have led to a decline in the nitrogen content of terrestrial plant leaves [18]. However, it remains unclear how soil warming affects the distribution of available nitrogen, which serves as the main nitrogen source for leaves [19].
Chinese fir is an important fast-growing and high-yield tree species in southern China, with its plantations accounting for 27.23% of the country’s dominant plantation species [20]. As a key carbon-sequestering species in subtropical regions [21], its leaves serve as the most active organs in the aboveground part of Chinese fir and are particularly sensitive to environmental changes. The response of leaf-available nitrogen to environmental changes reflects the plant’s adaptability and regulatory ability. Previous studies have revealed distinct nitrogen allocation strategies in Chinese fir leaves. Zhang observed that the young leaves of Chinese fir enhance photosynthetic capacity through preferential nitrogen allocation, while mature leaves accumulate NO3−-N as an osmotic regulator under soil-warming conditions [22]. These findings align with Hikosaka’s global meta-analysis of 393 plant canopies, which demonstrated decreasing leaf nitrogen concentration with increasing canopy depth and leaf age, coupled with significantly higher nitrogen-use efficiency in young leaves [23]. These differential strategies may be linked to soil temperature impacts on root growth and nitrogen uptake efficiency—for instance, soil warming has been shown to reduce root length and bleeding rate in crops, impairing nutrient transport and photosynthetic capacity [24]. However, whether and how soil warming modulates the uptake and distribution of specific nitrogen forms (e.g., NH4+-N vs. NO3−-N) in Chinese fir canopies, particularly between sunlit upper leaves and shaded lower leaves remains unaddressed. The differential nitrogen utilization between young and mature leaves may reflect a trade-off in resource allocation: young leaves may prioritize ammonium assimilation for protein synthesis to support their high metabolic activity, while mature leaves may utilize nitrate vacuolar storage for nitrogen buffering [25]. Despite these insights, significant knowledge gaps remain regarding how soil warming specifically affects the spatial distribution of available nitrogen (including NH4+-N, NO3−-N, and AA-N) across different leaf ages and canopy positions in Chinese fir. In view of this, we hypothesized that soil warming could lead to differences in the distribution of available nitrogen in leaves, and therefore further investigation was needed to elucidate these warming-induced allocation patterns and their implications for the species’ adaptation to climate change.
To illustrate the effects of soil warming on the distribution of available nitrogen in Chinese fir leaves, the contents of NO3−-N, NH4+-N, and AA-N in previous- and current-year leaves from the upper and lower canopy of Chinese fir were measured under both CK and W conditions. Our findings suggested that the allocation strategy of available nitrogen in leaves could serve as a crucial mechanism for plants to cope with the global heat effect. This discovery offers novel perspectives on plant environmental adaptability under global warming, shedding light on how plants reconfigure their physiological processes to thrive in a changing climate.

2. Materials and Methods

2.1. Study Area and Sites Description

The experimental site was located at the Chenda Observation Point of Fujian Sanming Forest Ecosystem National Field Station (26.19° N, 117.36° E), situated in a typical subtropical monsoon climate zone. Climatic conditions of this area are characterized by a mean annual temperature of 19.1 °C, annual precipitation of 1749 mm, annual evaporation of 1858 mm, and relative humidity of 81%. Geographically, the area features rugged, steep mountainous hills with an average altitude of 300 m and slope gradients ranging between 25 and 35 degrees. The predominant soil type is red soil derived from biotite granite, forming a key component of the region’s ecological landscape.

2.2. Experimental Design

The experimental plot measured 15 m × 15 m and contained 56 planted Chinese fir trees. The experimental design consisted of two treatments: warming (+4 °C above ambient temperature, W) and control (ambient temperature, CK), each with three replicates. In May 2015, heating cables were installed to implement the warming treatment: these cables were arranged in parallel at a depth of 10 cm with 20 cm spacing between them and looped around the plot’s perimeter to ensure uniform soil warming across the treatment area. The heating power density for the soil in warmed plots was 76 W·m−2. At the time of sampling, the Chinese fir trees in this plot were 14 years old, corresponding to the mid-aged stage of their growth cycle.

2.3. Sample Collection and Processing

Leaves were collected during the thinning period in December 2023. For each treatment (W and CK), a representative tree was randomly selected and felled. From each tree, five branches were harvested from both the upper and lower sections using sterilized pruning shears. Then, based on their age, the leaves were classified into two groups: previous- and current-year. The sampling locations are illustrated in Figure 1. All branches collected were immediately placed in sterile plastic bags and transported, on ice, to the laboratory for processing. Upon arrival, sterilized scissors were used to detach leaves from the branches, which were then pooled into paper bags and mixed thoroughly. Approximately 5 g of mixed leaves was weighed for measuring moisture content. The remaining leaves were surface-sterilized using 2% sodium hypochlorite for 10 min, flowed for 2 min in 75% ethanol, and then triple-washed using sterilized water.

2.4. Detection of Moisture and Available Nitrogen in Leaves

Approximately 5 g of fresh leaves was placed into a paper bag and oven-dried at 105 °C for 6–8 h until a constant weight was achieved. The leaf moisture was determined by calculating the weight difference between fresh and oven-dried samples. NO3−-N and NH4+-N in leaves were extracted using the improved water-immersion method [26]: 2 g of ground leaf was homogenized with 10 mL of deionized water in glass test tubes, sealed with a glass bulb, and extracted in a boiling water bath for 30 min. After cooling with tap water, the mixture was filtered, and the residue was repeatedly rinsed to collect the sample solution. NO3−-N was measured by UV spectrophotometry at 220 nm, while NH4+-N was determined using the indophenol blue colorimetric method. The amino acid nitrogen (AA-N) extraction is described in previous research [27]: 0.5 g of fresh leaves was added to 5 mL of 10% acetic acid, after which, the homogenate was centrifuged at 5000 rpm for 1 min, and 1 mL of the supernatant was transferred to 10 mL volumetric flask, then diluted to target volume with an acetate buffer (pH equals 5.4) to prepare the sample extract, and AA-N was quantified using the ninhydrin colorimetric method.

2.5. Statistical Analysis

As described in previous literature [28], one-way analysis of variance with Duncan’s test was performed by SPSS software (version 26.0, Chicago, IL, USA) to evaluate the significant difference (p < 0.05) in NO3−, NH4+, and free amino acid in previous- and current-year leaves from the upper and lower sections of Chinese fir between W and CK treatments. Before that, the normal distribution of data was checked using the Shapiro−Wilk test, and the homogeneity of variance was analyzed using Levene’s test.

3. Results

3.1. Leaf Moisture

The water content in previous- and current-year leaves from the upper and lower location of Chinese fir under W and CK treatments is presented in Table 1. For current-year leaves, the moisture of upper and lower leaves in CK was 61.64% and 59.69%, and 59.13% and 58.53% in W, respectively. In the previous-years leaves, CK showed 61.75% and 57.80% in the upper and lower locations of the tree, while W was 59.23% and 55.31%, respectively. No significant differences (p > 0.05) were observed between CK and W treatments, although a slight downward trend in water content was noted in the W treatment. These results indicate that long-term warming treatment for 8 years had no significant effect on the moisture content of leaves in different leaf positions on Chinese fir trees.

3.2. Leaf-Available Nitrogen

In order to investigate whether soil warming affected the contents of three available nitrogen types in Chinese fir leaves, their mean values (with 12 samples per treatment) were calculated. As depicted in Figure 2a, despite the fact that soil warming did not exert a significant influence on the NO3−-N or NH4+-N content within Chinese fir leaves, it did enhance their concentration to a certain degree. In contrast, warming not only failed to elevate AA-N content but, instead, caused a significant reduction (p < 0.01). Furthermore, the content of NO3−-N in leaves was substantially higher than that of NH4+-N and AA-N. Specifically, the amount of NO3−-N exceeded the latter two by more than 100-fold. This demonstrated that the effect of warming on the AA-N in Chinese fir leaves was the most pronounced, with an impact significantly surpassing those of NH4+-N and NO3−-N (Figure 2b). Moreover, this effect was negative, showing an approximate decrease of 28%. Conversely, soil warming positively influenced NO3−-N or NH4+-N, increasing their levels by 15% and 9%, respectively.

3.3. The Distribution Differences in Leaf NO3−-N

Soil warming potentially altered the distribution of NO3−-N in leaves. As depicted in Figure 3A, the relative differences in NO3−-N between current- and previous-year leaves (NiCP) were significantly influenced by the leaf positions. In both CK and W treatments, NiCP in the lower canopy of Chinese fir was significantly higher than that in the upper canopy. In the upper part of Chinese fir, the NiCP exhibited negative values, which were −20% and −10% under CK and W treatment conditions, respectively. Although soil warming reduced the disparity in NiCP, this reduction did not reach statistical significance (p > 0.05). Conversely, for lower leaves, the NiCP trend was inverted, with values of 34% and 48% under CK and W treatments, respectively. This result implied that soil warming expanded the disparity in NiCP in the lower part of the tree, though there was no obvious difference. In addition, leaf age also affected the relative differences in NO3−-N between upper and lower leaves (NiUL) of Chinese fir (Figure 3B). For current-year leaves, the NiUL were 14% in CK and 18% in W. By contrast, the NiUL in previous-year leaves increased significantly to 90% (in CK) and 96% (in W). Although long-term soil warming did not have a significant effect on the variation in these differences, these phenomena clearly demonstrated pronounced discrepancies in NO3−-N accumulation among leaves at different positions and leaf ages of Chinese fir. These results indicate that leaves at varying canopy positions and developmental stages respond differently to soil warming.

3.4. The Distribution Differences in Leaf NH4+-N

The relative differences in NH4+-N between current- and previous-year leaves (AmCP) varied in the upper and lower parts of Chinese fir (Figure 4A). In the upper canopy, the values of AmCP were negative, being −22% under the CK treatment and −13% under the W treatment, with no significant difference between them. However, when the leaves were in the lower canopy of Chinese fir, soil warming significantly increased the AmCP to 51%, which was more than three times higher than the increase in the CK treatment. This indicated that soil warming expanded the variation in AmCP in the lower canopy, which was closer to the soil. The relative difference in NH4+-N between upper and lower leaves (AmUL) of Chinese fir was affected by leaf age (Figure 4B). When considering solely current-year leaves, the value of AmUL was negative with the difference ratio reaching −27%, and soil warming enhanced the disparity in AmUL. Comparatively, the AmUL in the previous-year leaves was higher than that in current-year leaves, and soil warming exerted no significant effect on this difference. This demonstrates that soil warming altered the distribution of leaf NH4+-N, with distinct effects on different leaf positions and ages.

3.5. The Distribution Differences in Leaf AA-N

For upper leaves, the relative difference in AA-N between current-year and previous-year leaves (AACP) was 40% under the CK treatment (Figure 5A). Notably, under soil-warming conditions, the AACP value was −19%, indicating that soil warming reversed the distribution trend of AA-N in the upper canopy. By contrast, soil warming had no impact on this difference in lower leaves. When considering the AA-N difference between the upper and lower leaves (AAUL), it was found that soil warming had a significant impact on both current-year and previous-year leaves (Figure 5B). Focusing on current-year leaves, the AAUL value showed a significant contrast between treatments: it was −21% under the CK treatment, while this value shifted to 19% under the W treatment. This shift indicates that soil warming not only altered the magnitude of AAUL but also reversed their direction in current-year leaves. Interestingly, this warming effect was more pronounced in previous-year leaves, where AAUL values were −38% under CK and 98% under the warming treatment. This striking 136% increase (from negative to positive) suggests that soil warming promotes the translocation of AA-N to the upper canopy of Chinese fir, thereby prioritizing AA-N supply to upper leaves.

4. Discussion

Soil inorganic nitrogen enters plant leaves primarily through transpiration pull [16,28]. A stronger transpiration process leads to a more vigorous transpiration-driven water transport flow. Thus, inorganic nitrogen, dissolved as a solute in the plant’s water transport system, is carried in greater quantities from root systems to aboveground leaves by this transpiration-driven water stream [29,30]. Our results showed that soil warming increased the content of NH4+-N and NO3−-N in Chinese Fir leaves (Figure 2 and Figure S1), although the differences were not statistically significant. This increase may have been linked to enhanced transpiration driven by warming. However, some studies have reported that soil warming induces stomatal closure in leaves, thereby reducing transpiration [10,31]. We surmise that this discrepancy arises because those studies likely observed irreversible leaf damage caused by excessive summer heat [32], whereas our sampling was conducted in winter (December) when overall temperatures remained moderate, avoiding localized leaf overheating. Some studies suggest that NH4+-N plays a more critical role than NO3−-N in crop drought resistance [17]. However, our results showed that the trends in NH4+-N and NO3−-N in Chinese fir leaves were identical, which may indicate that NO3−-N plays a significant role in resisting soil warming stress in coniferous trees. Although we are unaware of the driving mechanism underlying this change, based on the sources of inorganic nitrogen, we can deduce that the increase in soil inorganic nitrogen and the enhanced uptake of inorganic nitrogen by the plants were likely to be significant contributing factors. Factually, previous studies [7,33] have demonstrated that soil warming can elevate the content of soil inorganic nitrogen by enhancing the microbial-driven mineralization process. The leaf nitrogen concentration of olive trees has also been found to exhibit a positive correlation with soil nitrogen [34]. Moreover, soil warming significantly increases fine-root production and turnover rates to increase the uptake of nutrients (e.g., N) and water [35,36]. One study has also confirmed that warming can enhance the uptake of NH4+-N and NO3−-N in Picea asperata and Abies fargesii var. faxoniana [37]. Some mycorrhizal fungi have also been reported to help plants absorb nitrogen in warming soil [38]. Overall, owing to the limitations of this study, the alterations in inorganic nitrogen within Chinese fir leaves resulting from soil warming require more comprehensive and in-depth investigation.
As one of the three major forms of available nitrogen, the transport of AA-N from soil to leaves is also influenced by transpiration, which should theoretically follow the same trend as inorganic nitrogen. However, our results showed that soil warming significantly decreased leaf AA-N rather than increasing it (Figure 2 and Figure S1). This suggested that leaf AA-N are not solely derived from transpiration-driven transport but primarily originate from the assimilation of inorganic nitrogen within the leaves themselves [15,39]. Under drought stress, NO3−-N assimilation in aboveground plant tissues decreases [40,41]. Similarly, Zhang observed in Chinese fir that heat stress induced shifts in resource-utilization strategies, prompting the species to allocate fewer resources to synthesizing small-molecule compounds and more to producing soluble proteins and phospholipid adaptations that protect leaves from damage [42]. In this study, soil warming increased leaf inorganic nitrogen content while reducing AA-N, providing evidence that leaf NO3−-N assimilation may be influenced by soil temperature changes. Future research should focus on how soil temperature modulates nitrogen transformation functions in leaves, particularly the biochemical pathways linking inorganic nitrogen uptake to organic nitrogen synthesis under changing climatic conditions.
The distribution differences in three types of available nitrogen between canopies and leaf ages (Figure 3, Figure 4 and Figure 5) indicate that the content of NH4+-N, NO3−-N, and AA-N in Chinese fir leaves is not only affected by transpiration, but also by nitrogen allocation strategies [36,42]. The reduction in leaf water content resulting from soil warming (Table 1) may be an important cause of this phenomenon. Water deficiency can increase leaf lifespan, prolong the duration of nitrogen utilization, and enhance nitrogen-use efficiency [43], but some studies have shown that water deficit reduces the amount of nitrogen involved in photosynthesis [19]. Factually, the light availability, transpiration, and microclimate vary among different tree canopy layers [10], and this can also affect the nitrogen allocation in leaves. As shown in Figure 3 and Figure 4, NH4+-N and NO3−-N levels in current-year leaves from the upper canopy of Chinese fir were lower than those in previous-year leaves, whereas the opposite trend occurred in the lower canopy. These results revealed that inorganic nitrogen transported to the top of Chinese fir was preferentially stored in previous-year leaves, whereas in the lower canopy, it accumulated more in current-year leaves. This allocation strategy could be attributed to the distinct physiological functions of leaves at different canopy positions and ages. Current-year leaves exhibit higher efficiency in resource acquisition and utilization, with their metabolic rates surpassing those of previous-year leaves [21]. Soil warming accelerates leaf respiration and transpiration rates, driving preferential allocation of more nutrients to the growth of younger leaves [44]. Concurrently, this process prompts older leaves to channel more photosynthates into internal tissue construction, enhancing their drought resistance [45]. Consequently, inorganic nitrogen transported upward in Chinese fir is preferentially stored in previous-year leaves to sustain their long-term physiological needs and support tissue maintenance. In contrast, lower-canopy leaves, being closer to warmed soil, are more susceptible to being influenced by soil warming. Additionally, current-year leaves in the lower canopy are in a stage of vigorous growth and development, requiring substantial nitrogen for cell division, elongation, and new tissue formation. Therefore, inorganic nitrogen is preferentially transported to these lower-canopy current-year leaves to meet their high demand during rapid growth.
Although soil warming reduced leaf AA-N, it simultaneously promoted the transport of AA-N to upper canopy leaves, especially upper previous-year leaves (Figure 5). This phenomenon is hypothesized to arise from two primary mechanistic pathways. Firstly, soil warming enhances transpiration rates, thereby facilitating the upward transport of amino acids from various plant tissues to the apex of Chinese fir via transpiration pull. This enhanced transpiration acts as a driving force, propelling the amino-acid-laden fluid upwards within the plant’s vascular system. Secondly, the observed pattern may reflect the elevated metabolic demands at the canopy apex. The apical regions of the canopy, being more exposed to sunlight and involved in crucial photosynthetic activities, have higher physiological requirements. This likely induces preferential allocation of amino acids to support the physiological requirements of upper-canopy tissues. The forest canopy plays a critical role in the photosynthesis of entire forest ecosystems [10,46], as upper-canopy leaves require continuous amino acid supply to maintain chloroplast physiological functions [47]. Amino acid accumulation in upper previous-year leaves could contribute to osmoregulation under high temperatures [48]. Additionally, the requirement for heat shock proteins formed in plant leaves in response to heat stress [49] may also be an important factor contributing to this phenomenon. This “apex-priority” strategy, which involves concentrating resources to maintain apical growth dominance while utilizing older leaves as temporary nitrogen storage, balances the short-term demands of rapid new tissue formation and the long-term adaptive needs for plant survival. Such a strategy is of critical importance for Chinese fir in adapting to heat or drought stress, as it integrates resource allocation for both immediate growth and sustained resilience in changing environments.
Nitrogen absorbed by plant leaves is present in various cellular structures of leaf cells and some free compounds. This pattern of nitrogen allocation determines the efficiency of leaf photosynthesis [50,51]. It also influences leaf toughness [52], and the intensity of chemical defense [53]. Generally, a greater proportion of nitrogen allocated to photosynthesis correlates with faster plant growth [54]. Plant leaves at different canopy layers and of different ages exhibit substantial differences in their physiological characteristics [10]. Investigating the variations in the distribution of available nitrogen across canopy layers and leaf ages can help us understand how plants adapt to environmental changes, particularly during soil warming. Although our study provides evidence that soil warming alters the distribution of available nitrogen in Chinese fir leaves across different canopy layers and ages, our findings cannot be directly extrapolated to other tree species or forest types. We advocate for future research to include these aspects to more comprehensively characterize the impacts of long-term warming at the ecosystem level.

5. Conclusions

In summary, soil warming had a significantly greater impact on leaf AA-N than on NH4+-N and NO3−-N, and the extent of its influence was affected by leaf position and leaf age. This study provides insights into how Chinese fir adapts to soil warming by allocating available nitrogen in its leaves. However, due to the lack of long-term monitoring and stratified sampling of the roots, stems, and leaves of Chinese fir, we were unable to determine whether the observed nitrogen adjustments were short-term phenomena or stable adaptation mechanisms over time, and we also could not, at that time, distinguish the nitrogen allocation and utilization patterns in various plant organs. Isotopic experiments targeting available nitrogen in roots, stems, and leaves will be designed in future experiments to elucidate the allocation mechanism of available nitrogen in soil-plant interactions under soil-warming conditions.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/f16060942/s1, Figure S1: Effects of long-term soil warming on the distribution of NO3−-N (a), NH4+-N (b), and free amino acid (c) in leaves from upper and lower location of the Chinese fir. CK and W represent control and soil-warming treatments, respectively. Different letters indicate significant difference (p < 0.05) between leaves (n = 3).

Author Contributions

Conceptualization, X.C. and Y.Z.; methodology, L.Z.; software, X.C.; validation, Z.Y. and Y.L.; formal analysis, X.C.; investigation, L.Z.; resources, Z.Y.; data curation, X.C.; writing—original draft preparation, X.C.; writing—review and editing, Y.Z. and C.S.; visualization, Z.T.; supervision, Z.Y.; project administration, Y.Z.; funding acquisition, X.C. and Y.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Initial Funding to Talent Introduction from Sanming University (grant number: 22YG15S), the Natural Science Foundation of Fujian Province (grant number: 2024J01908), and the National Natural Science Foundation of China (grant number: 32401561).

Data Availability Statement

The data that support the finding of this study are available from the corresponding authors upon reasonable request.

Acknowledgments

We would like to thank Doubao (https://www.doubao.com/ (accessed on 24 May 2025)) for their meticulous assistance in refining the language and correcting the grammatical aspects of our English thesis, which has significantly improved its clarity.

Conflicts of Interest

The authors declare no conflicts of interest.

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Forests 16 00942 g001
Figure 1. Schematic diagram of leaf sampling.
Figure 1. Schematic diagram of leaf sampling.
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Figure 2. Effects of long-term soil warming on available nitrogen concentration (a) and the associated proportional difference (b) in Chinese fir leaves. CK and W represent control and soil-warming treatments, respectively. AA-N stands for free amino acid nitrogen. Different lowercase letters indicate significant difference (p < 0.05) between leaves (n = 12), ** p < 0.01.
Figure 2. Effects of long-term soil warming on available nitrogen concentration (a) and the associated proportional difference (b) in Chinese fir leaves. CK and W represent control and soil-warming treatments, respectively. AA-N stands for free amino acid nitrogen. Different lowercase letters indicate significant difference (p < 0.05) between leaves (n = 12), ** p < 0.01.
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Figure 3. Effects of long-term soil warming on the relative difference in NO3−-N between current-year and previous-year leaves (A) and between upper and lower leaves of Chinese fir (B). CK and W represent control and soil-warming treatments, respectively. Different lowercase letters indicate that the differences in NO3−-N are significant (p < 0.05, n = 3).
Figure 3. Effects of long-term soil warming on the relative difference in NO3−-N between current-year and previous-year leaves (A) and between upper and lower leaves of Chinese fir (B). CK and W represent control and soil-warming treatments, respectively. Different lowercase letters indicate that the differences in NO3−-N are significant (p < 0.05, n = 3).
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Figure 4. Effects of long-term soil warming on the relative difference in NH4+-N between current-year and previous-year leaves (A) and between upper and lower leaves of Chinese fir (B). CK and W represent control and soil-warming treatments, respectively. Different lowercase letters indicate that the differences in NH4+-N are significant (p < 0.05, n = 3).
Figure 4. Effects of long-term soil warming on the relative difference in NH4+-N between current-year and previous-year leaves (A) and between upper and lower leaves of Chinese fir (B). CK and W represent control and soil-warming treatments, respectively. Different lowercase letters indicate that the differences in NH4+-N are significant (p < 0.05, n = 3).
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Figure 5. Effects of long-term soil warming on the relative difference in AA-N between current- and previous-year leaves (A) and between upper and lower leaves of Chinese fir (B). CK and W represent control and soil-warming treatments, respectively. Different lowercase letters indicate that the differences in AA-N are significant (p < 0.05, n = 3).
Figure 5. Effects of long-term soil warming on the relative difference in AA-N between current- and previous-year leaves (A) and between upper and lower leaves of Chinese fir (B). CK and W represent control and soil-warming treatments, respectively. Different lowercase letters indicate that the differences in AA-N are significant (p < 0.05, n = 3).
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Table 1. Effects of long-term soil warming on moisture content of Chinese fir leaves.
Table 1. Effects of long-term soil warming on moisture content of Chinese fir leaves.
SituationCK 1 (%)W 1 (%)
UpperCurrent61.64 ± 5.4359.13 ± 1.40
Previous61.75 ± 2.5159.23 ± 1.14
LowerCurrent59.69 ± 5.1458.53 ± 1.37
Previous57.80 ± 2.7555.31 ± 3.51
1 CK and W represent control and soil-warming treatments, respectively. Values are means ± standard error (n = 3). No significant differences between CK and W (p > 0.05).
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Chen, X.; Zhu, L.; Yang, Z.; Shen, C.; Li, Y.; Tang, Z.; Zhu, Y. Thermal Modulation of Leaf Nitrogen Forms in Chinese Fir Under Soil-Warming Conditions. Forests 2025, 16, 942. https://doi.org/10.3390/f16060942

AMA Style

Chen X, Zhu L, Yang Z, Shen C, Li Y, Tang Z, Zhu Y. Thermal Modulation of Leaf Nitrogen Forms in Chinese Fir Under Soil-Warming Conditions. Forests. 2025; 16(6):942. https://doi.org/10.3390/f16060942

Chicago/Turabian Style

Chen, Xing, Lijuan Zhu, Zhijie Yang, Caixia Shen, Yin Li, Zexuan Tang, and Yankun Zhu. 2025. "Thermal Modulation of Leaf Nitrogen Forms in Chinese Fir Under Soil-Warming Conditions" Forests 16, no. 6: 942. https://doi.org/10.3390/f16060942

APA Style

Chen, X., Zhu, L., Yang, Z., Shen, C., Li, Y., Tang, Z., & Zhu, Y. (2025). Thermal Modulation of Leaf Nitrogen Forms in Chinese Fir Under Soil-Warming Conditions. Forests, 16(6), 942. https://doi.org/10.3390/f16060942

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