We simulated drought through 50% isolated rainfall exclusion and collected fine root exudates of Cunninghamia lanceolata during dry and wet seasons. Combined with total organic carbon (TOC) measurements and untargeted metabolomics (LC-MS) analysis, we investigated the seasonal responses of exudation rate and chemical composition of fine root exudates to drought stress. The results showed that there were significant seasonal variations in drought effects on root exudates. During dry season, the unit root length exudation rate and TOC concentration of root exudates decreased by 72.7% and 74.6%. In wet season, they increased by 58.0% and 35.4%, respectively. Organic acids, phenols, and amino acids were the dominant types in the root exudates under drought conditions. In dry season, defensive secondary metabolites such as phenols (e.g., phloroglucinol), flavonoids (e.g., catechin), and phenolic compounds (e.g., methyl mandelate) significantly increased, contributing to enhanced antioxidant capacity and regulation of rhizosphere microbial communities. In contrast, primary metabolites like sugars (e.g., glucose, deoxyribose) and organic acids (e.g., palmitic acid, 2-methylglutaric acid) significantly increased in wet season, promoting osmotic regulation and soil nutrient activation in C. lanceolata. Our results suggest that C. lanceolata adopts a "dry season defense, wet season attack" carbon allocation strategy to cope with drought stress, employing "conservative defense" through defensive secondary metabolites in dry season and "active adaptation" via resource-acquisitive primary metabolites in wet season.

Biological nitrogen fixation (BNF) is a key process that supplies nitrogen (N) to terrestrial ecosystems, yet its capacity to supply N can be suppressed by global change drivers that increase soil N availability. For example, atmospheric N deposition (N+) can directly raise soil N concentrations, whereas decreases in precipitation (Rain-) may indirectly raise soil N concentrations by constraining plant and microbial N uptake. Biotic interactions, including N inputs or N uptake from plant roots and mycorrhizal fungi, can also interact to mediate N availability and BNF responses under these global change factors, though their interactions on BNF remain poorly understood. Here, we show that in a humid subtropical forest, N+ significantly reduced BNF rates by 43% (P=0.03) and nitrogenase gene (nifH) abundance by 57% (P <0.001), whereas Rain- increased BNF rates by 55% without altering nifH abundance (P=0.03). Notably, the combined N+Rain- treatment neutralized the inhibitory effect of N+, producing BNF rates similar to those under ambient conditions. Structural equation modeling revealed that Rain- indirectly enhanced BNF by increasing soil water-extractable organic carbon (WEOC), whereas N+ directly impaired diazotrophic activity, indicating a novel buffering mechanism that balances opposing effects of these global change drivers. Root and mycorrhizal exclusion treatments showed negligible effects on BNF or nifH abundance, and did not interact with N+ or Rain-, indicating that diazotrophic activity is largely independent of plant root and mycorrhizal inputs. Taken together, our findings highlight the nonlinear outcomes of multi-factor global changes: while N+ can suppress diazotrophic functioning, concomitant declines in precipitation may, paradoxically, sustain BNF via a carbon-mediated facilitation in humid subtropical soils. This apparent buffering capacity, related to WEOC dynamics, highlights how changes in precipitation can mitigate disruptions in N cycling caused by N deposition, with implications for incorporating hydroclimatic-carbon-nitrogen relationships into Earth system models.

Forest restoration drives profound reorganization of soil microbial communities, yet how abundant versus rare taxa differentially govern ecosystem recovery through community assembly and functional complementarity remains poorly understood. Here, we integrated null-model analysis, co-occurrence networks, and environmental threshold modeling across a subtropical forest chronosequence (39-180 years; Pinus massoniana to Castanopsis climax) to reveal taxon-specific assembly processes and their implications for carbon sequestration. Our results showed that rare taxa (92.8-94.4% OTUs) exhibited increased α-diversity and a stronger influence of deterministic assembly (homogeneous selection) with increasing restoration age, driven by soil dissolved organic carbon. Conversely, stochastic processes dominated the assembly of abundant taxa (4.3-6.3% OTUs), maintaining broader environmental thresholds. Microbial network patterns further demonstrated that keystone taxa in early restoration stages were predominantly abundant taxa (e.g., Chloroflexi, Planctomycetota), maintaining network centrality and stability, whereas with advancing restoration, rare taxa increasingly contributed to network modular integration and overall stability. Functionally, abundant taxa sustained resource acquisition and environmental responsiveness, whereas rare taxa exhibit higher carbon-related function potential such as Carbon metabolism and Glycolysis and Gluconeogenesis, indicating distinct ecological roles. Structural equation modeling further revealed stronger associations between rare taxa and soil organic carbon compared with abundant taxa. These results provide new insights into the microbial community composition in restored ecosystems and highlight the critical role of functional complementarity between abundant and rare taxa in jointly maintaining microbial diversity and network structural stability during forest restoration. We demonstrate that managing microbial complementarity can amplify carbon sequestration by 22-40% in restored forests, thereby providing a microbial toolkit for achieving Natural Climate Solutions.

Forest restoration is a key strategy for mitigating climate change, yet the underlying mechanisms of belowground microbial community succession, particularly across soils derived from contrasting parent materials (lithologies), remain poorly understood. This study examined soil microbial community dynamics and functional profiles along a restoration chronosequence (comprising Pinus massoniana plantations, conifer-broadleaf mixed forests, secondary and old-growth forests) developed on two contrasting parent material lithologies (granite vs. sandstone) in a typical subtropical eroded region of China. Using high-throughput sequencing of 16S and ITS rRNA genes coupled with functional prediction (FAPROTAX and FUNGuild). Mantel tests revealed a significantly stronger correlation between soil properties and fungal diversity and composition, whereas bacterial diversity and composition remained nearly unchanged across the different successional stages and lithologies. Lithology significantly mediated microbial life-history strategies and functional gene abundances. In sandstone soils, bacterial K:r ratios were closely linked to the abundance of carbon (C) cycling genes. In contrast, granite soils promoted a shift in fungal strategies from r- to K-selected, supporting higher microbial resource-use efficiency and more stable community structure, thereby accelerating soil organic C stabilization. Furthermore, nitrogen (N) cycling genes (e.g., N fixation, denitrification) showed divergent successional trajectories: sandstone soils were characterized by mineral N transformations, while granite soils exhibited an organic N-dominated pathway. Our findings demonstrate that parent material filters microbial communities and their life-history strategies, driving functional divergence in nutrient cycling during forest ecosystem recovery. This provides critical mechanistic insights into the role of lithology in shaping belowground ecological processes during forest restoration.

Reforestation with broadleaf tree species in degraded Pinus massoniana plantations is a widely adopted strategy to enhance soil organic carbon (SOC) sequestration in subtropical China. However, the specific microbial processes that mediate this effect, particularly regarding nutrient cycling and microbial physiological efficiency, remain poorly understood. Here, we compared single pine stands with stands where broadleaf species were introduced, across a restoration chronosequence (10, 20, and 41 years). We aimed to elucidate how tree species mixing influences SOC storage through its effects on soil properties, microbial communities (with a focus on arbuscular mycorrhizal fungi, AMF), extracellular enzyme activities, and microbial carbon use efficiency (CUE). We found that broadleaf introduction increased SOC, total nitrogen, and AMF biomass, while stimulating the activity of nitrogen-acquiring enzymes (leucine aminopeptidase). In contrast, mixed stands showed reduced activity of phosphorus-acquiring enzymes, indicating an alleviation of soil phosphorus (P) limitation. Changes in microbial carbon metabolism accompanied these shifts, reflected in the altered CUE patterns. Structural equation modeling confirmed that the introduction of broadleaf promoted SOC accumulation primarily by increasing AMF abundance and alleviating P limitation, which collectively reshaped microbial community structure and function. This multi-pathway process enhanced the transformation of organic matter and microbial residue formation, contributing to greater SOC stabilization. Our results demonstrate that rehabilitating degraded pine forests with broadleaf species enhances carbon sequestration by modifying microbial nutrient limitation and metabolic strategies. Microbial CUE in this context reflects microbial metabolic status rather than functioning as a direct driver of SOC accumulation, providing a critical mechanistic basis for targeted forest management in subtropical region.

Restoring degraded forests is essential for supporting soil biodiversity and multifunctionality by increasing tree diversity, yet the consequences for soil biodiversity and function remain poorly understood. Our study compared soil microbial communities (16S rRNA/ITS sequencing) in a pair of degraded and restored monoculture and mixed-species pine (Pinus spp.) plantations, restoration initiated in 1981 and 2000 in southern China. This 20-year age difference allowed us to assess how long-term restoration drives the cumulative complementarity effects of tree species mixing, and to examine the role of microbial communities in mediating soil multifunctionality. We analyzed their diversity patterns and functional roles in soil multifunctionality, which encompasses carbon (C), nitrogen (N), and phosphorus (P) cycling, as well as substrate quality—an index of microbial degradability, where lower values indicate a greater susceptibility to microbial decomposition. We found that mixed-species plantations (conifers and broadleaf trees), harbor higher soil biodiversity and multifunctionality than monocultures. Specifically, mixed plantations demonstrated stronger correlations between soil N/P cycling, substrate quality, and bacterial/fungal richness. Microbial diversity appeared to drive functional recovery through complementary associations with microbial functional groups (as indicated by the Shannon diversity of pathogenic fungi). Multiple regression models further suggested that soil multifunctionality in monocultures was primarily governed by litter carbon-to-nitrogen (C:N) ratios, whereas it was dually regulated by both soil C:N ratios and microbial diversity in mixed plantations. Together, our findings highlighted the importance of mixed-species plantations in enhancing soil multifunctionality, which should be considered in future sustainable forest restoration practices.

While tree species mixtures significantly influence soil carbon (C) cycling, the role of ectomycorrhizal (EM) vs. arbuscular mycorrhizal (AM) fungal strategies in driving C stabilization remains unresolved during forest restoration. This study employed a paired-plot design across 16 degraded sites, comparing EM coniferous monocultures with EM-AM coniferous-broadleaved mixtures. By integrating phospholipid fatty acid profiling, amino sugar-based necromass quantification, and lignin phenol analysis, we traced C flow through particulate organic C (POC) and mineral-associated organic C (MAOC) pools in soils. Introducing AM trees into EM plantations significantly increased total soil C by 35.3%. This shift was driven by coordinated increases in plantderived C (66.9-123.7%) and fungal necromass (56.5-63.8%). The mixture overcame the apparent C saturation in MAOC observed in monocultures through a dual mechanism: it accelerated vanillyl-type lignin depolymerization in POC by 2.1-fold, boosting microbial necromass production (+39.6%), while AM-driven nitrogen supply (+38.9%) stimulated lignin degradation in POC and microbial necromass accumulation, thereby promoting both POC and MAOC pools and ultimately enhancing SOC accumulation. Structural equation modeling confirmed the primacy of plant-mycorrhizal interactions, a process that concurrently increased mineral N availability by 106.7%, accelerated POC accumulation, and thus benefited MAOC and SOC accumulation. Our findings demonstrate that introducing AM broadleaved trees into EM pine monocultures can significantly enhance the potential of long-term soil C sequestration in these managed forests. These findings demonstrate that strategic mycorrhizal complementarity facilitates the transfer of lignin-derived C into mineral-stabilized microbial necromass, thereby increasing soil carbon stocks in degraded forests. This provides a clear management implication: moving beyond monocultures toward functionally diverse mixtures, informed by mycorrhizal traits, can be a direct and effective strategy for enhancing the climate mitigation potential and long-term soil health of managed forests.

Forest restoration and its capacity to enhance soil carbon sequestration constitute critical mechanisms for augmenting global soil carbon pools. However, the distinct regulatory roles of plant‐derived versus microbial‐derived carbon in restoration‐driven carbon sequestration remain poorly understood. Here, we collected whole‐profile soils from over 180 years of forest restoration sequences, including coniferous, mixed, secondary and old‐growth forests. We measured soil microbial necromass carbon (MNC) and lignin phenols to identify the key factors that influence the variation of carbon sequestration in surface (0–20 cm), subsurface (20–60 cm) and deep soil (60–100 cm). We systematically revealed depth‐dependent soil organic carbon (SOC) dynamics: Subsurface and deep SOC accumulation contributed disproportionately to total SOC stock increases during restoration, supplementing the conventional surface‐centric perspective. Through coupled measurements of MNC and lignin phenols, we uncovered divergent transformation pathways—while surface soils exhibited declining plant‐derived carbon proportions, microbial‐derived carbon progressively dominated deeper layers (27.4%–53.4% of SOC in late‐stage restoration). This microbial dominance is driven by depth‐specific conditions (nutrient scarcity, mineral protection) rather than being universal across all soil depths. Crucially, the coupling of lignin degradation and microbial assimilation in deep soils enhanced MNC stabilization, providing empirical evidence for the ‘microbial carbon pump’ theory through vertically resolved lignin–MNC linkages. Mechanistically, lignin distribution correlated with microbial‐mediated liberation and vertical transport, whereas depth‐specific nutrient‐microbe interactions governed MNC accumulation. The proportion of microbial‐derived carbon positively predicted SOC stocks in subsurface and deep soils ( R 2 = 0.63–0.80), contrasting with negative correlations of plant‐derived carbon in surface layers. Synthesis and applications . These findings establish a tripartite framework for precision restoration: (1) prioritizing subsurface carbon sequestration by alleviating nutrient limitations in deep soils, (2) steering plant–microbe‐derived carbon coupling through tree species mixtures that balance lignin inputs and microbial turnover and (3) optimizing vertical carbon allocation by synchronizing root traits with microbial stoichiometric needs. Our work redefines restoration strategies to harness depth‐specific biogeochemical processes, offering a pathway to maximize SOC sequestration across the whole soil profile.

The environmental impacts of reactive nitrogen (Nr) emitted from fertilized cropland present significant challenges for balancing food security, air pollution and climate change mitigation. As a leading agricultural producer, China requires high-resolution Nr emissions modeling within a comprehensive processed-based framework to address these issues effectively. In this study, we applied a process-based agroecological model (FEST-C*) to estimate daily Nr emissions at 0.25° in China during 2020 and analyzed the driving factors by using Structural Equation Modeling, Random Forest, and Dominance Analysis. The hotspots of annual Nr emissions were in North China, Southeast China, and Southwest China, collectively responsible for over 80 % of the total emissions. Approximately 81 % of the total Nr emissions were from wheat, maize, and rice fields. Timing and amount of basal and topdressing fertilization under different crop rotation systems determined the monthly and seasonal variations of Nr emissions. The impacts of various factors on Nr emissions varied with NH3 being mainly driven by fertilizer consumption and other Nr species (N2O, NO, and HONO) also affected by soil temperature and water content. The spatial distributions of monthly Nr emissions calculated by FEST-C* were more realistic than currently available emission inventories compared to satellite or field observations. These findings will enable policymakers to develop effective control measures that alleviate cropland Nr emissions while sustaining crop production in China.

Introducing broadleaf tree species into pine forests has been widely adopted as a strategy to mitigate forest degradation and enhance soil carbon sequestration. However, the microbial-mediated mechanisms underlying the effects of mixed coniferous-broadleaf forests on soil carbon sequestration in degraded forest ecosystems remain inadequately elucidated. This study investigated two previously degraded sites in southern China, presenting vegetation cover since 1981 and 2000, respectively. Within these sites, single Pine (Pinus massoniana) plantations and mixed plantations of pine and broadleaf species (Pine+Schima superba) were established as part of restoration efforts. Amino sugars were quantified as biomarkers of topsoil (0-10 cm) microbial necromass, and microbial community diversity and structure were assessed via high-throughput sequencing techniques (16S rRNA and ITS sequencing) and phospholipid fatty acids analysis. Our results showed that tree mixture increased soil microbial necromass, with the fungal necromass increasing more rapidly than bacterial necromass, suggesting that fungal necromass rapidly responds to tree species mixture effects, but bacterial necromass drives the sustained accumulation of microbial necromass C in the later restoration stages. Furthermore, mixed plantation soils yield higher bacterial alpha-diversity (Chao1), and ectomycorrhizal fungi abundance, all of which positively influence microbial necromass accumulation. Structural equation modelling results showed that tree mixture strengthens the integrated effects of litter quality and soil nitrogen status on microbial community composition (16S rRNA and ITS sequencing) and functions (FUNGuild) and thus enhancing microbial necromass. Importantly, this ecological restoration effect shifted microbial necromass regulation from dual independent pathways (physicochemical vs. microbial factors) to an integrated soil-microbial feedback loop, with mixed pine-broadleaf plantations enhancing the interaction between soil nutrient availability and microbial functional diversity to promote microbially-derived carbon accrual in soils. These findings demonstrate that introducing tree species with high-quality litter constitutes an effective vegetation restoration strategy, simultaneously accelerating soil organic matter sequestration in degraded pine forest ecosystems.

Global change factors like atmospheric nitrogen (N) deposition and drought pose threats to forest ecosystem including soil microbial diversity. However, how arbuscular mycorrhizal (AM) fungi associated with tree respond to N deposition and drought remains largely unknown. Here root- and soil-inhabiting AM fungi were examined in a field experiment involving N addition and simulated drought (precipitation exclusion) in a Chinese fir (Cunninghamia lanceolata) plantation. The results showed that precipitation exclusion significantly reduced AM fungal intraradical colonization rate in summer, while N addition had no significant effect on AM fungal morphological traits of intraradical colonization rate, hyphal and spore densities. However, seasonal changes significantly affected AM fungal morphological traits, with higher values were observed in summer than in winter. Neither N addition nor drought significantly affected AM fungal diversity or community composition, but AM fungal communities exhibited pronounced seasonal differences. In winter, both root- and soil-associated AM fungal community composition significantly correlated with the ratio of microbial biomass carbon and phosphorus (MBC/MBP), while in summer AM fungal communities were primarily associated with MBP and DOC. These findings highlight the importance of accounting for interaction of N addition and drought, and seasonal response difference on AM fungi in subtropical forest ecosystems.

Global warming may cause widespread soil carbon loss across multiple regions. Subtropical montane ecosystems store substantial soil organic carbon (SOC). However, the impacts of global warming on SOC pools with varying stability levels remain unclear. We simulated warming using an altitudinal translocation approach in Wuyishan Mountain, subtropical China. Specifically, soil columns were moved downslope from 1400 m (coniferous forest) to 1000 m (mixed forest), from 1000 m to 600 m (evergreen broadleaf forest), and from 600 m to 200 m (again, evergreen broadleaf forest). We investigated warming-induced changes in labile carbon, recalcitrant carbon, and total carbon pool in soils, along with the underlying mechanisms. Results showed temperature increases (1.45-2.11 degrees C) following downward translocation of soil cores. Warming reduced labile carbon across all elevations but decreased recalcitrant carbon only at higher elevations, with no net change in total carbon stocks. Mantel tests revealed stronger linkages between temperature, enzyme, and microbial communities at higher elevations. Structural equation modeling revealed that warming-induced changes in labile carbon, mediated by edaphic factors, directly influenced recalcitrant carbon. This cascaded effect reflects the integrated biotic-abiotic interactions driven by environmental change. Our findings identify high-elevation montane forests as critical hotspots of soil carbon vulnerability under warming, where disproportionate losses in mineral-associated carbon occur via thermal disruption of Fe-organic complexes. Protecting these elevational refugia requires urgent prioritization in climate-smart strategies. Specific management should target iron redox cycling, fungal functional traits, and oxidative enzyme activities to stabilize mineral-associated organic carbon.

China's subtropical forests are widely recognized as one of the world's largest natural sources of nitrous oxide (N2O), primarily due to high nitrogen (N) deposition from anthropogenic activities. Climate change has made precipitation reduction increasingly common in subtropical regions, significantly influencing N2O emissions. Denitrification is the main process contributing to N2O emissions in subtropical forest soils; however, most previous studies have focused on bacterial denitrification, often overlooking fungal denitrification. In this study, a factorial experiment was conducted using a randomized complete block design with four replicates per treatment, in a subtropical forest soil. We examine the effects of simulated N deposition, precipitation reduction, and their combination on the abundance of genes encoding nitrite reductase enzymes, including bacterial (nirK, nirS) and fungal (nirK) variants, with a focus on their seasonal dynamics during summer and winter. N deposition and precipitation reduction treatments showed distinct effects. N deposition significantly reduced fungal and bacterial nirK abundance in winter and decreased bacterial nirS abundance in summer. Precipitation reduction further suppressed bacterial nirK and nirS abundance in winter but had no effect on fungal nirK. In addition to treatment effects, seasonal variation also shaped gene abundances, with higher fungal nirK levels in winter and higher bacterial nirK in summer, while bacterial nirS remained seasonally stable. Predictor analysis using random forest models identified available phosphorus (AP) as the strongest driver of fungal nirK abundance. In contrast, bacterial nirK was primarily influenced by soil pH and AP, while ammonium was the key regulator of bacterial nirS. These results highlight the distinct responses of fungal and bacterial denitrifiers to seasonal changes, nitrogen deposition, and precipitation reduction, emphasizing the need to consider both microbial groups when examining biogeochemical cycles and their environmental controls under future climate scenarios. These insights are crucial for refining predictive models of N2O fluxes and for designing informed management practices in subtropical forest ecosystems.

With global warming,the pattern of extreme precipitation has changed considerably.How China's plantations,which are widely distributed and at a rapid growth stage,will respond to climate change;the prediction of their carbon sink functions;and the corresponding management decisions urgently need methods to monitor growth quickly and accurately.In this study,a 17-year-old middle-aged Chinese fir plantation was monitored three times semiannually from February 2019 to February 2020 by using UAV LiDAR.Three methods to estimate the individual tree Aboveground Biomass(AGB)via UAV LiDAR parameters were compared using ground survey data.The three methods were height and crown diameter regression,diameter at breast height-tree height regression,and diameter at breast height-tree height-crown diameter regression(D-HCD).Then,the seasonal growth changes in tree height,diameter at breast height,and AGB were estimated in a six-month interval.Results showed that D-HCD was the optimal method for estimating the individual AGB of Chinese fir(R2=0.77,root mean square error[RMSE]=15.99 kg).In the D-HCD method,the tree height and crown diameter extracted by UAV LiDAR were used to estimate the diameter at breast height(DBH),and AGB was calculated by substituting DBH and tree height into an allometric equation.The annual and even seasonal growth changes in the Chinese fir plantation in the fast-growing period could be accurately monitored by UAV LiDAR.The average total accuracy of individual tree identification in 16 plots reached 0.927,the estimated RMSE of tree height was only 0.13 m,the R2 between the estimated and measured annual individual AGB changes(AAGB)was 0.64,RMSE was equal to 1.87 kg,and the relative error(rRMSE)was 29.74％.When the individual trees were upscaled to plots,the relative error of AAGB estimation was reduced,and rRMSE decreased to 17.10％.During the study period,the average daily temperature in spring and summer was 7 ℃ higher than that in autumn and winter,and the amount of rainfall was more than three times that in autumn and winter.The seasonal distribution of rainfall in this year was seriously uneven,and the growth of Chinese fir showed obvious differences in dry and wet seasons.The average increments in individual tree height in wet and dry seasons were 0.50 and 0.13 m,respectively,and the biomass increments were 5.12 and 1.37 kg,respectively.Individual AAGB increased with DBH class during annual and seasonal growth;the larger the individual was,the more advantageous the growth was.In particular,the growth of dominant trees in the dry season was much better than that of other diameter classes,indicating a strong drought tolerance.However,the growth of individuals whose DBH was much smaller than the average level almost stopped.

Global warming is widely expected to alter nitrogen (N) cycling in terrestrial ecosystems by accelerating N transformations in soils. However, it is unclear how warming will affect plant-soil N cycling in subtropical ecosystems. Here, we measured the N transformations including net ammoniation, nitrification, nitrous oxide emissions and nitrate in soil solution throughout the plant-soil continuum with 2 years of experimental soil warming (+5 degrees C) in a young subtropical Chinese fir mesocosm. Seasonal variations of soil and plant (foliage and root) N concentrations and isotopes (delta N-15), foliar water use efficiency and arbuscular mycorrhizal colonization rate were measured. Soil warming significantly increased net ammonization and nitrification of the soil, together with the transient positive response observed in inorganic N of the soil. Warming increased nitrate N fluxes in soil solution and nitrous oxide emissions in the first year but not in the second year, suggesting N losses through leaching and gaseous in the initial period of warming. Warming primarily induced enrichment of N-15 in foliage relative to the soil, which was attributed to the trade-offs of persistent increases in plant N uptake caused by enhanced tree growth and a decrease in N losses with continuous warming. Furthermore, young trees' growth and N uptake capacity can rapidly acclimate to climate warming as a result of warming-induced increases in arbuscular mycorrhizal colonization and foliar water use efficiency. Our findings highlight that warming accelerates the plant-soil N cycle and promotes young trees' growth and N uptake, which in turn reduces soil N lost from this subtropical ecosystem. Therefore, our study suggests that the competition for N between plants and microbes governs whether subtropical forests are opened or closed N cycle systems under climate warming. We predict that subtropical young forests can still maintain their high productivity because young trees can maintain their N uptake capacity and adequate soil N supply in facing future climate warming.Read the free Plain Language Summary for this article on the Journal blog.

Understory vegetation has an important impact on soil organic carbon (SOC) accumulation. However, little is known about how understory vegetation alters soil microbial community composition and how microbial diversity contributes to SOC chemical composition and persistence during subtropical forest restoration. In this study, removal treatments of an understory fern (Dicranopteris dichotoma) were carried out within pine (Pinus massoniana) plantations restored in different years in subtropical China. Soil microbial community composition and microbial diversity were measured using phospholipid fatty acids (PLFAs) biomarkers and high-throughput sequencing, respectively. The chemical composition of SOC was also measured via solid-state 13C nuclear magnetic resonance (13C NMR). Our results showed that fern removal decreased alkyl C by 4.2 % but increased O-alkyl C by 15.6 % on average, leading to a decline of alkyl C/O-alkyl C ratio, suggesting altered chemical composition of SOC and lowered SOC recalcitrance without fern. Fern removal significantly lowered the fungi-to-bacteria ratio, and it also reduced fungal and bacterial diversity. Partial correlation analysis revealed that soil nitrogen availability was a key factor influencing microbial diversity. Bacterial diversity showed a close relationship with the Alkyl C/O-alkyl C ratio following fern removal. Furthermore, the microbial community structure and bacterial diversity were responsible for 18 % and 55 % of the explained variance in the chemical composition of SOC, respectively. Taken together, these analyses jointly suggest that bacterial diversity exerts a greater role than microbial community structure in supporting SOC persistence during understory fern removal. Our study emphasizes the significance of understory ferns in supporting microbial abundance and diversity as a means of altering SOC persistence during subtropical forest restoration.

To date, it is unclear how differences in litter quality affect soil organic matter (SOM) decomposition through a phenomenon called 'priming effects' (PEs), especially for low-fertility forest soils under field conditions. Here, the effects of low- and high-quality leaf litter on PE and microbial metabolism of litter-derived carbon (C) were explored in a low-fertility pine (Pinus massoniana) plantation. A 185-day in situ incubation experiment was carried out by adding two 13C-labeled leaf litters to the pine soil—a low-quality (high lignin: nitrogen) litter sourced from pine and a broadleaved species Schima superba produced high-quality (low lignin: nitrogen) litter. To determine the key microbial groups contributing to PEs, the abundance of 13C-labelled litter enrichment in soil phospholipid fatty acids (13C-PLFAs) was quantified. We found that high-quality litter decomposed more rapidly than low-quality litter, with both litter-derived CO2 efflux reaching a plateaued level during the experimental period. Low-quality litter induced net positive PEs, while high-quality litter induced net negative PEs during the litter decomposition processes. Bacterial groups governed the negative PEs induced by high-quality litter, whereas fungal communities targeted the positive PEs induced by low-quality litter. Random forest model and variation partitioning analysis demonstrated that the direction and magnitude of PEs were driven by litter-induced changes in key microbial groups rather than the structure of the microbial community. Our results demonstrate that microorganisms preferentially utilized litter-derived C in high-quality litter treatment and SOM in low-quality litter treatment, respectively. In contrast to low-quality litter, adding high-quality litter promoted the microbial metabolism of litter-derived C, reducing SOM decomposition (strong negative PEs). Taken together, this study provides isotope-based suggestions for the improvement of degraded pine forests—introducing tree species that produce high-quality litter may benefit soil C sequestration by reducing soil C losses due to PE in nutrient-poor pine forests.

The priming effects (PEs) of soil organic carbon (SOC) is a crucial process affecting the C balance of terrestrial ecosystems. However, there is uncertainty about how PEs will respond to climate warming. Here, we sampled soils along a subtropical elevation gradient in China and conducted a 126-day lab-incubation experiment with and without additions of C-13-labeled high-bioavailability glucose or low-bioavailability lignin. Based on the mean annual temperature (MAT) of each elevation (9.3-16.4 degrees C), a temperature increase of 4 degrees C was used to explore how PEs mediate the decomposition of SOC in response to warming. Our results showed that the magnitude of glucose-induced PEs (PEglu) was higher than lignin-induced PEs (PElig), with both PEs linearly increasing with MAT. Across the MAT (i.e., elevation) gradient, warming had consistent negative effects on PEglu, whereas rising MAT exacerbated the negative effects of warming on PElig. Moreover, the temperature sensitivity of SOC decomposition decreased after adding glucose and lignin across the MAT gradient, suggesting that fresh C inputs may prime microbial breakdown of labile SOC under warming. Taken together, warming alleviated the SOC loss due to PEs through varying mechanisms depending on substrate bioavailability, since warming mediated the PEglu by increasing available nitrogen and weakening microbial nitrogen-mining but inhibited the PElig by switching from microbial nitrogen-mining to microbial co-metabolization. Our findings highlight the role of warming in regulating the PEs and suggest that incorporating the suppression effect of warming on PEs can contribute to the accurate prediction of soil C dynamics in a warming world.

[Objective]Soil microbial metabolic efficiency profoundly affects soil nutrient cycling and soil carbon sequestration in the recovery process of eroded and degraded areas.However,it is unclear how tree species mixture affects soil extracellular enzyme stoichiometry and microbial carbon-utilization efficiency in single Pinus massoniana plantation in the eroded red soil area.[Methods]In this study,we used single P.massoniana plantation,and P.massoniana and Schima superba mixed plantation with different restoration years,determined the soil physicochemical properties,extracellular enzyme activity and phospholipid fatty acid markers of different forests,and calculated the soil enzyme stoichiometric ratio and microbial carbon-utilization efficiency(CUE).[Results](1)With the increase of restoration years,soil acid phosphatase(AP)and β-glucosidase(βG)activities were significantly reduced,while C∶NEEA(βG∶NAG),vector length(VL)and vector angle(VA)of the mixed forest were lower than that of pure forest,while C∶PEEA(βG∶AP)and N∶PEEA(NAG∶AP)showed the opposite trend,indicating that long-term mixing is beneficial to alleviate the carbon limitation and phosphorus limitation of microorganisms.(2)Mixing was beneficial to improve CUE and significantly increased with the increase of forest age.Specifically,compared with that of Y39-CF and Y19-MF,CUE of Y39-MF significantly increased by 200.00％and 136.84％,respectively.(3)By analyzing the characteristics of soil enzyme activity and the relationship between CUE and environmental factors,it was found that soil carbon,nitrogen and phosphorus availability was an important influencing factor,mainly affecting AP activity,C∶PEEA,N∶PEEA,VL,VA and CUE,while the stoichiometric ratio of soil nutrients mainly affected βG activity and C:NEEA.(4)The results of redundancy analysis(RDA)showed that the ratio of fungi and bacteria mainly affected the characteristics of soil enzyme activity and CUE in the mixed forest,while the main influencing factors in pure forest were actinomyces and Gram-negative bacteria.[Conclusion]Mixing tree species significantly improves forest soil nutrients,affects the release of soil extracellular enzymes,and improves CUE,and it is conducive to soil carbon sequestration.Therefore,the differences between pure forest and secondary forest should be considered in future management and research of P.massoniana forest.

Vegetation restoration of degraded land affects litter quality by changing the composition of tree species, providing direct effects on regulating the dynamic of soil organic C (SOC) through the priming effect (PE). However, it is unclear how the combined effects caused by vegetation restoration and input of different quality litters on PE-related C loss and gain. Here, we collected soils from an unrestored site and a site restored for 20 years, adding 13C-labeled low-quality (with high C/nitrogen [N] and lignin/N) and high-quality (with low C/N and lignin/N) litters to the soil, respectively. Our results revealed that adding high- and low-quality litter in two sites produced positive PEs after 150-day laboratory-based incubation. The PE induced by high-quality litter was lower than that of low-quality in two sites, which can be interpreted as low-quality litter has higher C/N that aggravates the nutrient imbalance of microorganisms and enhances their demand for N, prompting microorganisms to accelerate the mineralization of SOC through the "N mining". High-quality litter inputs can boost microbial C use efficiency and alleviate soil C loss due to PE in unrestored and restored pine forests. Moreover, high-quality litter input has a greater positive effect on SOC gain in unrestored lands than in restored lands, suggesting that litter with higher nutrient availability or fertilization is especially needed for the restoration of degraded soil fertility and C formation. Taken together, this study highlights the importance of tree species producing high-quality litter in mediating SOC decomposition and formation during degraded lands restoration, which is beneficial for the restoration of degraded lands and the enhancement of soil C sequestration.