Impacts of drought events and nitrogen (N) deposition on forests are increasingly concerning in the context of global climate change, but their effects, in particular, their interactive effects on soil respiration and its components remain unclear. A two-factor random block field experiment was conducted at a subtropical Moso bamboo forest in Southwest China to explore the response of soil respiration (R_s), autotrophic respiration (R_a), and heterotrophic respiration (R_h) to throughfall re-duction and N addition. Our results showed that throughfall reduction significantly decreased R_s, which is mainly attributed to the decrease in R_a as a result of the decline in fine roots biomass. The N addition led to microbial carbon limitation hence significantly decreased R_h, and thus R_s. We also observed the negative effect of throughfall reduction on R_s was exacerbated by N addition, which is attributed to the significant reduction in R_a under the interaction between throughfall reduction N addition. Our findings suggest that R_a tended to respond more sensitively to potential drought, while R_h responds more sensitively to N deposition, and consequently, increased soil N availability caused by N deposition might aggravate the negative effect of expected drought on soil carbon cycling.

Soil extracellular enzyme stoichiometry (EES) is the essential predictor in nutrient status and resource limitation of soil microorganisms, whose metabolism has a vital role in biogeochemical cycling and ecosystem function. However, little is known about how N₂-fixer tree species with different planting patterns affect soil nutrient resources in terms of extracellular enzyme activity (EEA) or EES within aggregates in degraded karst ecosystems. In this study, we evaluated soil EEA and EES related to carbon (C), nitrogen (N), and phosphorus (P) cycles across two eight-year-old pure plantations of legume species [Dalbergia odorifera T. Chen (PD) and Acrocarpus fraxinifolius Wight ex Arn. (PA)] and a mixed plantation of the two tree species listed above (MP). Meanwhile, a nearby undisturbed shrubland was used as a control (CK). We concluded that the activities of C-, N-, and P-acquiring enzyme increased to different degrees in the N₂-fixer tree species stands (particularly in MP) compared to CK in all aggregates. Compared to CK, MP significantly increased by 39.0%, 54.0%, 39.3%, and 24.8% in total C-acquiring EEA, 41.1%, 60.5%, 47.8%, and 12.5% in total N-acquiring EEA, and 100.4%, 79.7%, 69.2%, and 56.4% in total P-acquiring EEA within >2 mm, 1–2 mm, 0.25–1 mm, and <0.25 mm aggregates, respectively. Furthermore, the logarithmic transformed ratio of C-, N-, and P-acquiring enzyme activities was 1.20:1.08:1, which deviated from the global ratio (1:1:1). Vector analysis of EEA showed that the vector length (VL) within aggregates was significantly lower than that of CK in all stands of N₂-fixer species except PD; while in all treatments, vector angle (VA) was <45° for all aggregate sizes, except in MP, where VA reached 45° for <0.25 mm aggregate. These indicated soil microbes were limited by C and N together. However, MP significantly alleviated microbial C and N limitation than CK (p < 0.05). There were obvious positive relationships between enzyme C:N, C:P, and N:P ratios. VL was markedly negatively linked to VA. EES was markedly related to most soil nutrients and microbial biomass stoichiometry ratios. Changes in soil EEA and EES were primarily driven by available phosphorus (AP), microbial biomass carbon (MBC), soil C:N and MBN:MBP ratios. Together, our results demonstrate the influences after introducing N₂-fixer tree species (particularly MP) for vegetation recovery on soil microbial nutrient limitation and ecological processes in aggregate level and will contribute to the development of ecological restoration practices and fertility management in degraded karst ecosystems of southwest China.

Nitrogen (N)-fixing plants play an important role in stimulating soil nitrogen supply, but the effect of N-fixing plants on soil organic carbon (SOC) sequestration has not been well documented. In this study, we measured rhizospheric carbon components and N content of first to second (1–2)-order and third to fifth (3–5)-order roots of N-fixing and non-N-fixing plants in a montane tropical rainforest in Hainan Island, China, to examine the effects of N-fixing plants on SOC and soil heavy fraction carbon (HFC), and to ascertain the main regulating factors. The results showed that rhizospheric SOC between (1–2) and (3–5)-order roots was 14.2% and 10.5% greater, respectively, for N-fixing plants compared with non-N-fixing plants. Similarly, the HFC was 0.9% (1–2) and 3.6% (3–5) greater for N-fixing plants than non-N-fixing plants. Redundancy analysis showed that spatial variance in rhizospheric SOC and HFC associated with 1–2-order roots was explained by specific root length (SRL) of second-order roots (55.8%) and specific leaf area (SLA) (14.6%), whereas spatial variance was better explained for 3–5 roots by total soil N (44.6%), diameter of fifth-order roots (16.4%), and leaf C (17.9%), respectively. Within the rhizospheric soil of 1–2 roots, N-fixing plants with smaller SRL of second-order roots presented greater SOC and HFC, whereas smaller SLA was associated with greater HFC. By contrast, rhizospheric soil of 3–5 roots under N-fixing plants had greater SOC and HFC levels in environments with higher total soil N and smaller diameters of fifth-order roots. Our study revealed that N-fixing plants enhanced rhizospheric SOC and HFC compared with non-N-fixing plants. The rhizospheric SOC and HFC were correlated strongly by root morphology traits, leaf morphology, and soil properties. Study Implications: In this article, we compared rhizospheric soil of N-fixing and non-N-fixing species in a montane tropical rainforest in Hainan Island, China, to examine the effects of N-fixing plants on SOC and HFC. Our results suggest the N-fixing plants enhance and stabilize rhizospheric SOC and HFC; rhizospheric SOC and HFC for first- to second-order roots of N-fixing plants were mostly negatively regulated by SRL of second-order roots; and N-fixing plants had positive effects on rhizospheric SOC and HFC for third- to fifth-order roots through increased soil N. Understanding these mechanisms could improve and accumulation of soil C sink in tropical areas.

Understanding the relationship between biodiversity and ecosystem multifunctionality remains a central challenge in ecology. Although biodiversity is generally associated with enhanced multifunctionality, this relationship can vary across ecosystems and environmental contexts. It remains unclear how concurrent global changes, such as drought and nitrogen deposition, modulate biodiversity–multifunctionality relationships, in particular the moso bamboo forests in subtropical China. We conducted a field manipulation experiment combining throughfall reduction and nitrogen addition to simulate drought and nitrogen deposition, and applied amplicon sequencing to examine how these global change factors independently and interactively influence soil microbial diversity, multifunctionality, and their interrelationships. We found that throughfall reduction significantly reduced soil multifunctionality, likely due to lower soil water availability, and nitrogen addition exacerbated this negative effect. However, environmental changes did not affect bacterial and fungal α-diversity, nor was soil multifunctionality correlated with microbial diversity. Instead, shifts in soil multifunctionality were closely linked to changes in microbial community composition and co-occurrence network structure. In particular, increasing relative abundances of Acidobacteriota, Myxococcota, and Chytridiomycota were positively associated with higher soil multifunctionality. Our findings demonstrate that soil multifunctionality responses to throughfall reduction and nitrogen addition are primarily mediated through microbial community reorganization rather than diversity loss, providing new insights into the mechanisms linking global change drivers, microbial communities, and ecosystem functioning.

The roles of multiple global change are expected for many terrestrial ecosystems in future. As two main global change factors, the impact of drought and nitrogen deposition and their interaction on soil respiration and its components (R) remains unclear. To explore the responses of soil respiration (Rs), autotrophic respiration (Ra) and heterotrophic respiration (Rh) to multiple global change factors, we established a field experiment of throughfall reduction and nitrogen additions in a subtropical Moso bamboo (Phyllostachys heterocycla) forest in the Southwest China, using a 4 × 4 completely randomized design. Results showed that bivariate exponential equation with soil temperature (T) and soil moisture (SWC) (R=a.ebT.SWCc) was fitted to predict Rs, Ra and Rh. Throughfall reduction, nitrogen additions and their interaction had no effect on annual mean Rs and Ra, but nitrogen additions significantly depressed annual mean Rh. Nitrogen additions significantly decreased contribution of Rh to Rs and increased contribution of Ra to Rs, however, the contributions were non-responsive under throughfall reduction. The more positive effect of nitrogen additions on the contribution of Ra to Rs was appeared compared with that of throughfall reduction, thereby more negative effect on the contribution of Rh to Rs. The fine root biomass, fine root carbon and nitrogen storage regulated Rs, while fine root phosphorus storage determined Ra. The Rh was negatively correlated with vector lengths, thus suggesting that microbial carbon limitation caused the decline of Rh. Our findings demonstrate that the nitrogen additions played overriding role than throughfall reduction in affecting the contribution of Ra and Rh to Rs. Moreover, the negative response of temperature sensitivity of Rs and Rh to nitrogen additions, suggesting that that the nitrogen additions may weaken the positive response of soil CO2 emission to global climate warming. Our study highlights asymmetrical responses of Rs, Ra and Rh to throughfall reduction and nitrogen additions and could enhance accurate predictions of soil carbon dynamics in response to multiple global climate change in future.

Background: Microbial necromass carbon (MNC) is a critical component of persistent soil organic carbon (SOC). However, the elevational patterns of MNC accumulation and its contribution to SOC across different soil layers remain poorly understood in climate-sensitive alpine ecosystems. We investigated these dynamics across topsoil and subsoil layers along a subtropical elevational gradient (1700–3500 m) in southwest China, which encompasses five vegetation types from evergreen broadleaved forests to alpine meadows. Results: The results showed that MNC content increased linearly with elevation, with a more pronounced accumulation in topsoil. This pattern resulted from a clear depth-dependent shift in primary regulators, where fine root biomass and mean annual temperature dominated topsoil MNC accumulation while total phosphorus dominated subsoil accumulation. Notably, the contribution of MNC to SOC remained consistent across soil depths (averaging 29.9 ± 3.3%) and exhibited a quadratic elevational trend, peaking in high-elevation meadows. Structural equation modeling revealed that iron oxides and total phosphorus exerted significant direct effects on the MNC contribution to SOC, while fine root biomass had an indirect effect by enhancing phosphorus availability. Conclusions: These findings highlight alpine meadows as key reservoirs of MNC and reveal distinct depth-dependent mechanisms governing its accumulation. This study provides a mechanistic framework for predicting SOC dynamics under climate change in vulnerable alpine ecosystems.