Plants and microbes are the primary drivers in affecting the formation and accrual of soil organic carbon (SOC) for natural ecosystems. However, experimental evidence elucidating their underlying mechanisms for SOC accumulation remains elusive. Here, we quantified plant and microbial contributions to SOC accrual in successional subtropical forests by measuring leaf-, root-, and microbial biomarkers, root and leaf litter inputs, and microbial C decomposition. The long-term monitoring results showed that SOC accumulated rapidly at the early-successional stage, but changed little at the mid- and late-successional stages. SOC accrual rate was positively correlated with fine-root production and microbial C turnover, but negatively with annual litterfall. Biomarker data exhibited that the rapid SOC accumulation was jointly driven by root- and microbe-derived C inputs from the early- to mid-successional stages. In contrast, aboveground litterfall considerably contributed to soil C accrual from the mid- to late-successional stages compared to belowground processes, although SOC accumulation is low. Our study revealed the importance of root production and microbial anabolism in SOC accrual at the early stages of forest succession. Incorporating these effects of belowground C inputs on SOC formation and accumulation into earth system models might improve model performance and projection of long-term soil C dynamics.

Understanding the sensitivity of ecosystem respiration (ER) to increasing temperature is crucial to predict how the terrestrial carbon sink responds to a warming climate. The temperature sensitivity of ER may vary on a diurnal basis but is poorly understood due to the paucity of observational sites documenting real ER during daytime at a global scale. Here, we used an improved flux partitioning approach to estimate the apparent temperature sensitivity of ER during the daytime (E_0,day) and nighttime (E_0,night) derived from multiyear observations of 189 FLUXNET sites. Our results demonstrated that E_0,night is significantly higher than E_0,day across all biomes, with significant seasonal variations in the day–night discrepancy in the temperature sensitivity of ER (ΔE₀ = E_0,night/E_0,day) except for evergreen broadleaf forest and savannas. Such seasonal variations in ΔE₀ mainly result from the effect of temperature and the seasonal amplitude of NDVI. We predict that future warming will decrease ΔE₀ due to the reduced E_0,night by the end of the century in most regions. Moreover, we further find that disregarding the ΔE₀ leads to an overestimation of annual ER by 10~80% globally. Thus, our study highlights that the divergent temperature dependencies between day- and nighttime ER should be incorporated into Earth system models to improve predictions of carbon–climate change feedback under future warming scenarios.

Classical theory posits that late-successional plants develop symbiotic mycorrhizal networks to improve plant nutrient use efficiency and soil nutrient conservation, particularly of nutrients like phosphorus (P), which become depleted during plant community development. However, experimental test of these hypothesized functions are lacking. Here, we conducted a experiment of trenches lined with mesh screens of varying sizes across subtropical succession and a regional survey of 14 subtropical forest sites to explore mycorrhizal hyphal effects on soil nitrogen (N) and P cycling and plant use efficiency. Results revealed that later successional plants had greater P use efficiency, showing lower leaf P concentration and a higher N:P ratio than early successional ones. Ectomycorrhizal (EcM) fungal abundance increase with succession and largely explain the variation in plant PUE and leaf N: P ratio. At late successional stage, these fungi promote soil P conservation through enhancing soil P adsorption and microbial biomass P: N ratio, while simultaneously stimulating nitrogen (N) cycling through the greater release of N-related relative to P-related enzymes. Regionally, EcM abundance was positively correlated with soil N: P enzyme ratio, and nonlinearly correlated to leaf N:P ratio after controlling for soil nutrients, confirming its role in enhancing soil N cycling and P conservation. Our findings highlight EcM fungal critical role in balancing forest N and P cycling, underscoring the need to integrate mycorrhizal effects into nutrient management strategies for subtropical forests.

：Ecological succession and restoration rapidly promote multiple dimensions of ecosystem functions and mitigate global climate change. However, the factors governing the limited capacity of forests to sequester soil carbon (C) in old ecosystems are poorly understood. Ecological theory predicts that plants and microorganisms jointly evolve into a more mutualistic relationship to accelerate detritus decomposition and nutrient regeneration in old than young ecosystems, likely explaining the changes in C sinks across forest succession or rewilding. To test this hypothesis, we conducted a field experiment of root-mycorrhizal exclusion in successional subtropical forests to investigate plant-decomposer interactions and their effects on soil C sequestration. Our results showed that soil C accrual rate at the 0-10 cm soil layer was higher in early-successional arbuscular mycorrhizal (AM) forests with an average value of 1.26 mg g−1 yr−1 than that in the late-successional ectomycorrhizal (EcM) forests with nonsignificant change. A transition from early-successional AM to late-successional EcM forests increase fungal diversity, especially EcM fungi. In the late-successional forests, the presence of ectomycorrhizal hyphae promotes soil C decomposition and nutrient cycle by increasing soil nitrogen and phosphorus degrading enzyme activity as well as saprotrophic microbial richness. Across early- to late-successional forests, mycorrhizal priming effects on soil C decomposition explain a slow-down in the capacity of older forests to sequester soil C. Our findings suggest that a transition from AM to EcM forests supporting greater C decomposition can halt the capacity of forests to provide nature-based global climate change solutions. Keywords: Belowground C allocation; Ectomycorrhizal richness; Enzyme activity; Mycorrhizae-saprotroph interaction; Nutrient competition