Afforestation results in a wide range of soil resources with carbon (C), nitrogen (N), and phosphorus (P) levels that rarely meet microbial elemental demands. Such stoichiometric imbalances result in the limitation of microbial activity by nutrients, and have consequences for microbial C and nutrient use efficiency and ultimately the fate of soil C. However, how microorganisms cope with stoichiometric imbalances following afforestation and how their responses regulate microbial-driven C emissions remain unclear. We compared sites along a 42 year Robinia pseudoacacia afforestation chronosequence on the Loess Plateau of China, to quantify soil microbial nutrient limitation and explore the mechanisms underlying microbe-mediated C dynamics under conditions of stoichiometric imbalance. Soil available nutrients, potential activities of C-, N-, and P-acquiring enzymes, microbial biomass, microbial community composition and diversity, as well as microbial respiration were measured. Results showed that stoichiometric imbalances increased soil enzymatic activities targeting the mobilization of limiting nutrients at different afforestation stages. Specifically, soil microbial communities were limited by C in farmland, co-limited by N and P at the 10-year site, and became more limited by P as stand age increased. Reductions in stoichiometric imbalance along the afforestation chronosequence corresponded to an increased microbial alpha diversity and fungi-to-bacteria ratio. Stoichiometric imbalances were more strongly associated with changes in soil bacterial beta diversity than fungal beta diversity. Bacterial communities transitioned from being oligotrophic (Actinobacteria dominant) to copiotrophic (Proteobacteria dominant) during forest development, and this was significantly related to stoichiometric imbalance. However, no significant correlation was detected between stoichiometric imbalance and the dominant fungal phyla (i.e., Ascomycota, Basidiomycota, and Zygomycota). The synergistic responses of enzymatic stoichiometry and microbial community properties to stoichiometric imbalance following afforestation led to reduced microbial threshold elemental ratios, which elevated microbial C use efficiency and increased biomass turnover time, further suppressing microbial respiration. Such collaborative-adaptations imply that more C will be diverted into microbial biomass rather than losing, thus could be favorable to soil C storage. Collectively, these findings highlight the importance of stoichiometric imbalances in regulating microbial-driven C emissions and contribute to an improved understanding of how substrate quality changes induced by revegetation influences terrestrial C flows in ecologically fragile areas.

植树造林产生的土壤资源种类繁多，其中碳（C），氮（N）和磷（P）的水平很少满足微生物的元素需求。这种化学计量的不平衡导致养分限制了微生物的活动，并影响了微生物C和养分的利用效率，最终影响了土壤C的命运。然而，造林后微生物如何应对化学计量的不平衡以及它们的反应如何调节微生物驱动的C排放仍不清楚。我们比较了42年刺槐的站点黄土高原地区绿化造林时序，定量计算化学计量失衡条件下土壤微生物养分的含量，探讨微生物介导的碳动态变化的机理。测量土壤有效养分，获取C，N和P的酶的潜在活性，微生物生物量，微生物群落组成和多样性以及微生物呼吸。结果表明，化学计量失衡增加了土壤酶活性，其目标是在不同造林阶段动员有限养分。具体而言，在农田中，土壤微生物群落受到碳的限制，在10年期土壤中受到氮和磷的共同限制，并且随着林分年龄的增加，土壤微生物群落也受到磷的限制。沿绿化时间顺序减少化学计量失衡对应于增加的微生物α多样性和真菌与细菌的比率。化学计量失衡与土壤细菌β多样性的变化比真菌β多样性更密切相关。细菌群落从贫营养化（在森林发展过程中，放线菌占主导地位，而嗜营养菌则占优势（Proteobacteria占优势），这与化学计量失衡显着相关。但是，在化学计量失衡与主要真菌门（即子囊菌，担子菌菌和合子菌）之间未发现显着相关性。）。绿化后酶促化学计量和微生物群落特性对化学计量失衡的协同响应导致微生物阈值元素比降低，从而提高了微生物的C利用效率和生物量转换时间，进一步抑制了微生物的呼吸作用。这种协作适应意味着更多的碳将被转移到微生物生物质中而不是损失，因此可能有利于土壤碳的存储。总的来说，这些发现突出了化学计量失衡在调节微生物驱动的碳排放中的重要性，并有助于人们更好地了解由植被引起的基质质量变化如何影响生态脆弱地区的陆地碳流动。