Long-term warming of a forest soil reduces microbial biomass and its carbon and nitrogen use efficiencies

Global warming impacts biogeochemical cycles in terrestrial ecosystems, but it is still unclear how the simultaneous cycling of carbon (C) and nitrogen (N) in soils could be affected in the longer-term. Here, we evaluated how 14 years of soil warming (+4°C) affected the soil C and N cycle across different soil depths and seasons in a temperate mountain forest. We used H218O incorporation into DNA and 15N isotope pool dilution techniques to determine gross rates of C and N transformation processes. Our data showed different warming effects on soil C and N cycling, and these were consistent across soil depths and seasons. Warming decreased microbial biomass C (−22%), but at the same time increased microbial biomass-specific growth (+25%) and respiration (+39%), the potential activity of β-glucosidase (+31%), and microbial turnover (+14%). Warming reduced gross rates of protein depolymerization (−19%), but stimulated gross N mineralization (+63%) and the potential activities of N-acetylglucosaminidase (+106%) and leucine-aminopeptidase (+46%), and had no impact on gross nitrification (+1%). Microbial C and N use efficiencies were both lower in the warming treatment (−15% and −17%, respectively). Overall, our results suggest that long-term warming drives soil microbes to incorporate less C and N into their biomass (and necromass), and to release more inorganic C and N to the environment, causing lower soil C and N storage in this forest, as indicated by lower soil C and total N contents. The decreases in microbial CUE and NUE were likely triggered by increasing microbial P constraints in warmed soils, limiting anabolic processes and microbial growth and promoting pervasive losses of C and N from the soil.全球变暖可影响陆地生态系统的生物地球化学循环，但目前尚不清楚长期尺度下土壤碳（C）与氮（N）的协同循环会受到何种影响。本研究以温带山地森林为研究对象，探究了14年持续土壤增温（+4℃）下，不同土层深度与季节的土壤碳、氮循环响应特征。本研究采用H₂¹⁸O掺入DNA法与¹⁵N同位素池稀释技术，测定了土壤碳、氮转化过程的总速率。研究结果显示，增温对土壤碳、氮循环的影响存在显著分化，且该效应在不同土层深度与季节间保持一致。增温使微生物生物量碳降低22%，但同时提升了单位微生物生物量的生长速率25%与微生物呼吸速率39%，还提高了β-葡萄糖苷酶的潜在活性31%以及微生物周转速率14%。增温降低了蛋白质解聚的总速率19%，但促进了总氮矿化作用63%，同时提升了N-乙酰氨基葡萄糖苷酶与亮氨酸氨基肽酶的潜在活性，增幅分别达106%与46%；而对总硝化作用无显著影响（变化幅度为1%）。增温处理下的微生物碳利用效率（CUE, carbon use efficiency）与氮利用效率（NUE, nitrogen use efficiency）均有所降低，分别下降15%与17%。综合来看，本研究结果表明，长期土壤增温会促使土壤微生物将更少的碳、氮同化至自身生物量（及残体）中，同时向环境释放更多无机碳与无机氮，最终导致该森林生态系统的土壤碳、氮储量下降，这一点可通过土壤总碳与总氮含量降低得到印证。微生物碳、氮利用效率的下降，可能源于增温土壤中微生物受到的磷限制加剧，进而限制了同化代谢过程与微生物生长，并最终加剧了土壤碳、氮的全域性流失。