<b>Phosphorus limitation rather than temperature governs microbial carbon use efficiency in cropland soils</b>

Soil microbial carbon use efficiency (CUE), an integrated metric defining carbon allocation between microbial growth and respiration, plays a central role in regulating the first steps of carbon accumulation or depletion in soil. The environmental dependment of microbial CUE remain uncertain due to the paucity of large-scale data, particularly in agricultural ecosystems. Here, we conducted a large-scale survey of microbial CUE across Chinese croplands (spanning ~29º of latitude) and combined it with metagenomics to elucidate the most important factors governing microbial CUE.There was significant spatial heterogeneity of microbial CUE (ranged from 0.19 to 0.60), with an average of 0.31. Challenging the theoretical speculation of inverse temperature-efficiency relationship, microbial CUE in croplands is independent with temperature. However, the spatial variability of microbial CUE is primarily governed by phosphorus limitation and secondarily by microbial community composition. In phosphorus limitation soils, soil microbiomes allocate more carbon in gene expression and enzymatic investment to phosphorus-acquisition at the expense of growth efficiency. Our large-scale study provides the first solid evidence that microbial phosphorus limitation governs the microbial CUE in cropland soils, which is critical for policy-making for soil carbon sequestration in cropland by microbial approach.土壤微生物碳利用效率（CUE）是定义微生物生长与呼吸作用之间碳分配的综合指标，在调控土壤碳积累或损耗的初始过程中发挥核心作用。由于大规模数据的匮乏，微生物CUE的环境依赖性仍不明确，这一不确定性在农业生态系统中尤为突出。本研究针对跨越约29个纬度的中国农田开展了大规模微生物CUE调研，并结合宏基因组学（metagenomics）技术，阐明了调控微生物CUE的关键影响因子。研究结果显示，微生物CUE存在显著空间异质性，范围为0.19至0.60，平均值为0.31。本研究挑战了“温度与效率呈负相关”的理论推测，农田土壤的微生物CUE与温度并无显著关联。然而，微生物CUE的空间变异主要受磷限制调控，其次受微生物群落组成影响。在磷限制土壤中，土壤微生物会将更多碳投入基因表达与酶合成以获取磷资源，却以牺牲生长效率为代价。本大规模研究首次提供了确凿证据，表明农田土壤的微生物磷限制是调控微生物CUE的核心因素，这一发现对于通过微生物途径制定农田土壤固碳相关政策具有重要指导意义。