Soil microbial C:N:P stoichiometry and microbial maintenance respiration (i.e. metabolic quotient, qCO₂) were monitored along a nutrient gradient in soils from a 52-year space-for-time chronosequence of reclaimed agricultural land after brown-coal mining. Land reclamation produced loess soils of initially low (0.2%) SOC. Consecutive agricultural land management led to a gradual recovery of SOC contents. Our data revealed sudden shifts in microbial stoichiometry and metabolic quotient with increasing SOC at a critical value of 1% SOC. As SOC increased, accrual rate of C into microbial biomass decreased, whereas microbial N increased. Simultaneously, metabolic quotient strongly decreased with increasing SOC until the same critical value of 1% SOC and remained at a constant low thereafter. The microbial fractions of the soil in samples containing < 1% SOC were out of stoichiometric equilibrium and were inefficient at immobilising C due to high maintenance respiration. Increasing SOC above the threshold value shifted the soil microbes towards a new equilibrium where N became growth limiting, leading to a more efficient acquisition of C. The shift in microbial N accrual was preluded by high variation in microbial biomass N in soils containing 0.5–0.9% SOC indicative of a regime shift between microbial stoichiometric equilibria. Our data may help in establishing a quantitative framework for SOC targets that, along with agricultural intensification, may better support feedback mechanisms for a sustainable accrual of C in soils.

土壤微生物C：N：P化学计量和微生物维持呼吸（即代谢商，qCO _2）在褐煤开采后，经过52年的开垦后农用土地的时空时序，沿着土壤中的养分梯度对土壤进行了监测。土地开垦产生的黄土土壤最初的SOC低（0.2％）。连续的农业土地管理导致SOC含量逐渐恢复。我们的数据显示，在1％SOC的临界值下，微生物化学计量和代谢商会随着SOC的增加而突然变化。随着SOC的增加，C在微生物生物量中的累积率降低，而微生物N则增加。同时，代谢商随SOC的增加而强烈降低，直到相同的临界值（1％SOC）为止，此后一直保持在恒定的低水平。样品中土壤微生物含量< 1％SOC处于化学计量平衡之外，由于维持呼吸较高，因此无法有效地固定C。SOC增加到阈值以上，使土壤微生物趋向于一个新的平衡，在该平衡下N成为生长的限制，从而导致更有效地获取C。微生物N累积的变化是由于微生物生物量N在含0.5-0.9的土壤中变化较大而导致的SOC的百分比表示微生物化学计量平衡之间的变化。我们的数据可能有助于建立SOC目标的定量框架，与农业集约化一起，可以更好地支持可持续地累积土壤中C的反馈机制。从而导致更有效地获得碳。微生物N累积的变化是由于土壤中SOC为0.5–0.9％的土壤中微生物生物量N的高变化所致，表明微生物化学计量平衡之间的变化。我们的数据可能有助于建立SOC目标的定量框架，与农业集约化一起，可以更好地支持可持续地累积土壤中C的反馈机制。从而导致更有效地获得碳。微生物N累积的变化是由于土壤中SOC为0.5–0.9％的土壤中微生物生物量N的高变化所致，表明微生物化学计量平衡之间的变化。我们的数据可能有助于建立SOC目标的定量框架，与农业集约化一起，可以更好地支持可持续地累积土壤中C的反馈机制。