Microbe-mediated carbon (C) transformation plays a crucial role in the accumulation of soil organic carbon (SOC). However, microbial conversion efficiency of newly-added C and native SOC to necromass remain under-investigated, especially in relationship to nutrient availability, limiting our understanding of how ‘microbial C pump’ functions in the overall sequestration of soil C. Here we collected the rhizosphere and non-rhizosphere soils under broadleaved and coniferous trees of varying nutrient availability, and conducted an 80-day soil incubation with ¹³C-labeled glucose to evaluate the efficiencies of microbial biomass (phospholipid fatty acids) synthesis (i.e., C use efficiency, CUE′) and necromass accumulation (i.e., amino sugar accumulation efficiency, AAE) relative to respiration. We found that microbial AAE and CUE′ had different variation patterns and influencing factors. The best predictor of AAE was the ratio of inorganic nitrogen (N) to added glucose (reflecting N limitation), while ratios of fungi to bacteria, C to N, and soil pH were important for predicting CUE′. These findings suggest that the efficiency of microbial necromass accumulation, strongly influenced by N availability, can be decoupled from that of biomass synthesis. Furthermore, we found that glucose addition stimulated microbial transformation of native SOC into necromass in C- but not N-limited soils (with a high AAE) without invoking a priming effect, potentially enhancing microbe-mediated SOC sequestration. In contrast, glucose addition may lead to enhanced degradation of SOC or specifically  SOC-derived necromass in soils under N limitation. These findings suggest that promotion of microbe-mediated SOC sequestration should not only increase C input but also consider nutrient availability. We also argue that AAE is a more reliable indicator to assess the efficiency of microbial C pump fueled by labile C. More research is needed to verify how soil microbes convert new C inputs and native SOC in diverse soils.
 

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