Linking microbial life history strategy to carbon use efficiency under changing substrate and nutrient conditions

Microbial carbon use efficiency (CUE)-the allocation of carbon (C) between growth and respiration-is a key regulator of soil carbon turnover and microbial feedbacks to climate change. Yet, the microbial traits and environmental drivers shaping CUE in forest soils remain insufficiently understood. Here, we combined 18Olabeled DNA quantification of microbial growth with measurements of respiration, extracellular enzyme activities, and community composition across a 3842 km forest transect to assess how microbial ecological strategies influence CUE under contrasting resource conditions. Microbial CUE was estimated as the ratio of 18O-derived growth to total C uptake during controlled incubations. Microbial traits were characterized by extracellular enzymes involved in C, N, and P acquisition, as well as life-history strategies indicated by bacterial rRNA gene copy number and copiotrophic vs. oligotrophic taxa of microorganisms. We found that higher carbon availability stimulated C-acquiring enzyme production and microbial growth in high-latitude forests, resulting in elevated CUE. In contrast, N- and P-acquiring enzymes were associated with increased respiration in low-latitude forests, reflecting greater energy investment in nutrient acquisition at the expense of biomass production. Moreover, communities dominated by oligotrophic taxa exhibited reduced growth and enhanced respiration, consistent with lower CUE under low-resource conditions. Together, these findings indicate that shifts in microbial lifehistory strategies mediate resource-dependent trade-offs in carbon allocation, with important implications for soil carbon cycling across environmental gradients.