Microbial carbon and energy use efficiency after a centennial history of land use

Aim: Soil microorganisms rely on coupled fluxes of carbon (C) and energy to fuel their maintenance, switch from dormancy to activity and grow in microhabitats characterized by dynamic environmental conditions. Although C and energy fluxes in microbial metabolism are closely linked, there is a lack of cases investigating how they jointly affect microbial growth under land use change. Methods: We assessed C use efficiency (CUE) using ¹⁸O-labelled water on 0–20 cm soils collected from hundred years of continuous land use, combined with substrate-induced growth respiration and microcalorimetry to monitor land-use effects on microbial growth and heat release. Results: The shorter lag-phase and higher fractions of growing microorganisms, aligned with microbial proliferation induced faster microbial turnover and 11–27% higher CUE in grassland. Respiration and heat release curves exhibited a coupled pattern. However, the heat release peak occurred 3–5 h earlier irrespective of land uses. Specifically, grassland soil had greater heat release by 12–39% than cropland and bare fallow soils. After the heat release peak, the heat rate decreased in grassland, while bare fallow soil exhibited a more gradual decrease. This was attributed to the continual decrease in C availability (i.e., dissolved organic C) and the dormancy of microorganisms (i.e., lower microbial biomass) in bare fallow limited the utilization of substrate by decomposers and lowered microbial growth rate, and prolonged heat release. Conclusion: We conclude that sustaining fungal-mediated processes and substrate quality is vital for enhancing soil C sequestration under land-use intensification.