Microbial carbon use efficiency, biomass residence time and temperature sensitivity across ecosystems and soil depths

Decomposition of soil organic matter by microorganisms is a fundamental mechanism driving the terrestrial carbon (C) cycle. Microbial C use efficiency (CUE), microbial biomass residence time (MRT), and soil C temperature sensitivity (Q10) co-determine the fate of soil C in a changing climate. In order to reveal the effect of soil depth and varying hydrologic properties on CUE, MRT, and Q10 of microbial respiration, we incubated soils from three ecosystems (wetland, grassland, and forest) and soil depths (0–10, 20–30, and 50–60 cm) at two temperatures (10 and 30 °C). Microbial CUE was estimated using a substrate-independent method by incorporating 18O from labeled water into microbial DNA with the simultaneous measurement of microbial respiration. CUE ranged from about 0.2 to 0.7 with a mean value of 0.5 ± 0.1, MRT ranged from 4 to 73 days with a mean value of 26 ± 19 days, and Q10 ranged from 1.8 to 2.9, averaging 2.3 ± 0.3 across all samples. We found that CUE increased but MRT and Q10 decreased along the wetland-grassland-forest hydrologic gradient; and they all increased with soil depth. Moreover, CUE and MRT were lower at 30 °C than that at 10 °C. Although there were some differences in factors regulating the variation in CUE, MRT or Q10 among soil depths and ecosystem types, both within individual ecosystems and depths, CUE, MRT, and Q10 were strongly correlated to available C:N ratios, clay content, and C quality, respectively. In conclusion, our findings emphasize the importance of stoichiometry and C quality of available substrates in predicting the variation in microbial C use efficiency and soil C temperature sensitivity in different soil depths and along a hydrologic gradient.