Microbial respiration per unit microbial biomass depends on litter layer carbon-to-nitrogen ratio

Soil microbial respiration is a central process in the terrestrial carbon (C) cycle. In this study, I tested the effect of the carbon-to-nitrogen (C : N) ratio of soil litter layers on microbial respiration in absolute terms and per unit microbial biomass C. For this purpose, a global data set on microbial respiration per unit microbial biomass C – termed the metabolic quotient (qCO2) – was compiled from literature data. It was found that qCO2 in the soil litter layers was positively correlated with the litter C : N ratio and was negatively correlated with the litter nitrogen (N) concentration. The positive relation between qCO2 and the litter C : N ratio resulted from an increase in respiration with the C : N ratio in combination with no significant effect of the litter C : N ratio on the soil microbial biomass C concentration. The results suggest that soil microorganisms respire more C both in absolute terms and per unit microbial biomass C when decomposing N-poor substrate. The reasons for the observed relationship between qCO2 and the litter layer C : N ratio could be microbial N mining, overflow respiration or the inhibition of oxidative enzymes at high N concentrations. In conclusion, the results show that qCO2 increases with the litter layer C : N ratio. Thus, the findings indicate that atmospheric N deposition, leading to decreased litter C : N ratios, might decrease microbial respiration in soils.