Respiration and carbon use efficiency characteristics of soluble protein-derived carbon by soil microorganisms: A case study at afforested sites

Soluble protein plays a significant role in the release of carbon dioxide (CO2) in soils. However, our understanding of the respiration and C use efficiency (CUE) characteristics of soluble protein-derived C by soil microorganisms is limited. To address this issue, we sampled surface soils (0-10 cm) from seven tree monocultures and examined the temporal dynamics of turnover of C-14-labelled soluble protein-derived C by soil microorganisms. Two double first-order exponential kinetic decay models were applied to analyze the mineralization data (i.e., with and without abiotic protein-surface interactions). The model incorporating the immobilization of protein on the non-living solid phase exhibited the best fit to the experimental data (R-2 > 99.6%). Our results suggest that 66.1-73.9% of the soluble protein-derived C was immobilized by the non-living solid phase in soils. After uptake by the soil microbial community, 8.0-13.8% of the C was rapidly respired as CO2, while 15.0-20.8% was used in anabolic processes, resulting in a CUE of 55.1-70.2%. However, there was little effect of forest type on protein turnover rate in the soil. The C:N ratio of soil microbial biomass (C/N-mic) was positively related to the CUE of protein and exhibited less variation within a forest type. Compared with soil microbial biomass C and N, C/N-mic could serve as a better indicator of the CUE of protein by soil microorganisms. This study sheds light on the respiration and CUE characteristics of soluble protein-derived C by soil microorganisms at afforested sites and enhances our understanding of the trade-off between the metabolism of protein-derived C by soil microorganisms and its immobilization by the non-living solid phase in soils.