Increasing plant species diversity benefits soil protein accumulation in a subtropical forest

Abstract Protein, derived from plant and microbial residues, constitutes a significant portion of soil nitrogen or organic carbon pool and plays a crucial role in determining soil nitrogen and carbon sequestration. Though diversifying plant species has been regarded as a promising strategy for promoting soil nitrogen or organic carbon accumulation during vegetation restoration or afforestation, how increasing plant species diversity (PSD) would influence soil protein pool remains unknown. We selected 45 plots covering a natural gradient of PSD, which was reflected by the Shannon index and varied from 0.15 to 3.57, in a subtropical karst forest. The major objective was to investigate the impact of increasing PSD on soil protein abundance, with soil protein depolymerization rate and multiple biotic and abiotic variables being measured to explore the underlying mechanism. The relative abundance of soil protein, that is proportion in soil organic matter varied between 24.1 and 34.0% with an average of 29.2 ± 2.3% across the 45 plots. Soil protein abundance increased on average by 1.56% with each unit of PSD increase. Nevertheless, soil protease activity, protein depolymerization rate and microbial amino acid uptake rate all significantly ( p < 0.05) decreased as PSD increased. The mechanism underlying the increase of soil protein abundance in response to higher PSD included three aspects, that is stimulating microbial biomass and subsequent microbial residue input to the soil, promoting mineral protection of soil protein via elevating levels of soil exchangeable calcium and magnesium, and decreasing soil protease activity and protein depolymerization due to aggravated soil microbial phosphorus limitation. Synthesis and application : Our study suggests that increasing PSD is an efficient way to promote soil protein accumulation. Given that protein is a major component of soil nitrogen or organic carbon pool, diversifying plant species during vegetation restoration or afforestation would hence benefit soil nitrogen and organic carbon sequestration.