Excluding Roots or Mycorrhizal Hyphae Alters the Microbial Community and Function by Decreasing Available C and N in a Subtropical Chinese Fir Forest

Abstract

Carbon (C) inputs, primarily from roots and associated mycorrhizal hyphae, serve as crucial energy sources for microbial-driven C and nitrogen (N) cycling in the soil. However, our understanding of how soil microbial diversity, function, and associated soil properties respond to the exclusion of roots and their associated mycorrhizal hyphae remains limited. In our study, we conducted an experiment with no exclusion of roots or mycorrhizal hyphae (Control), exclusion of roots and retention of mycorrhizal hyphae (NR), and exclusion of roots and mycorrhizal hyphae (NRH) in a Chinese fir (Cunninghamia lanceolata) forest, the most important plantation in China. The soil properties, microbial community diversity and composition, and microbial function were investigated after 2 years of experiment exclusion. We found that exclusion of roots and hyphae significantly decreased DOC, DON, NH4+-N, and NO3−-N, but not SOC, TN, and TP, indicating that the exclusion of roots and mycorrhizal hyphae mainly reduced available C and N concentrations. Meanwhile, the species richness and Chao1 of bacteria and fungi were significantly reduced, primarily due to the decrease in available C and N levels. These findings suggest that the removal of roots and mycorrhizal hyphae results in a decrease in C and N availability, subsequently leading to a loss of microbial diversity. Compared to after the CT treatment, the relative abundances of Proteobacteria and Actinobacteria phyla were reduced after exclusion of roots and hyphae. However, the relative abundances of the phyla Acidobacteria, WPS2, Rozellomycota, and Glomeromycota showed an increase in exclusion treatments. Furthermore, the relative abundances of genes for C degradation (e.g., malQ, malZ, chi, rfbB, bglX, and ablA), C fixation (e.g., accA, icd, korA, and korB), and N fixation (nifS) were increased; conversely, the N degradation genes (e.g., nasA, nirB, ureC, and gdh2) were decreased in treatments involving excluding roots and hyphae. These results, in conjunction with the strong relationships between functional genes and DOC, DON, NH4+-N, and NO3−-N, suggest that microorganisms regulate functional genes to enhance C and N fixation or organic matter decomposition in response to C or N limitation resulting from root and mycorrhizal hypha exclusion. Collectively, our study revealed that the changes in roots-derived C directly altered available C and N in soil, which influenced the microbial community and function, and, in turn, regulated microbial-driven nutrient cycling in forest soils.