Impacts of successive Chinese fir plantations on soil carbon and nitrogen dynamics: Conclusive insights from metagenomic analysis.

Abstract

Chinese fir forests play a significant role both economically and ecologically, contributing to soil and water conservation while also serving as an efficient timber-producing species that brings economic benefits. However, the issue of soil degradation due to continuous Chinese fir planting cannot be overlooked. Continuous planting leads to a decrease in soil nutrients, a reduction in microbial diversity, and changes in microbial community composition, which in turn affect the abundance of carbon and nitrogen cycle functional genes in Chinese fir forest soils. We utilized metagenomic sequencing technology to investigate the dynamics of microbial community composition and carbon and nitrogen-related functional genes in the soils of Chinese fir forests across different plantation generations, exploring their relationship with soil carbon and nitrogen nutrients. We found that the relative abundance of bacterial communities is dominant in both phylum and genus levels within microbial communities. The partial least squares path models (PLS-PM) indicated that planting generations had a negative effect on dissolved organic carbon (DOC), nitrate nitrogen (NO₃^--N), and microbial biomass nitrogen (MBN), with a significant negative impact on microbial residual carbon (MRC). Easily utilizable carbon nutrient (DOC) in Chinese fir forest soil showed a significant positive effect on the abundance of carbon fixation functional genes (direct effect = 0.91, p < 0.01), and on the abundance of methane metabolism functional genes (direct effect = 1.27, p < 0.01). Nitrogen nutrients (NO₃^--N, MBN) in the soil also had a significant positive effect on the abundance of carbon fixation functional genes (direct effect = 0.90, p < 0.01). Bacterial communities (Acidobacteria and Verrucomicrobia) had significant negative effects on carbon and nitrogen cycling processes. The abundance of nasA and nirA genes generally showed a decreasing trend with increasing plantation generations. The decrease in available nitrogen nutrients with increased plantation generations was influenced by Assimilatory nitrogen reduction to ammonia (ANRA), an energy-consuming process. In summary, the continuous planting of Chinese fir had significant impacts on the carbon and nitrogen nutrient cycling processes, and it influenced the composition of microbial communities and the spatial distribution of functional genes. Clarifying the changes in carbon and nitrogen nutrient cycling processes in Chinese fir continuous planting provides a reference for maintaining the productivity of Chinese fir plantations.