N-cycle gene abundance determination of N mineralization rate following re-afforestation in the Loess Plateau of China

Assessing the function of forest ecosystems requires an understanding of the mechanism of soil nitrogen mineralization. However, it remains unclear how soil N-cycling genes drive soil nitrogen mineralization during afforestation. In this study, we collected soil samples from a chrono-sequence of 14, 20, 30, and 45 years of Robinia pseudoacacia L. (RP14, RP20, RP30, and RP45) with a sloped farmland (FL) as a control. Through metagenomic sequencing analysis, we found significant changes in the diversity and composition of soil microbial communities involved in N-cycling along the afforestation time series, with afforestation effectively increasing the diversity (both alpha and beta diversity) of soil microbial communities. We conducted indoor culture experiments and analyzed correlations, which revealed a significant increase in both soil nitrification rate (Rn) and soil nitrogen mineralization rate (Rm) with increasing stand age. Furthermore, we found a strong correlation between soil Rm and soil microbial diversity (both alpha and beta diversity) and with the abundance of soil N-cycling genes. Partial least squares path modeling (PLS-PM) analysis showed that nitrification genes (narH, narY, nxrB, narG, narZ, nxrA, hao, pmoC-amoC) and denitrification genes (norB, nosZ, nirK) had a greater direct effect on soil Rm compared to their effect on soil microbial communities. Our results reveal the relationships between soil nitrogen mineralization rate and soil microbial communities and between the mineralization rate and functional genes involved in N-cycling, in the context of Robinia pseudoacacia L. restoration on the Loess Plateau. This study enriches the understanding of the effects of microorganisms on soil nitrogen mineralization rate during afforestation and provides a new theoretical basis for evaluating soil nitrogen mineralization mechanisms during forest succession.