Vegetation Restoration Alters the Diversity and Community Composition of Soil Nitrogen-Fixing Microorganisms in the Loess Hilly Region of China

In this study, we determined the variations in the diversity and structure of nitrogen-fixing microorganisms during afforestation and how this variability correlates with the soil physicochemical factors. Soils were sampled from replicated plots representing three revegetation types (Robinia pseudoacacia L. [Rp], Caragana korshimkii Kom [Ck], and abandoned land) with adjacent farmland as the control at a research site in Ansai District, Yan'an City, Shaanxi Province, China. Using high-throughput sequencing of the nifH rRNA gene, we found that revegetation significantly increased the Shannon diversity index, and for the richness index Chao1 was highest in Ck and have no significant difference in other three lands. From farmland to vegetation types abandoned land, Ck, and Rp, the dominant phylum of nifH microorganisms changed from Actinobacteria to Proteobacteria. Furthormore, variation in the Chao1 richness index was significantly influenced by pH (p = 0.002), soil organic carbon (SOC; p = 0.025) and total nitrogen (TN; p = 0.015). The Shannon diversity index was significantly influenced by SOC, nitrate nitrogen (NN), C to P ratio, TN, AN, N to P ratio, TP, and sand. The changes of nifH microorganism composition at the phylum level were significantly correlated with the soil N to P ratio, which explained 53.1% of the variation among environmental factors (F = 11.8, p = 0.002). The ratio of N to P (interpretation rate 65.2%, F = 18.8, p = 0.002) was also a key factor that influenced the composition at the genus level. Based on the diversity and composition, Ck was a more advantageous recovery method to increase the capacity for N fixation. This study provides a basis for the study of N-fixation characteristics in the Loess Hilly Region.