SOC mediates the contribution of generalists and specialists to changes in soil nirK bacterial diversity: Evidence from apple orchards in main production areas of China

nirK-Denitrifying bacteria are closely related to N2O emissions and they have been studied widely in agro-ecosystems. However, the mechanisms associated with maintenance of the diversity of nirK bacterial communities in agro-ecosystems are unclear. In particular, the ecological roles of “generalists” and “specialists” in nirK bacterial communities under different soil organic carbon (SOC) levels have not been characterized. In this study, we divided 102 soil samples (0–20 and 20–40 cm) from 62 apple orchards in China's main apple producing areas (Shaanxi and Shandong provinces) into four groups according to the SOC content and soil depth, i.e., high organic carbon soils in the 0–20 cm depth, low organic carbon soils in the 0–20 cm depth, high organic carbon soils in the 20–40 cm depth, and low organic carbon soils in the 20–40 cm depth. In the nirK bacterial community, 4187 operational taxonomic units (OTUs) (∼51.32 %) were classified as specialists and 1781 OTUs (∼21.83 %) as generalists. The α-diversity of generalists was higher in the high SOC soils than the low SOC soils, which was consistent with the α-diversity of the whole nirK bacterial community. However, the observed number of specialist species was lower in high SOC soils than low SOC soils. Analysis based on the Spearman's correlation coefficients also showed that the α-diversity and relative abundances of generalists and specialists responded differently to environmental factors. Both deterministic and stochastic processes contributed to the assembly of generalists and specialists. Among the deterministic processes, variable selection was important for the assembly of the generalist community, whereas homogeneous selection was important for the assembly of the specialist community. The niche breadth of generalists was higher than that of specialists, whereas the niche overlap of specialists was higher than that of generalists. The niche breadth of generalists was higher in high SOC soils than low SOC soils. In both networks, generalists had higher degrees than specialists, although the number of generalists was much lower than that of specialists. Our findings demonstrate the contributions of generalists and specialists to the changes in the diversity of the nirK bacterial community at different SOC levels as well as providing new insights into the mechanisms responsible for maintaining the diversity of the nirK bacterial community.