Correlations Between the Soil Bacterial-Feeding Nematodes, Bacteria, and Nitrogen in the Cropland of the Upper Yangtze River, China

Bacterial-feeding nematodes affect bacteria-mediated nitrogen (N) cycling processes by their predation on bacteria. However, the correlations between bacterial abundance, diversity, community composition, and the abundance of bacterial-feeding nematodes in the presence of organic combined with inorganic fertilizers, and their effects on soil N availability are unclear. Here, four field fertilization treatments, including unfertilized control (CK), chemical N, phosphorus (P) and potassium (K) fertilizers (NPK), NPK plus crop straw (SNPK), and NPK plus pig manure (MNPK), were performed for studying relationships among bacterial abundance, diversity, community composition, and the abundance of bacterial-feeding nematodes along with corresponding effects on soil N availability. Results showed that nitrate nitrogen (NO 3 − -N) content was significantly upregulated in the SNPK and MNPK treatments compared to that in the NPK treatment and showed a significant positive relationship with bacterial abundance and community composition. As revealed by partial least squares structural equation model (PLS-SEM), bacterial community composition had a stronger effect on the NO 3 − -N content, and bacterial-feeding nematodes indirectly affected the NO 3 − -N content by directly positively affecting the bacterial community composition, among which the bacterial-feeding nematode Eucephalobus played a stronger role in the production of NO 3 − -N. Overall, the findings of this study suggested that bacterial-feeding nematodes can improve soil N availability through impacting bacterial community composition rather than bacterial diversity and abundance. These results suggest that bacterial-feeding nematodes may play an important role in improving soil N availability in farmlands.