Effects of conventional and reduced N inputs on nematode communities and plant yield under intensive vegetable production

As the largest group of soil mesofauna, nematodes occupy all consumer trophic levels in soil food webs, and may serve as a proxy for soil food web structure and composition. The present study was conducted in an intensively managed, solar greenhouse vegetable-production system to investigate the effects of nitrogen management on soil nematode communities. We conducted two experimental trials. The first trial was a field survey in vegetable greenhouses with various cultivation histories (1, 2 and ≥5 years) and open grain fields. The second trial was a series of nematode community analyses over four years from a long-term N management experiment with three treatments: NN (no nitrogen input), RN (reduced N fertilization) and CN (conventional N application). In the field survey, we found that soil total N significantly increased with planting age. After one year of cultivation, greenhouse soil had a significantly lower Shannon–Wiener diversity index (H) (1.55) and a higher abundance of root knot nematodes (RKNs) (292 nematodes per 100 g dry soil) compared to the soil in the open fields. With increasing time of cultivation, there were further decreases in H and increases in RKNs with H reaching 1.03 and RKNs 1254 after five or more years of vegetable planting. Analyses of soil nematode community in the N management experiment indicated that the abundance of RKNs significantly decreased by 55.9% to 770 per 100 g dry soil in the RN treatment compared to 1745 per 100 g dry soil in the CN treatment. The maturity index of the soil nematode community was negatively correlated with the amount of N input and soil total N. Further, tomato fruit yield was not affected by reduced N input in the RN treatment in contrast to the CN treatment. Our results clearly demonstrate that reduced N input had two benefits; (1) reducing the risk of nitrate pollution associated with excessive N input, (2) decreasing the abundance of RKNs and improving the soil nematode community for vegetable production systems.