The Response of Soil Nematode Community to Nitrogen, Water, and Grazing History in the Inner Mongolian Steppe, China

Nutrient nitrogen and water availability are co-limiting factors for grassland productivity in the Inner Mongolian steppe. The objective of this study was to evaluate the interactive effects of water and nitrogen (N) addition on soil abiotic factors and soil nematode community composition. A 3-year experiment with addition of water (with and without irrigation simulating wet year precipitation) and nitrogen (0, 25, and 50 kg N ha(-1)) was conducted at two sites in Inner Mongolia with histories of heavy grazing (HG) and moderate grazing (MG). The results showed that HG had several lower nematode ecological indicators; namely, the Simpson index (P < 0.01), maturity index (MI, including free-living nematodes), and plant parasite index (PPI, including plant-parasitic nematodes). In addition, HG had a lower proportion of omnivores-predators and higher proportion of bacterivores (P < 0.01), with reduced soil moisture, total nitrogen and total carbon (C), and increased C/N ratio. A relatively low addition of N did not influence soil total carbon and total nitrogen and had only a slight effect on the composition and structure of the soil nematode community. Water addition increased the proportion of plant parasites, reduced the proportion of bacterivores and omnivores-predators and increased the soil total carbon and total nitrogen. Redundancy analysis revealed that grazing history alone explained more variation in nematode taxon composition (17.4%, P < 0.01) and PPI (35.6%, P < 0.01) and MI (17.5%, P < 0.01) and Simpson index (10.5%, P < 0.01). Water addition explained more variation in the nematode feeding group (24.4%, P < 0.01), total nematodes (7.9%, P < 0.01) and PPI (78.7%, P < 0.01). Nitrogen addition explained variation in the nematode taxon composition (6.2%, P < 0.01). These results suggested that HG caused soil degradation, and water addition facilitated plant parasites and enhanced soil properties, whereas application of small amounts of nitrogen to N-limited semiarid grasslands did not induce clear changes in this system in the short term.