Effects of Nitrogen Addition Frequency on Soil Nematodes in a Soybean Cropland

Soil nematodes play an essential role in maintaining the stability of terrestrial ecosystems and promoting substance recycling and energy flow. The application of nitrogen fertilizer is an essential factor affecting soil nematodes in farmland, though it is an effective method to increase crop yield and improve economic benefits. However, previous studies have mostly focused on the impact of nitrogen fertilizer addition on soil nematodes, there is still a lack of knowledge about the effects of frequency of nitrogen addition on soil nematode communities in farmland ecosystems. Soybean crops in farmland ecosystem were selected as the research object, three treatments including control (C), low-frequency nitrogen application (NL), and high-frequency nitrogen application (NH) were set up to study the response of soil nematode communities to different frequency nitrogen additions, and to analyze the response mechanism of soil nematode community structure to different frequency nitrogen addition in farmland, providing basic data and scientific basis for farmland ecosystem management. The results show that: (1) After nitrogen application, there was no significant change in the species richness and density of soil nematodes. In the 0-10 cm soil layer, low-frequency nitrogen application significantly increased the relative abundance of fungivores by 197.33 %, while the relative abundance of plant-parasites decreased by 64.51 %. High-frequency nitrogen application significantly increased the relative abundance of bacterivores by 26.49 %, while the relative abundance of plant-parasites decreased by 47.13 % (P < 0.05). (2) In the 0-10 cm soil layer, low-frequency nitrogen application significantly increased the proportion of cp-1 taxa and significantly decreased the proportion of cp-3 taxa. High-frequency nitrogen application significantly increased the proportion of cp-1 taxa (P < 0.05) (cp-1 represents a nematode community with a cp value of 1). (3) Low-frequency nitrogen application significantly decreased the nematode channel ratio of 0-10 cm soil layer, significantly increased the basal index of nematode communities in soil layers >10-20 cm, and significantly decreased the structural index of nematode communities in soil layers 0-10 and >10-20 cm (P < 0.05). In the 0-10 cm soil layer, the enrichment index of nematode communities was significantly increased by low- and high-frequency nitrogen application (P < 0.05). (4) Redundancy analysis (RDA) show that ammonium nitrogen, moisture content and nitrate nitrogen are the main environmental factors which drive the change of soil nematode community. Overall, nitrogen application changed the soil environment and the structure of nematode trophic taxa. Nitrogen application significantly increased the number of cp-1 taxa which were r-strategists, accelerated the generational alternation and energy flow of soil nematode communities, and reduced the stability of soil ecological communities. Low-frequency nitrogen application significantly decreased the nematode channel ratio, indicating that the fungal decomposition path in the soil decomposition path was enhanced.