Effects of fertilizers application on soil nematode communities and oat yield in saline-alkali dryland

Utilizing saline-alkali land is of great significance for enhancing cultivated land quality. Nematodes are fauna critical to the soil food web that are morphologically and functionally diverse and widely used to indicate soil quality. However, the ecological indicator function of the nematode community on oat saline-alkali land, particularly the application of industrial waste on saline-alkali land, has less been reported. In this study, soil nematodes were investigated as an indicator of the soil ecological environment to explore the effects of the long-term application of different fertilizers on the micro-food web of saline-alkali barren soil under dry farming. Based on a 5-year experiment, seven treatments were set: combined application of vinegar residue and fly ash (VRF), vinegar residue application (VR), organic manure application (OM), bacterial manure application (BM), combined application of organic manure and chemical fertilizer (OCF), chemical fertilizer application (CF), and no fertilization (CK). Soil samples were collected from the plow layer at the seedling, heading, and harvesting stages of oats, and the soil nematode community, soil traits, and oat productivity under long-term application of different fertilizers were analyzed. The results showed that most fertilizer treatments reduced soil pH and bulk density and significantly increased contents of organic matter, nitrate nitrogen, and microbial quantity (bacteria, fungi, and actinomycetes). With the exception of BM, the yield of oats increased significantly by 44.5%−92.3% under other treatments, and OM and CF had the highest oat yield (P<0.05). The total number of soil nematodes significantly increased due to fertilization; OM was highest at the seedling and heading stages, and VR was highest at the harvesting stage (P<0.05). The enrichment index (EI) of each treatment and the structure index (SI) of most treatments were lower than 50, indicating that the soil nutrients were poor and the soil food web was in a stressed or degraded state in this experimental field. Although the nematode community under OM application reached a structured state at harvesting stage (SI>50), the food web was relatively stable, and the proportion of plant-parasitic nematodes was higher (36.2%). Compared to other fertilizers, the number of dominant species was significantly increased, and the nematode diversity was the lowest (P<0.05) under CF treatment. The combination of OM and CF (OCF) compensated for these shortcomings to some extent. VRF and VR significantly increased the proportion of microbial-feeding nematodes and decreased the proportion of plant-feeding nematodes. The Wasilewska index (WI) under the VRF and VR treatments was higher than that of CK during the three growth periods; and the maturity index (MI) of free-living nematodes under the VRF treatment was always the highest (P<0.05), which indicated that organic carbon mineralization was dominated by microbial-feeding nematodes under the VRF and VR treatments and that the food web was mature and stable with less disturbance under the VRF treatment. Soil organic matter, available P, and microorganisms were the main factors affecting the number of soil nematodes. Soil pH, EC, and bulk density were the main limiting factors for oat yield, but there was no significant correlation between the number of nematodes and oat yield, but β-glucan content was negatively correlated with the number of plant parasitic nematodes (r=−0.52^*). The combined application of organic manure/chemical fertilizer and vinegar residue/fly ash was beneficial to the health and stability of the soil microecological environment.