Soil biotic communities play vital roles in enhancing soil nutrient cycling and soil fertility.  Long-term excessive nitrogen (N) application is disadvantageous to the stability of soil food webs and affects arable soil health and sustainable utilization.  Proper organic substitution is essential to improve soil health and alleviate the disadvantages of excessive chemical fertilization.  However, the biological effects of various organic amendments on soil micro-food webs are poorly understood.  In order to explore the effects of various organic amendments including stover, biochar and manure on soil micro-food webs (microbial and nematode communities), a field plot experiment with maize having five treatments viz., 100% urea (100% N), 70% urea (70% N), 70% urea plus stover (Stover), 70% urea plus cattle manure (Manure) and 70% urea plus biochar (Biochar) was conducted.  Manure treatment increased the carbon (C) to N use efficiency of soil microbes, which contributed to the retention of soil C, while Biochar treatment elevated soil organic C (SOC) and soil pH.  Additionally, Biochar treatment mitigated the negative effects of soil acidification on the soil micro-food web and reduced the abundance of plant parasites.  Overall, the biological effect of organic amendments was distinguished from chemical fertilization (100% N and 70% N) through principal co-ordinates analysis.  Negative relationships among soil properties, microbial and nematode biomass in the 100% N treatment were diminished in treatments where chemical fertilizer was reduced.  The bottom-up effects on soil food webs were observed in organic substitution treatments.  In conclusion, organic amendments improved soil fertility by regulating soil microbial and nematode communities in the cropland ecosystem, alleviated the negative effects of chemical fertilizer on the micro-food webs and controlled the trophic cascades among soil biota.

Land use influences soil biota community composition and diversity, and then belowground ecosystem processes and functions. To characterize the effect of land use on soil biota, soil nematode communities in crop land, forest land and fallow land were investigated in six regions of northern China. Generic richness, diversity, abundance and biomass of soil nematodes was the lowest in crop land. The richness and diversity of soil nematodes were 28.8 and 15.1% higher in fallow land than in crop land, respectively. No significant differences in soil nematode indices were found between forest land and fallow land, but their network keystone genera composition was different. Among the keystone genera, 50% of forest land genera were omnivores-predators and 36% of fallow land genera were bacterivores. The proportion of fungivores in forest land was 20.8% lower than in fallow land. The network complexity and the stability were lower in crop land than forest land and fallow land. Soil pH, NH₄⁺-N and NO₃^--N were the major factors influencing the soil nematode community in crop land while soil organic carbon and moisture were the major factors in forest land. Soil nematode communities in crop land influenced by artificial management practices were more dependent on the soil environment than communities in forest land and fallow land. Land use induced soil environment variation and altered network relationships by influencing trophic group proportions among keystone nematode genera.