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A field experiment was carried out at the Shenyang Experimental Station of Ecology (CAS) in order to study the effects of slow-release urea fertilizers high polymer-coated urea (SRU1), SRU1 mixed with dicyandiamide DCD (SRU2), and SRU1 mixed with calcium carbide CaC2 (SRU3) on urease activity, microbial biomass C and N, and nematode communities in an aquic brown soil during the maize growth period. The results demonstrated that the application of slow-release urea fertilizers inhibits soil urease activity and increases the soil NH4+-N content. Soil available N increment could promote its immobilization by microorganisms. Determination of soil microbial biomass N indicated that a combined application of coated urea and nitrification inhibitors increased the soil active N pool. The population of predators/omnivores indicated that treatment with SRU2 could provide enough soil NH4+-N to promote maize growth and increased the food resource for the soil fauna compared with the other treatments.

Labile organic carbon (LOC) is a fraction of soil organic carbon (SOC) with rapid turnover time and is affected by soil fertilization. This investigation characterized the SOC content, LOC content and LOC distribution in the treatment plots of surface soil erosion at five levels (0-, 5-, 10-, 20-and 30-cm erosion). The soil had received contrasting fertilizer treatments (i.e., chemical fertilizer or chemical fertilizer + manure) for 6 years. This study demonstrated that both SOC and various LOC fractions contents were higher in the plots with fertilizer + manure than in those with fertilizer alone under the same erosion conditions. The SOC and LOC contents decreased as the erosion depth increased. Light fraction organic carbon, particulate organic carbon, easily oxidizable organic carbon (KMnO 4-oxydizable organic carbon), and microbial biomass carbon were 27%-57%, 37%-47%, 20%-25%, and 29%-33% higher respectively in the fertilizer + manure plots, than in the fertilizer alone plots. Positive correlations (p < 0.05) between SOC content and different fractions contents were observed in all plots except the correlation between total SOC content and water-soluble organic carbon content in the different fertilization treatments. Obviously, fertilizer + manure treatments would be conducive to the accumulation of LOC and SOC in the Black soil of Northeast China.