Effects of Organic and Inorganic Fertilization on Soil Organic Carbon and Enzymatic Activities

The overapplication of mineral nitrogen (N) fertilizer has become a scientific problem to be solved in the North China Plain; meanwhile, most organic manure cannot be recycled into the soil. To make full use of organic manure and decrease the applied rate of mineral N fertilizer, a four-year fertilization experiment was carried out to assess the effects of substituting mineral fertilizer with organic manure on soil organic carbon (SOC), labile organic carbon (LOC), stable organic carbon (UOC), microbial biomass carbon (MBC), and soil enzymatic activity. There were six treatments including unfertilized control (CK); mineral fertilizer without organic manure (FR); and mineral nitrogen substituted by organic manure at 20%, 40%, 60%, and 80% (FM1, FM2, FM3, and FM4), respectively. The results showed that SOC, UOC, and LOC were significantly increased by 13.30~40.56%, 41.10~121.92%, and 4.91~15.57% with increasing amounts of organic manure, respectively. Adding organic manure also enhanced urease, catalase, sucrase, and cellulase activities as well as MBC in surface soil (0–20 cm). The present study revealed the activities of urease, catalase, sucrase, and cellulase were closely related to SOC and its components. Following organic manure application, MBC and enzymatic activities had active effects in enhancing LOC and UOC sequestration based on path analyses. Additionally, organic manure increased SOC by direct and indirect enhancement of UOC and LOC, respectively, and improved SOC sequestration as compared to FR treatment. Finally, the study revealed that SOC quantity and quality were both significantly improved by combined application of mineral fertilizer and organic manure, and this effect was enhanced with the increase of organic manure application. Considering the improvements in soil properties and grain yields, substituting mineral fertilizer with organic manure might be a promising alternative in the North China Plain.