Effect of long-term application of inorganic fertilizer and organic amendments on soil organic matter and microbial biomass in three subtropical paddy soils

Soil organic matter (SOM), microbial carbon (Cmic), and microbial nitrogen (Nmic) status affected by the application of inorganic fertilizer and organic amendments in subtropical paddy soils were investigated. Soil samples were collected from the plow layer of three long-term (17 years) field experiments at Xinhua, Ningxiang, and Taojiang counties in Hunan Province, China. Results showed that, compared to the control, application of inorganic fertilizer alone showed no significant effect on soil organic C (SOC), total N (Ntot), Cmic and Nmic. The application of inorganic fertilizer along with manure or straw significantly increased SOC and Ntot and soil Cmic and Nmic contents for all three sites, while following an application of inorganic fertilizer along with straw only for two sites. Cmic and Nmic were closely correlated with SOC and Ntot, respectively. In conclusion, application of inorganic fertilizer along with manure or straw is an effective way of enhancing SOM and microbial biomass in subtropical paddy soils.