[Response of black soil organic carbon, nitrogen and its availability to longterm fertilization]

Based on the long-term fertilization experiments, effects of various fertilization practices on the soil organic carbon (SOC) and total nitrogen (TN) in the surface (0-20 cm) and subsurface (20-40 cm) black soil in northeast China were studied. Results showed that, compared with the CK, long-term application of organic manure, especially the combination of mineral fertilizers and organic manure significantly increased the organic SOC and TN in the surface soil. Application of mineral fertilizers plus organic manure with conventional (NPM) and high application (N2P2M2) rate increased SOC significantly by 24. 6% and 25.1% , and TN by 29.5% and 32.8%, respectively. However, there was no significant difference among the treatments for SOC and TN at the subsurface. Compared with the CK (CKh), mineral fertilizer plus organic manure (NPM and N2P2M2) did not only increase the soil microbial biomass carbon (SMBC) and nitrogen (SMBN) , dissolved organic carbon (DOC) and nitrogen (DN), but also significantly increased the ratio of SMBC and DOC to SOC, SMBN and TN to TN. Application of the NPM and N2P2M2 increased the value of SMBC/SOC by 0.36 to 0.59 and SMBN/TN by 1.21 to 1.95 percentage points, respectively. The value of DOC/SOC and DN/TN ranged from 0.53% to 0.72% and 1.41% to 1.78%, respectively. This result indicated that SMBC, SMBN, DOC, DN and SMBC/ SOC, SMBN/TN, DOC/SOC, DN/TN were more sensitive than SOC and TN to long-term fertilization in the soil profile, and were better indicators for the impact of long-term fertilization soil fertility. The concluded that the application of manure especially manure plus mineral fertilizers can increase soil nutrients activity in the surface and subsurface black soil, acting as a helpful practice to improve soil fertility and the ability of nutrient supply, while it may cause potential environment pollution on carbon and nitrogen loss in the agroecosystem.