Changes of soil carbon and nitrogen and characteristics of nitrogen mineralization under long-term manure fertilization practices in black soil

Soil nitrogen mineralization is an essential index for soil nitrogen fertility and one of the important factors of regulating crop productivity. Based on a longterm experiment in black soil, we analyzed changes of soil total and labile organic carbon and nitrogen after 32 years of various fertilizations. Soil nitrogen mineralization after 32 years of fertilization practices was estimated by anaerobic incubation method. Results show that the fertilization could significantly increase both the concentrations and ratios of dissolved organic carbon and nitrogen (DOC and DON). The application of chemical NPK fertilizers combined with organic manure significantly decreases the ratio of soil microbial biomass nitrogen to soil total nitrogen, increases soil microbial biomass C/N ratio (SMBC/SMBN), and promotes the biological fixation of soil nitrogen. The amounts of soil mineralized nitrogen are increased by 7.2 and 9.2 times under the moderate and high application rates of manure, respectively. However, the application of chemical fertilizers (N or NPK) has no effect on black soil nitrogen mineralization. After the application for 32 years, soil nitrogen mineralization amounts (Nt) are largely enhanced with the application of moderate and high rates of manure alone. The application of organic manure could significantly enhance soil nitrogen mineralization rate (Nt/TN). However, compared with the manure fertilization alone, additional application of chemical N fertilizer and NPK fertilizers decreases the Nt/TN rate by 23.5%-32.1% and 14.1%-17.8%, respectively. There are strong linear correlations between soil mineralized nitrogen amount and soil organic matter, soil nitrogen and labile fractions. The soil nitrogen mineralization rate is improved with the increase of organic matter and total nitrogen stock, and then is stable at 0.4% level. These results indicate that, even the amount of soil nitrogen mineralization is directly related to the amount of organic matter, soil organic matter quality also regulates the nitrogen mineralization capacity. Moreover, organic nitrogen application is the most important practice of enhancing soil nitrogen fertility.