Effect of manure application on distribution of organic C and total N in bulk black soil and particle-size fractions

The black soil fertilized with different amount of pig manure for 21 years was sampled from Gongzhuling long-term station. Fine clay( less than or equal 0.2μm), coarse clay (0.2-2μm), silt(2-53μm), fine sand(53-250μm) and coarse sand (250-2000μm) fractions were obtained by ultrasonic dispersion, centrifugation and wet sieving separation. Soil organic carbon(SOC) and total nitrogen(TN) were detected in bulk soil and particle-size fractions. The results show that soil organic matter mostly exist in the silt plus clay, and the ratios of carbon to nitrogen become narrow with particle size decreasing. Soil organic carbon in fine clay fraction is characterized by a more rapid turnover than in the coarse clay fraction. Except for the fine sand, the SOC, TN concentrations in bulk black soil and other size fractions increase significantly with the increase of pig manure application. The application of pig manure resulted in the increase of the enrichment coefficients of SOC and TN in coarse sand, while the decrease in clay fraction, especially in coarse clay. Moreover, C / N ratios in silt and sand fractions narrowed markedly. Change of pig manure application amount affected the concentrations of SOC, TN significantly, while C / N ratios slightly in both bulk soil and particle-size fractions.