Distribution of nitrogen in aggregates of black soil and red soil under long-term fertilization

In order to study the change dynamics of soil nitrogen and effect of long-term fertilization on N distribution in soil aggregates, the total nitrogen（TN）, soil microbial biomass nitrogen(SMBN) and nitrogen accumulation in four size aggregates were analyzed in black soils and red soils based on 22 year longterm fertilization experiments. The results show that the TN contents decrease at a rate of 0.015 g/(kga) for both the CK and the NPK treatments in black soil, while the contents increase at 0.025 g/(kga) for the NPKM treatment(P 0.05). The TN contents of red soils do not changed significantly for all treatments over the fertilization period. Compared with the NPK treatment, the NPKM treatment increases SMBN by 15% and 43% and TN by 43% and 45% in black soils and red soils(P 0.05), respectively. The highest nitrogen concentrations are detected in 2-53 m microaggregates in black soils(0.73 -1.21 g/kg) and 2 m microaggregates in red soils(0.46-0.98 g/kg) among the four sizes aggregates. Compared with the NPK treatment, the NPKM treatment results in 4.3 and 5.1 percentage increases for the ratio of N contribution in 250-2000 m macroaggregates to total N, in black soils and red soils, respectively. The nitrogen contribution in 2-53 m aggregates of red soils are significantly decreased by 5.9 and 9.7 percentages under the NPKM and NPKS treatments, respectively, compared with the NPK treatment. However, there is no significant change for the N contribution in black soils except 250-2000 m macroaggregates. It suggests that it is 2-53 m aggregates N which lead to different soil N amounts between the two soil types under the long-term fertilization. Combination of chemical fertilizer with manure improves the N distribution in 250-2000 m macroaggregates, which has higher ability to apply N to crop than to microaggregates. Therefore, chemical fertilizer in combination with manure is a strongly recommended fertilization to improve soil fertility and develop sustainable agriculture.