Distribution Of Nitrogen In Aggregates Of Black Mollisols and Red Typic Hapludults Soils Under Long-Term Fertilization.

Soil aggregates with various degree of stability has important implications in nutrient storage and supply for plant growth. This study evaluated the effect of long-term fertilization on N distribution in aggregates. The total N (TN)and accumulation in four size fractions (250−2000 µm, 250−53 µm, 53−2 µm and <2 µm) were analyzed in two soils: black (high OM) soil (Mollisols) from northeast China and red (acidic) soil (Typic Hapludults) from south China, from a 22-year fertilization experiment. The treatments included unfertilized control, chemical fertilization (NPK), manure application (70% N supply) with NPK (NPKM), and NPK with corn-stover returned (NPKS). The portion of N in a specific aggregate relative to the total N amount in all aggregates and differences under various fertilizations were estimated. The results showed that the TN content decreased at 0.015 g/kg/yr for both control and NPK treatments, but increased at 0.025 g/kg/yr for the NPKM treatment in black soil (P < 0.05). The TN content of red soil, however, did not change significantly for all treatments over the fertilization period. The highest TN concentrations were detected in the 53−2 µm microaggregates in the black soil (0.73−1.21 g/kg), and <2 µm microaggregates in red soil (0.46−0.98 g/kg) among the four size fractions of aggregates. There was significant decrease of N portion in the 53−2 µm aggregates of the red soil by 6% and 10% under NPKM and NPKS treatment, respectively, compared to the NPK treatment. However, there was no significant change for the N portion in the 53−2 µm aggregates in the black soil. This data may suggest that soil property change from long-term fertilization further impacted soil N storage or availability.