Effects of long-term ridge tillage on distributions of different nitrogen forms of soil water stable aggregates

Soil aggregates are the most important structures, and different sizes of aggregates have different functions in maintaining, supplying and transforming nutrients. A 18-year long-term field experiment was carried out to study distributions of nitrogen forms and urease activities of soil aggregates under conventional tillage (CT) and ridge tillage (RT) practices. The results show that there are similar distribution patterns of total nitrogen, microbial nitrogen, NO3--N and urease activities in soil water stable aggregates under CT and RT. Total nitrogen and microbial nitrogen are mainly distributed in soil aggregate whose size is large than 0.25 mm, followed by those in the 0.25–0.053 mm aggregate, and those in the silt and clay component (0.053 mm) are the least. Urease activities are the highest in the size of 2.0–0.25 mm aggregate, and are the least in the size of 0.25–0.053 mm aggregate. NH4+-N and NO3--N concentrations are the highest in the size of larger than 2.0 mm aggregate. Total nitrogen, microbial nitrogen, NO3--N and urease activities under RT are higher than those under CT. For instance, total nitrogen concentration within 2.0–0.25 mm aggregate under RT is 23.1% higher than that under CT, NO3--N in the size of larger than 2.0 mm aggregate is 26.7% higher under RT than that under CT. In conclusion, total nitrogen, microbial itrogen concentrations and rease activities in different sizes of aggregates under RT are significantly higher than those under CT, the distribution patterns of nitrogen concentrations and urease activities are dependent on the structures of aggregates, while tillage methods had no significant effects on them.