Characteristics of N forms and N-transforming bacteria in paddy soil under different tillage patterns

Paddy soil samples were collected in layers (0-5, 5-12, and 12-20 cm) during rice growth period to investigate the characteristics of the N forms and N-transforming bacteria in the soil profile under different tillage patterns (no-tillage with straw returning, NTS; conventional tillage with straw returning, CTS; no-tillage, NT; and conventional tillage, CT). In the whole rice growth period, ammonifying bacteria in 0-5 cm soil layer had the highest number under NTS, and nitrosobacteria in 0-5 cm and 5-12 cm soil layers were more abundant but in 12-20 cm soil layer were lesser under CT than under NT. Nitrosobacteria and denitrobacteria in 0-20 cm soil layer were lesser under NTS than under CTS. At elongating and ripening stages, anaerobic N-fixing bacteria in 0-5 cm soil layer were more abundant under NT than under CT. In the whole rice growth period, the alkali-hydrolyzable N and total N contents in 0-5 cm soil layer were significantly higher but in 5-12 cm and 12-20 cm soil layers were lower under NT than under CT, and the NH4+-N and NO3-N contents in 0-20 cm soil layer were higher under NTS but in 12-20 cm soil layer had no significant differences between NT and CT. Correlation analysis and multiple polynomial regression analysis further revealed that there were significant relationships between soil NH4+-N and soil ammonifying bacteria, nitrosobacteria and denitrobacteria, and between soil alkali-hydrolyzable N and soil anaerobic N-fixing bacteria. Among the test tillage patterns, NTS could be the more desirable one for the N supply and fertility maintenance of paddy soil.