[Effects of straw returning amount and type on soil nitrogen and its composition]

Straw returning to soil can supplement soil nutrients required for crop growth, fertilize soil, and improve soil quality. To explore the long-term effect of straw returning on soil total nitrogen and its composition, herein, five treatments including no rice straw + no wheat straw returning (NRW), no rice straw + all wheat straw returning (W), all rice straw + no wheat straw returning (R), half rice straw + half wheat straw returning (HRW), and all rice straw + all wheat straw returning (ARW) were conducted in triplicate in Taihu Lake region, China. The effects of both straw amount and type were examined. Compared with the results obtained in 2007, the results herein obtained in 2017 showed that after 10 years of straw returning, soil total nitrogen and heavy fraction nitrogen increased, while light fraction organic matter decreased. Among the five treatments, ARW had the largest decrease in light fraction nitrogen of 8.09 g·kg^-1; the R treatment had the highest contents of both total and heavy fraction nitrogen, and also the highest contents of ammonium and nitrate. There was no significant difference in alkali-hydrolyzable nitrogen among the five treatments. These results indicated that crop straw was the critical material source for soil nitrogen, and that the effects of straw returning on soil nitrogen depended on the type and amount of crop straw returned to soil. The changes of light fraction nitrogen were more sensitive to straw returning, while the heavy fraction nitrogen was relatively stable, which was the key fraction sustaining soil fertility. With the prolonging of straw returning, the relationship between the total nitrogen and diffe-rent nitrogen components changed. The processing manner of all rice straw returning + no wheat straw returning was the way that could most significantly enhance soil nitrogen content.