Abstract:A continuous in situ field experiment from 2017 to 2018 in the cold and black soil region of Heilongjiang Province was carried out to explore the effect of straw returning and different N application rates in successive years on nitrogen composition in surface soil of paddy field under controlled irrigation mode. Two straw returning levels: 6t/hm2 and 0t/hm2, and four N application rates: N0(0kg/hm2), N1(85kg/hm2), N2(110kg/hm2) and N3(135kg/hm2) were set, a total of eight treatments. The changes of ammonium nitrogen (NH+4N), nitrate nitrogen (NO-3N), soluble organic nitrogen (SON), δ15N content and total soluble nitrogen distribution ratio in paddy soil surface after straw returning were analyzed based on nitrogen stable isotope technique. The results of 2-year experiment showed that under controlled irrigation condition, straw returning increased the ratio of SON to total soluble N, N mineralization and the content of δ15N in soil surface layer. The SON content in the surface soil layer of treatments with straw was lower than those without straw at different N application rates. Among all the treatments, the NH+4N and NO-3N contents in the surface soil of N3 treatments were decreased by 40.3% and 38.7% compared with those treatments without straw at the same N application rate. Straw returning promoted the existence of total soluble nitrogen in the soil surface in a more stable form of SON. Straw returning improved soil nitrogen supply capacity compared with treatments without straw returning, there was no significant difference in N mineralization in the surface soil between N0 with straw and maximum N mineralization of the treatment without straw (P>0.05). With the increase of N application, the N mineralization in the surface layer of soil treated with straw returning was significantly higher than that without straw (P<0.05). The high content of δ15N in straw promoted the enrichment of δ15N in soil surface. There was no significant difference in the content of soil surface δ15N between the treatments of N1 and N2 with straw and treatments of N2 and N3 without straw. The content of soil surface δ15N of the treatment with straw was significantly higher than that without straw (P<0.05). Moreover, the correlation between total soluble nitrogen and NH+4N, NO-3N, SON and δ15N in the soil surface was changed after two years of straw returning. The results of this study can provide scientific basis for the feasibility of straw returning in Northeast China, which had important significance for ensuring the sustainable utilization of agricultural water and soil resources in Northeast China.
