Effects of straw incorporation on nitrogen absorption of split fertilizer applications and on rice growth

A greenhouse experiment investigated the effect and mechanism of straw incorporation (0 and 6 t ha(-1)) on the absorption of fertilizer nitrogen (N) of split applications and on the growth of rice shoots and roots. N fertilizer was split into: (1) base fertilizer (BF), incorporated before transplanting; (2) tillering fertilizer (TF), broadcasted at tillering stage; and (3) panicle-formation fertilizer (PF), broadcasted at panicle-formation stage. N-15-urea was used as one of three splits. To evaluate the hypothesis that straw incorporation influences the N-15 absorption of one split by enhancing N-15 immobilization or changing root growth, we measured the N-15 residual rate in soil at maturity and root growth throughout the rice-growing season. Straw incorporation significantly decreased N-15 absorption of BF, accompanied by significantly higher N-15 immobilization in soil than in the no-straw treatment. However, straw incorporation significantly increased N-15 absorption of top-dressing (both TF and PF), accompanied by significantly greater dry matter and length of surface roots (0-5 cm depth) throughout the rice-growing season (top-dressing N-15 was seldom found in soils > 5 cm). Shoot dry matter with straw incorporation decreased significantly in the early stage but increased in the late stage, compared with the no-straw treatment, and shoot total N with straw incorporation decreased significantly throughout the rice-growing season (this negative effect had decreased by the late stage). To better synchronize N supply with rice demand and reduce the risk of water eutrophication, N levels of BF and top-dressing should be reviewed when straw is incorporated.