Effects of long-term fertilization and straw returning on distribution and mineralization of nitrogen in paddy soils in subtropical China

To clarify the effects of long-term fertilization and straw returning on nitrogen cycle of paddy soil ecosystem and the ways of maintaining soil nitrogen supplying potentials, a long-term fertilization experiment of paddy soils was setup in Taoyuan country, Hunan province from 1990. Distributions of soil nitrogen and C/N ratio in soil profiles, soil nitrogen mineralization curves and differences under different temperatures, and relationship between cumulative mineralization and effective accumulated temperature were studied. The results show that nitrogen accumulations in soil profiles are significantly improved under the long-term amendments of chemical fertilizer or crop straw, especially in 020 cm soil layer, and effects are better with combined application of fertilizer and straw. Curves of soil mineral nitrogen and cumulative mineralization are similar under the two temperatures. Soil mineralization rates at 10℃ may require more time to reach stable state those at 30℃; Compared with the 10℃ treatments, significant differences of all treatments are found at 30℃, and nitrogen cumulative mineralization at 30C during 56d is about two times of 10C. Higher cumulative mineralization is found in the NP (chemical N, P) and NP+C(chemical N, P + straw) treatments, demonstrates that a more contributive effect of crop straw and fertilizer than chemical fertilizer only. The relationship between cumulative mineralization and effective accumulated temperature well fit the effective accumulated temperature model (EATM) (p0.01) in all treatments at 30℃, and the K values are similar to those of EATM in chemical fertilizer treatments and CK, but increasing of K is found in the straw and fertilizer combined treatments. The NPK+C treatment (chemical N, P, K + straw) has higher mineralization potential and nutrient supplying capacity. Decrease of absolute values of n in EATM are found in the straw and fertilizer combined treatments, demonstrates that cumulative mineralization of these treatments maintain a stable range with incubation time.