Effects of adding different exogenous nitrogen on rice straw decomposition and nutrient release

Abstract: A straw returning technique has been one of the most important measures for the comprehensive utilization of straw in ecological agriculture in the world. The soil fertility can also be improved to protect the environment during the sustainable development of farmland. The amount of straw returning is ever increasing in China in recent years. It is necessary to optimize the straw returning for higher efficiency during resource utilization. The nitrogen content can also be added to adjust the carbon-nitrogen ratio of straw for the rapid decomposition, thus improving the nutrient release rate with the better straw returning. In this study, different exogenous nitrogen sources were added to determine the dynamic changes of rice straw during decomposition. The suitable nitrogen source was also screened. Indoor cultivation was adopted at the constant humidity. Four treatments were set, including the control (CK), the application of urea (PU), the application of urea ammonium nitrate (UAN), and the application of lime nitrogen (CaCN2). The results showed that the highest decomposition rate of rice straw was achieved at 0-5 d with 0.39-0.47 g/d, followed by the 5-30 d with 0.12-0.16 g/d, and the lowest was 0.045-0.050 g/d during 30-150 d. The addition of exogenous nitrogen significantly increased the cumulative decomposition rate of rice straw (P<0.05), compared with the CK. There was no significant difference in decomposition rate, whereas, a relatively significant difference was found in the decomposition characteristics of rice straw at different stages among the three treatments of exogenous nitrogen. Specifically, the decomposition rates of rice straw in the PU, UAN, and CaCN2 treatments in 0-30 d were 0.076, 0.077, and 0.078 g/d, respectively. The decomposition rates of the PU, UAN, and CaCN2 treatments at 30-150 d were 0.046, 0.046, and 0.050 g/d, respectively. The cellulose, hemicellulose, and lignin were attributed to the decomposition rates of rice straw at different stages. The maximum decomposition rates of cellulose treated with the PU, UAN, and CaCN2 were 13.23%, 10.65%, and 11.29% higher than those with the CK, respectively. The PU treatment also presented the best promoting effect on cellulose decomposition. The decomposition rates of hemicellulose treated with the PU, UAN, and CaCN2 were 6.40%, 5.89%, and 4.74% higher than those with the CK, respectively. The PU and UAN presented the best promoting effects on hemicellulose decomposition. The PU and UAN treatments contributed the most to improving the early decomposition rate of rice straw. The maximum decomposition rates of lignin treated with the PU, UAN, and CaCN2 were higher than those with the CK, respectively. The CaCN2 treatment presented the best promoting effect on the lignin decomposition, thus improving the decomposition rate of rice straw in the late stage. Consequently, the different types of exogenous nitrogen can be combined to achieve the best decomposition. This finding can provide a theoretical basis to improve the nutrient utilization of rice straw for the feasibility and sustainability of straw returning.