Effects of straw mulching combined with nitrogen application on soil organic matter content and atrazine digestion

To explore the response of atrazine (AT) degradation rate, soil organic matter (SOM) distribution and the relationship between them to straw mulching and nitrogen application, field experiments were conducted to study the distribution of SOM content and AT degradation rate under different straw returning modes combined with nitrogen fertilization in 2 years in Hetao Irrigation District. No (N₀), low (N_L), medium (N_M), and high (N_H) levels of N fertilization were incorporated into the soil combined with the surface coverage straw (Treatment BN₀, BN_L, BN_M, BN_H, respectively) and the deeply buried straw (Treatment SN₀, SN_L, SN_M, SN_H, respectively). The traditional cultivation was used as a control treatment (Treatment CK). The results showed that SOM content of Treatment B was accumulated in 0~20 cm soil layer. The largest SOM content of Treatments B in 0~20 cm soil layer was found in BN_H treatment, with an average increase of 14.2% and 24.1% significantly when compared with those in CK and SN_H (P < 0.05), respectively. The SOM content of Treatments B increased with the increase of nitrogen application and decreased with the deepening of soil depth. The SOM content of Treatment S was accumulated in the soil layer (20~40 cm and 40~60 cm) near the inter-layer. With the increase of nitrogen application and depth of soil layer, the SOM content increased firstly and then decreased. SN_M had the largest SOM content in the soil layer of 20~40 cm and 40~60 cm, with an average increase of 82.6% and 67.7% when compared with Treatment CK (P < 0.05). In the soil layer over 60 cm, there was no significant difference in SOM content of different straw returning methods under the same nitrogen level (P > 0.05). Straw returning methods and nitrogen application level significantly affected AT digestion rate and digestion half-life, with significant differences among treatments (P < 0.05). It found that treatment SN_M had the highest digestion rate and the shortest half-life of AT. Compared with Treatment CK, the digestion rate of Treatment SN_M was increased by 5.3% on average, and the half-life was shortened by 3.9 days on average. Single regression and stepwise regression analysis of the half-life of AT degradation and SOM content in different soil layers (0~20 cm and 20~40 cm) showed that the degradation of AT was greatly affected by SOM content of 20~40 cm soil layer. Based on the comprehensive analysis, the effect of straw deep burial combined with medium nitrogen application rate (Treatment SN_M) was best, which could achieve the goal of increasing SOM content and shortening the half-life of AT digestion. The research provided a technical support for straw resource utilization, alleviated AT pollution and improved farmland ecological environment in Hetao Irrigated District.