Effects of Water-Nitrogen Coupling on the Mineralization of Organic Carbon and Nitrogen for Mulched Farmland Soils in the Arid Regions of Northwest China

This study aimed to clarify the effects of irrigation and nitrogen application on soil fertility, to explore the characteristics of organic carbon and nitrogen mineralization and their influencing factors in mulched farmland soils under different water and nitrogen conditions, so as to provide the theoretical basis for water and nitrogen control measures for mulched farmland crops in the Northwest of China.

In this study, on the basis of five consecutive years of field trials of winter wheat-summer maize with nitrogen application in mulching, the soil samples were collected under three nitrogen application levels of 0 (N0), 180 kg·hm^-2 (N1) and 360 kg·hm^-2 (N2), and three soil moisture gradients, namely, 40% of the field holding capacity (W0), 60% of the field holding capacity (W1), and 100% of the field holding capacity (W2), were set up for indoor organic carbon and nitrogen mineralization. Then, the effect of water-nitrogen coupling on soil organic carbon and nitrogen mineralisation in mulched farmland were analyzed.

Increasing water content significantly increased the cumulative soil carbon mineralization (C_min), carbon mineralization rate, cumulative net soil nitrogen mineralization (N_min), nitrogen mineralization rate, and potential mineralized nitrogen (N_p). C_min, N_min and N_p all showed a tendency to increase and then decrease with increasing nitrogen application. At the end of incubation, C_min was the highest under N1W1 treatment (1 781.00 mg·kg^-1), which was significantly higher than that under other treatments (N0W0, N0W1, N0W2, N1W0, N1W2, N2W0, N2W1, N2W2) by 8.8% to 51.8%, respectively, and its N_min was also maintained at a relatively high level (29.52 mg·kg^-1), while the potential mineralized carbon (5 883.79 mg·kg^-1) and N_p (30.74 mg·kg^-1) were also maintained at a relatively high level. The random forest algorithm indicated that soil microbial carbon (MBC), soil microbial nitrogen, dissolved organic carbon, organic carbon, and total dissolved nitrogen were the important factors affecting C_min and N_min. MBC showed a tendency of increasing and then decreasing with the increase of soil moisture, and the MBC content under W1 significantly increased by 60.1%-340.0% and 3.1%-6.7%, respectively. The structural equations showed that soil moisture had a direct positive effect (0.70) and an indirect positive effect (0.55) on soil carbon mineralization, while the nitrogen application had a direct positive effect (0.90) and an indirect negative effect (0.24) on soil nitrogen mineralization.

From the perspective of soil carbon and nitrogen mineralization, this study recommended 60% field capacity and 180 kg N·hm^-2 as suitable water and nitrogen regulation strategies for mulched farmland in the dryland of Northwest China.