Effects of nitrogen forms on decomposition and nutrient release of rapeseed straw

Excess rape straw causes unreasonable straw processing, such as burning, resulting in resource waste and serious environmental pollution. Returning straw to the soil is an effective way to improve this situation. Previous studies have found that nitrogen can regulate the soil microbial carbon-nitrogen ratio to promote straw decomposition. At present, much research has focused on the effects of nitrogen rate on straw decomposition, whereas the effects of different forms of nitrogen have rarely been proposed. In this experiment, rape straw was returned to soil for 120 days with different forms of nitrogen:urea (PU), urea ammonium nitrate (UAN), and lime nitrogen (CaCN₂), at a 90 kg·hm^-2 application rate in nylon net bags. The effects of different forms of nitrogen on the decomposition and nutrient release of rape straw were studied. The straw weight and carbon, nitrogen, phosphorus, and potassium contents of straw residues were investigated at 5, 10, 30, 60, 90, and 120 d after straw mixtures were buried into soil to explore characteristics of straw decomposition and nutrient release with the application of different nitrogen forms. The aim of the study was to optimize straw incorporation into soil to improve straw utilization efficiency. The results showed that the decomposition of rape straw was fast in the early stage (0-30 d) and slow in the later stage (30-120 d). At 120 d, the cumulative decomposition rate of rape straw was 46.08%-52.34%. The release rates of carbon, nitrogen, phosphorus and potassium were 44.25%-51.52%, 51.19%-54.87%, 52.82%-58.45%, and 96.61%-97.46%, respectively. The addition of nitrogen significantly promoted the decomposition of rape straw. At 120 d, the cumulative decomposition rate of nitrogen treatments increased by 10.80%-13.59% than that of the control treatment. Different forms of nitrogen have different effects on straw decomposition and release rates of carbon, nitrogen, phosphorus and potassium. Among them, the PU treatment decomposition process was divided into two stages, rapid decomposition (0-30 d) and slow decomposition (30-120 d); at 30 d, the decomposition rate of rape straw reached 40.39%. After 30 d, the decomposition rate gradually became stable and reached 51.06% at 120 d. The UAN treatment decomposition process was divided into three stages, rapid decomposition (0-30 d), slowing decomposition (30-60 d), and slow decomposition (30-120 d); the decomposition rate reached 40.67% at 30 d. The decomposition rate increased by 7.54% from 30 to 60 d, and it was 51.63% at 120 d. The decomposition effect of the CaCN₂ treatment was mainly manifested after 60 d. At 60 d, the decomposition rate of rape straw reached 44.37%, and was 52.34% at 120 d. Overall, the effect of UAN was more significant. Compared with no nitrogen treatment, the cumulative decomposition rate of straw in the UAN treatment increased by 12.04%; the cumulative release rates of carbon, nitrogen, phosphorus increased by 9.33%, 7.19%, and 6.97%, respectively. The effect of each treatment on the potassium release rate of straw was not significant. Therefore, this study provided the basis for enhancing rape straw decomposition to promote straw resource utilization.