Abstract: We conducted an experiment with the rice cultivar “Kengeng 8” as the research material following a two-factor randomized block design. There were four nitrogen application levels: N₀(no nitrogen application), N₁ (75 kg·hm^-2), N₂(125 kg·hm^-2), N₃(150 kg·hm^-2), and two decomposition agent levels: F₀(no decomposition agent) and F₁ (with decomposition agent:2 kg·t^-1 straw decomposition agent combined with Trichoderma harzianum). By measuring rice straw degradation rate, nutrient release rate, and rice yield, we investigated the effects of coupling different nitrogen application rates with decomposition agent on rice straw decomposition under straw return conditions in cold region, to assess the optimal nitrogen application rate. The results showed that the straw degradation rate and nutrient release rate under treatments with nitrogen and decomposition agent were higher than those without nitrogen and without decomposition agent. The F₁N₂ treatment exhibited the best performance. Compared with F₀N₀, straw degradation rate during the entire rice growth period increased by 19.8%-196.1%, and the nutrient release rates of straw C, N, P, and K increased by 17.3%-54.7%, 12.2%-44.8%, 14.7%-48.2%, and 4.4%-43.4%, respectively. The combination application of nitrogen fertilizer and decomposing agent significantly regulated straw C/N ratio and shortened the duration of straw decomposition. The F₁N₂ treatment was optimal, with C/N ratio of 44.09, and its decomposition parameter reached the threshold value of 0.6 at the heading stage, shortening decomposition duration by 20 d compared with F₀N₀. The F₁N₂ treatment increased spikelets per panicle, seed setting rate, and rice yield by 79.1%, 40.4%, and 134.7%, respectively. In conclusion, the F₁N₂ treatment significantly promoted straw decomposition and nutrient release under straw return conditions, thereby facilitating soil fertility and rice yield in cold region.

Key words: rice, nitrogen application rate, straw decomposition agent, straw turnover