Effects of returning method and nitrogen application rate on fallow weeds decomposition, soil fertility, and rice yield

This study aimed to investigate fallow weed decomposition and nutrient release responses to the methods of returning fallow weeds to the field and nitrogen fertilizer management, as well as their impacts on rice yield and soil fertility, to provide technical support for rational natural resource utilization in paddy fields, soil fertility improvement, and grain production capacity enhancement. A two-year fixed-site experiment was conducted in Yueyang City, Hunan Province, China, using fallow weeds as the research object. By combining the nylon mesh bag method with field experiments, the effects of two return-to-the-field methods – straw mulch (SM) and straw burial (SB) – and three nitrogen application levels – no nitrogen application (N0), 150 kg·hm⁻² (N1), and 300 kg·hm⁻² (N2) – on the decomposition of fallow weeds returned to the field, nutrient release, soil fertility, and rice yield were studied. The results showed that the cumulative decomposition rate of fallow weeds in each treatment ranged from 69.11%–84.54%. Specifically, at the same nitrogen application rate, the decomposition rate of fallow weeds in the SB treatment was significantly 8.67% higher than that in the SM treatment. Under the same returning method, the decomposition rates of fallow weeds in nitrogen application treatments (N1 and N2) significantly increased by 9.65% compared to that in the N0 treatment, with no significant difference observed between the two nitrogen application groups. The cumulative release rates of carbon, nitrogen, phosphorus, and potassium from the fallow weeds in each treatment were 54.74%−65.07%, 36.43%−49.59%, 56.23%−70.13%, and 91.70%−94.80%, respectively. At identical nitrogen rates, the cumulative release rates of carbon, nitrogen, and phosphorus from fallow weeds in the SB treatment were significantly higher than those in the SM treatment. With uniform returning methods, the nitrogen application treatments (N1 and N2) increased the cumulative release rates of carbon, nitrogen, and phosphorus from fallow weeds by an average of 8.41%, 16.82%, and 10.53%, respectively, compared to those in the N0 treatment, whereas the two nitrogen-application treatments remained statistically indistinguishable. Under the same nitrogen application rate, the SB treatment markedly increased the soil contents of alkali-hydrolyzable nitrogen, available phosphorus, and available potassium in the soil in contrast to the SM treatment. Across the same returning method, nitrogen-applied treatments exhibited higher levels of these nutrients than those in the non-nitrogen application treatment, with no considerable variation between the two nitrogen application regimes. The rice yield in the SB treatment was 5.54% higher than that in the SM treatment at consistent nitrogen application rates. The nitrogen application treatments (N1 and N2) boosted rice yields by 18.36% relative to the N0 treatment when the returning method remained constant. No significant divergence was detected between the two nitrogen-application scenarios. The increase in rice yield was primarily attributed to significant increases in the number of effective panicles per hm², spikelets per panicle, and total spikelets per hm². Notably, there were no significant differences in the decomposition rates of fallow weeds, cumulative release rates of carbon, nitrogen, and phosphorus from fallow weeds, soil fertility, and rice yield under the interaction of returning methods and nitrogen application rates. The results of the correlation analysis showed that the decomposition rate of fallow weeds and the release rates of carbon, nitrogen, and phosphorus were significantly positively correlated with soil total nitrogen, alkali-hydrolyzable nitrogen, available phosphorus, and available potassium, and were also significantly positively correlated with rice yield. In conclusion, straw burying and returning to the field combined with 150 kg·hm⁻² nitrogen fertilizer application can promote the decomposition of fallow weeds returned to the field and the release of nutrient elements, significantly improve the characteristics of soil fertility, and are beneficial for the coordinated improvement of soil fertility in paddy fields and rice yield.