Returning low C/N crop straw to the paddy field can achieve the dual benefits of reduced methane emissions and enhanced yield stability

Chuanhai Shu, Binbin Liu, Yu Li, Qiqi Chen, Leilei Li, Yuanqing Shi, Hongkun Xie, Zhonglin Wang, Qin Liao, Qingyue Cheng, Feijie Li, Na Li, Zongkui Chen, Yongjian Sun, Zhiyuan Yang, Jun Ma

Agriculture, Ecosystems and Environment,Volume 393 , 1 November 2025 （IF=6.4）

Abstract

The combination of upland-paddy rotation and straw returning is a common practice in China's subtropical regions. However, the effect and mechanisms of returning the upland crop residues to the paddy field on methane (CH₄) emissions during the rice (Oryza sativa L.) season under different upland-paddy rotation systems remain scarce. To this end, a two-year field experiment (2023–2024) compared five of the most popular rotation systems: wheat-rice, rapeseed-rice, green vegetable-rice, Vicia villosa var.-rice, and fallow-rice in the Sichuan Basin. CH₄ emissions, rice yields, greenhouse gas intensity, and soil methanogenic and methanotrophic gene abundances were measured. Dry-season crops received distinct fertilizer inputs, while rice-season management remained uniform. The results showed that: wheat-rice had the highest abundance of the methanogenic gene (5.00 × 10⁷ and 2.27 × 10⁷ copies g⁻¹ soil in 2023 and 2024, respectively) during the rice growth period, resulting from high carbon-to-nitrogen ratio straw inputs (43.91), which enhanced methanogen activity, leading to the largest cumulative CH₄ emissions (683.41 and 238.65 kg hm⁻²) and higher rice yields (7741.83 and 8231.04 kg hm⁻²). Therefore, the greenhouse gas intensity of wheat-rice (2.22 and 0.74 kg CO₂-eq kg⁻¹ grain yield) was significantly higher than that of other systems. In contrast, the Vicia villosa var.-rice (147.56 and 39.55 kg hm⁻²) rotation with low carbon-to-nitrogen ratio straw (11.28) reduced cumulative CH₄ emissions compared to wheat-rice through balancing methanogenic and methanotrophic gene abundances (ratio 0.61–1.19), while maintaining stable yields (7668.22–8614.37 kg hm⁻²). The yield of the Vicia villosa var.-rice rotation system was not significantly different from green vegetable-rice (8117.98 and 9203.85 kg hm⁻²) and was higher than that of the other systems. Notably, Vicia villosa var.-rice achieved the lowest greenhouse gas intensity (0.48–0.12 kg CO₂-eq kg⁻¹) by synergistically optimizing carbon-nitrogen dynamics and soil organic carbon sequestration. The above results indicate that the Vicia villosa var.-rice rotation system exhibits superior performance in terms of stable yield and emission reduction. This study highlights that the straw carbon-to-nitrogen ratio plays a pivotal role in regulating methane metabolism and maintaining carbon-nitrogen balance, thereby presenting a climate-smart strategy for rice cultivation. It provides a robust scientific foundation for optimizing crop rotation systems and contributes to mitigating agricultural greenhouse gas emissions.