Controlled irrigation can mitigate the greenhouse effects of rice paddy fields with long-term straw return and stimulate microbial necromass carbon accumulation

Context and problem

The overall greenhouse effects of rice paddy fields are influenced by the balance between greenhouse gas (GHG) emissions and soil organic carbon sequestration (SOCS). Studies on how straw return impacts GHG emissions and SOCS under different water regimes—specifically, conventional irrigation (CI) and alternate wetting and moderate drying (AWMD)—are crucial for developing strategies to mitigate the greenhouse effect in rice paddy fields.

Objective

This study aimed to develop a strategy for decreasing GHG emissions, improving SOCS, and increasing grain yield of rice paddy fields under long-term straw return.

Methods

Different water regimes were introduced after six years of straw return in the rice paddy field, and there were four treatments: straw removal and CI (N-CI), straw removal and AWMD (N-AWMD), straw return and CI (R-CI), and straw return and AWMD (R-AWMD). We studied various traits related to soil organic carbon sequestration capacity and GHG emissions over three years to investigate the effects of combination of AWMD and straw return on the GHG emission from paddy field.

Results

Straw return significantly increased net greenhouse gas emissions (NGHGE) and seasonal soil total organic carbon sequestration rate (TOCSR) due to substantial quantity of straw inputs. On average, straw return increased NGHGE by 2.125 t CO₂ ha^–1 and TOCSR by 393.2 kg C ha^–1, respectively. AWMD could mitigate the greenhouse effects caused by straw return by decreasing NGHGE by 28.0 %, primarily attributed to the reduction in CH₄ emissions (-27.0 %), which outweighed the effects of increased N₂O and CO₂ emissions. Although AWMD did not increase the overall soil organic carbon (SOC) content, it optimized the composition of SOC by increasing the percentage of microbial-derived C, including fungal necromass C (FNC) and bacterial necromass C (BNC), which are more stable than plant-derived C. The aerobic environment in AWMD combined with straw return enhanced the activities of microbes, which promoted the conversion of plant residue C to FNC and BNC and improved soil carbon sequestration.

Conclusions

The combination of straw return with AWMD can reduce GHG emission, and optimize soil carbon sequestration by stimulating microbial necromass carbon accumulation.

Implication

This study offers valuable insights into mitigating GHG emissions and enhancing soil organic carbon sequestration in high-yielding rice system through the combined adoption of AWMD and straw return.