Iron amendment decreases methane emissions from subtropical paddies by altering soil microbial communities

Methane (CH₄) emissions from flooded paddy fields, exacerbated by excessive nitrogen (N) fertilizer application, trigger serious climate challenges. The impact of reducing N fertilization rate combined with iron (Fe) amendment on CH₄ emissions remains unclear. This 4-year field study (2020-2023) investigated the effects of 100%, 80%, 60%, and 0% of the conventional N (urea and commercial organic manure) fertilization rate (100%N, 80%N, 60%N, and 0%N) as well as 80%, 60%, and 0% of the conventional N with the Fe powder (≥99% purity) amendment (80%N + Fe, 60%N + Fe, and 0%N + Fe) on CH₄ emissions from subtropical rice paddies. The results revealed that 60%N + Fe treatments decreased cumulative CH₄ emissions by 43.79% compared to the non-amended treatment, and by 57.33% in relative to the 100%N treatment in the 2023 rice season (P < 0.05). Meanwhile, Fe amendment significantly lowered the mcrA/pmoA ratio, which facilitated the decrease in CH₄ emissions. Community assembly analysis showed that Fe amendment enhanced stochastic processes in methanogens at 60% of conventional N but reduced dispersal at 80% of conventional N, with opposite trends for methanotrophs. Co-occurrence networks demonstrated increased connectivity and reduced modularity under Fe amendment. Moreover, soil Fe²⁺ content and methanogen community structure, as critical drivers, were negatively correlated with CH₄ flux and cumulative emissions (P < 0.05). Taken together, Fe amendment is a potent strategy to mitigate CH₄ emissions under reduced N fertilization, offering a green production solution for global paddy systems.

This study clarified the effects of Fe amendment on CH₄ emissions from subtropical paddy fields under various N fertilization rates through a 4-year in situ field experiment. We found that the Fe amendment combined with reduced N fertilization rates decreased CH₄ emissions, in particular under the 60% of conventional N fertilization rate. Furthermore, the Fe amendment lowered the mcrA/pmoA ratio. Moreover, the Fe amendment increased connectivity while reducing modularity in co-occurrence networks of methanogen communities. Soil Fe²⁺ content and methanogen community structure were key drivers of CH₄ emissions. The findings provide an insight into the microbial mechanisms of mitigating CH₄ emission from flooded paddy soils through the Fe amendment.