Microalgae biofertilizer reduces methane emissions associated with lower soil dissolved organic carbon and mcrA abundance in paddy soil

Low-carbon technologies are pivotal for boosting rice yields while cutting greenhouse gas emissions in paddies. Microalgae biofertilizer offers great potential for enhancing soil fertility and crop productivity. However, few studies have addressed its effects on greenhouse gas emissions and the underlying mechanisms in paddy soil. Here, a greenhouse pot experiment was conducted to investigate the effects of microalgae biofertilizer on greenhouse gas emissions from paddy soil. The experiment designed four treatments, including N100 (full chemical nitrogen fertilizer), N80 (20% reduced nitrogen fertilizer), N80 + LM and N80 + HM (N80 with low- and high-dose microalgae, respectively). The results showed that the cumulative CH₄ emissions of N80 + LM and N80 + HM were significantly reduced by 37.8% and 55.3% in late rice than that of N80, respectively. This was associated with the reductions of soil dissolved organic carbon concentration, mcrA gene abundance, and mcrA/pmoA ratio, collectively reducing the CH₄ emissions. However, cumulative N₂O emissions exhibited no significant variation across treatments, potentially related to unaffected microbial functional genes and invariant soil nitrate availability. In addition, compared with N80, N80 + LM and N80 + HM significantly increased rice yield by 11.2% and 13.1% in late rice and by 13.4% and 16.3% in early rice, respectively, whereas decreased GHGI by 41.2% and 53.9% in late rice and by 19.4% and 24.5% in early rice, respectively. However, no significant difference in the effects was observed between different dose microalgae biofertilizer. These results suggest that, under the tested greenhouse pot conditions, N80 plus microalgae biofertilizer held the potential to reduce CH₄ emissions and maintain rice yield. Nonetheless, field-scale validation is required prior to agronomic recommendations in rice paddy.