Greenhouse gases emissions from aquaculture ponds: Different emission patterns and key microbial processes affected by increased nitrogen loading.

Global aquaculture production is expected to rise to meet the growing demand for food worldwide, potentially leading to increased anthropogenic greenhouse gases (GHG) emissions. As the demand for fish protein increases, so will stocking density, feeding amounts, and nitrogen loading in aquaculture ponds. However, the impact of GHG emissions and the underlying microbial processes remain poorly understood. This study investigated the GHG emission characteristics, key microbial processes, and environmental drivers underlying GHG emissions in low and high nitrogen loading aquaculture ponds (LNP and HNP). The NO flux in HNP (43.1 ± 11.3 μmol m d) was significantly higher than in LNP (-11.3 ± 25.1 μmol m d), while the dissolved NO concentration in HNP (52.8 ± 7.1 nmol L) was 150 % higher than in LNP (p < 0.01). However, the methane (CH) and carbon dioxide (CO) fluxes and concentrations showed no significant differences (p > 0.05). NO replaced CH as the main source of Global Warming Potential in HNP. Pond sediments acted as a sink for NO but a source for CH and CO. The △NO/(△NO + △N) in HNP (0.015 ± 0.007 %) was 7.7-fold higher than in LNP (0.002 ± 0.001 %) (p < 0.05). The chemical oxygen demand to NO-N ratio was the most important environmental factor explaining the variability of NO fluxes. Ammonia-oxidizing bacteria driven nitrification in water was the predominant NO source, while comammox-driven nitrification and nosZII-driven NO reduction in water were key processes for reducing NO emission in LNP but decreased in HNP. The strong CH oxidization by Methylocystis and CO assimilation by algae resulted in low CH emissions and CO sink in the aquaculture pond. The Mantel test indicated that HNP increased NO fluxes mainly through altering functional genes composition in water and sediment. Our findings suggest that there is a significant underestimation of NO emissions without considering the significantly increased △NO/(△NO + △N) caused by increased nitrogen loading.