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

Abstract

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 N₂O flux in HNP (43.1 ± 11.3 μmol m^-2 d^-1) was significantly higher than in LNP (-11.3 ± 25.1 μmol m^-2 d^-1), while the dissolved N₂O concentration in HNP (52.8 ± 7.1 nmol L^-1) was 150 % higher than in LNP (p < 0.01). However, the methane (CH₄) and carbon dioxide (CO₂) fluxes and concentrations showed no significant differences (p > 0.05). N₂O replaced CH₄ as the main source of Global Warming Potential in HNP. Pond sediments acted as a sink for N₂O but a source for CH₄ and CO₂. The △N₂O/(△N₂O + △N₂) 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 NO₂-N ratio was the most important environmental factor explaining the variability of N₂O fluxes. Ammonia-oxidizing bacteria driven nitrification in water was the predominant N₂O source, while comammox-driven nitrification and nosZII-driven N₂O reduction in water were key processes for reducing N₂O emission in LNP but decreased in HNP. The strong CH₄ oxidization by Methylocystis and CO₂ assimilation by algae resulted in low CH₄ emissions and CO₂ sink in the aquaculture pond. The Mantel test indicated that HNP increased N₂O fluxes mainly through altering functional genes composition in water and sediment. Our findings suggest that there is a significant underestimation of N₂O emissions without considering the significantly increased △N₂O/(△N₂O + △N₂) caused by increased nitrogen loading.

Keywords: Comammox; Environmental factor; Methane; Microbial community; Nitrous oxide; nosZ.