Crayfish–Fish Aquaculture Ponds Exert Reduced Climatic Impacts and Higher Economic Benefits than Traditional Wheat–Rice Paddy Cultivation

In pursuit of higher economic profits, an increasing number of conventional rice paddies are being converted into aquaculture ponds in Southeast China. Due to the lack of field observations, the greenhouse gas (GHG) emissions caused by this change are not clear. A parallel field experiment in Southeast China was performed to compare CH4 and N2O emissions from rice paddies and rice-paddy-converted freshwater crayfish–fish aquaculture ponds that had previously been rice paddies. The annual fluxes of CH4 and N2O fluxes from inland crayfish–fish aquaculture averaged 0.36 mg m−2 h−1 and 45.55 μg m−2 h−1, which amounted to 31.50 kg CH4 ha−1 and 3.99 kg N2O ha−1, respectively. Compared with traditional rice paddies, such conversions significantly reduced the emissions of CH4 and N2O emissions by 46.4% and 67.5%, respectively, but greatly increased the net ecosystem economic budget (NEEB) by 485%. The fluxes of both CH4 and N2O fluxes from aquaculture ponds were positively correlated with water/sediment temperature and dissolved organic carbon in the sediment, but were negatively correlated with the concentration of oxygen that is dissolved in the water. In addition, the emissions of CH4 and N2O were closely associated with the chemical oxygen demand of water and the content of N in the sediment, respectively. The results of this study suggest that converting rice paddies to freshwater crayfish–fish aquaculture ponds could cause a reduction in the impacts on the climate and result in greater economic benefits. There is an urgent need worldwide for more field studies on the emissions of CH4 and N2O emissions from aquaculture ponds, including more types of fish species and management practices. These results will help researchers to comprehensively evaluate whether such conversions of agricultural land use are ecologically and economically feasible.