Carbon budgets of two typical polyculture pond systems in coastal China and their potential roles in the global carbon cycle

The role of aquaculture systems in the global carbon cycle is poorly understood to date. We investigated the carbon budgets in 2 polyculture systems (PM: swimming crab Portunus trituberculatus with kuruma shrimp Marsupenaeus japonicus; and PMR: crab, shrimp and shortnecked clam Ruditapes philippinarum) during the farming season (125 d in total). The main carbon input occurred via water (PM: 55.06%; PMR: 62.50%), followed by that via feed. PM absorbed 21.55 g C m(-2) across the air-water interface, accounting for 13.27% of the total input. The carbon output via water was the main output in both PM (89.17 %) and PMR (46.43%). PMR emitted 53.00 g C m(-2) into the atmosphere during the farming season, accounting for 32.53% of the total output. The carbon output by harvested animals in PMR accounted for 19.48% of the total output, which was much higher than that in PM (5.71 %). Carbon accumulation in the sediment was significantly higher in PM than in PMR (p < 0.05), but the value of both represented a small portion of the total output. Clam farming in the polyculture system profoundly altered the carbon budgets by changing the CO2 flux direction, reducing carbon accumulation in water and sediments and increasing the carbon output by harvested animals. In terms of the global warming potential, which was calculated from the results of CO2 and CH4 fluxes across the air-water interface, PM had the potential for mitigating the greenhouse effect on the 20 yr horizon, whereas PMR exacerbated global warming. Our results suggest that PM acted as a net carbon sink in the global carbon cycle, while PMR acted as a carbon source.