Water Cycling and Nutrient Leaching from Paddy Field as Affected by Rice-crab Co-culture

Rice-crab co-culture is a characteristic industry for comprehensively promoting rural revitalization in China, especially in the northeast. However, the differences in water cycling and soil-water environmental characteristics between rice-crab fields and rice fields are not known. In order to quantitatively characterize the water cycle and soil-water environmental characteristics of rice-crab aquaculture, 12 typical fields were set up in two large-scale rice-crab irrigation areas (Yingkou Irrigation Area and Dawa Irrigation Area) at the estuary of the Liaohe River, and the daily ET, daily percolation, irrigation quota, times of irrigation, nutrient dynamics of the water and the topsoil, nutrient leaching, and the yields of rice and crab in rice-crab fields were quantitatively researched by using the rice field as a control. The results show that the rice-crab co-culture model had no significant effect on rice yield, but the increase in the productive value of crabs, the comprehensive economic benefit of rice-crab field was significantly increased by 38.89% to 76.49% compared with rice field (P<0.05). Compared with rice fields, irrigation quota and total leakage of crab paddy fields were significantly increased by 3.29% to 20.93% (P<0.05) and 5.66% to 10.83% (P<0.05), respectively, and water productivity was significantly decreased by 1.85% to 9.28% (P<0.05). However, the comprehensive output value per unit of water use increased significantly by 33.34% to 70.23%. Compared with rice fields, the contents of NH₄⁺-N, NO₃^--N, available phosphorus and available potassium in surface water of rice-crab fields were significantly decreased by 19.99% to 37.24%, 35.25% to 56.22%, 18.69% to 20.30% and 8.12% to 9.36%, respectively (P<0.05). The potential risk of surface runoff nutrient non-point source pollution is reduced. Compared with rice fields, the contents of NH₄⁺-N, NO₃^--N, available phosphorus and available potassium in surface soil of rice-crab fields were significantly increased by 60.00% to 83.88%, 18.21% to 44.97%, 28.34% to 54.56% and 10.47% to 21.24% (P<0.05), and the soil fertility was improved. Compared with rice fields, leaching losses of NH₄⁺-N, NO₃^--N, available phosphorus and available potassium in rice-crab fields were significantly increased by 20.60% to 25.20%, 26.24% to 44.75%, 14.15% to 18.83% and 4.31% to 16.94%, respectively (P<0.05), which promoted nutrient transfer to deep soil profile (20 to 100 cm). The study revealed that the rice-crab co-culture system has great potential to increase the integrated output value per unit of water, reduce the risk of runoff pollution, improve the land fertility and increase the comprehensive economic benefit of the rice field, but the problem of promoting nutrient leaching in the paddy field should not be ignored. The study can provide theoretical basis and technical support for green sustainable development of crab irrigation area and farmers' income increase.