Effects of improved traditional rice-fish system productivity on field water environment

World agriculture currently faces great challenges in producing sufficient food while minimizing negative environmental effects of crop cultivation, requiring us to rethink current agricultural production processes. Traditional agricultural systems based on local species diversity and interactions have contributed to food and livelihood security throughout the world for centuries. As human population increased, however, traditional agricultural systems faced daunting challenges in food demand and supply due to the lower economic benefits of the systems. Thus improving the productivity of traditional agricultural systems in a sustainable way has been of great concerned to producers and decision-makers. To determine the feasibility of increasing traditional agricultural productivity without corresponding increase in negative environment impacts, two experiments were conducted at a traditional rice-fish co-culture site that had been operated for over 1 200 years in southern Zhejiang Province. In the first experiment, three treatments without pesticide, including rice monoculture (RM), rice-fish co-culture (RF) and fish monoculture (FM), were compared. In addition to measured rice yield (in RM and RF), fish yield (in RF and FM), and surveyed input-output economics, focus was put on total nitrogen (TN), total phosphorus (TP), total ammonia nitrogen (NH₄⁺-N) and chemical oxygen demand (COD) in field water. Four fish density treatments, including with target fish yields in rice-fish co-culture fields of 750 kg·hm^-2 (RF750), 1 500 kg·hm^-2 (RF1500), 2 250 kg·hm^-2 (RF2250) and 3 000 kg·hm^-2 (RF3000), were designed in the second experiment. Rice and fish yields, input and output of economics, and TN, TP, NH₄⁺-N and COD were measured in the field water. The first experiment showed no significant difference in rice yield between RM and RF. There was also no significant difference in fish yield between RF and FM in the experiment. However, total and net income of the system were higher in RF than in RM and FM. Field water contents of TN, TP, NH4+-N and COD were not significantly different between RF and RM. The second experiment showed that rice yield, fish yield, and total and net economic output increased with increasing fish stock density and fish feed input. Increase in fish stock density increased target yield of fish by 1 500 kg·hm^-2 (50%), significantly increased net income of rice-fish co-culture systems by 25.2%~101.4%, and also increased COD, TN, TP and NH₄⁺-N content in field water. When target yield was 3 000 kg·hm ^-2, TP and COD contents became significantly higher than in other treatments; which enhanced risk of environmental pollution. Economic analysis indicated that target fish yield of 2 250 kg·hm^-2 gave higher economic income and with little impact on field water environment.