Characteristics of heavy metal (cadmium) transformation, migration and cycling in rice-duck mutual ecosystem

Heavy metal pollution has been being the subject of attention because it endangers food security and agro-ecological environment. Raising ducks in paddy fields is a Chinese traditional agriculture mode of integrated planting and breeding in paddy fields in subtropical regions. Neither chemical pesticides nor herbicides are applied throughout the growing season in the mutual rice-duck ecosystem and surface water is maintained about 10 cm depth during the period of raising ducks. Field experiments on Cd transformation, migration and cycling in mutual rice-duck ecosystems were conducted in 2014–2015 in double cropping rice regions in Hunan Province and a conventional rice field used as the control. The aim of the experiment was to explore heavy metal pollution risk of products of rice-duck mutual ecosystem, and provide references for adjustment and improvement of management strategies of fertilizer and feed, optimization of food chain, input-output structure rationalization and Cd pollution control of the rice-duck mutualism. In 2014, 17-day-old ducks were introduced into the paddy field (at a holding capacity of 675 ducks per hectare) 23 days after early rice seedling transplanting; in 2015, 20-day-old ducks were introduced into the paddy field (at a holding capacity of 675 ducks per hectare) 21 days after early rice seedling transplanting. The ducks were retrieved at the end of heading stage of early rice in the experiments in both years. Input-Output Analysis method was used to analyze heavy metal Cd cycling in the mutual rice-duck ecosystem using collected data in field experiments. Input included materials embodied in fertilizers, feed, seedling, duckling and irrigation, while output covered seed and duck in the rice-duck mutualism. The return materials consisted of feces, rice straw, rice root, weeds and insects. The results showed that Cd input in the mutual rice-duck ecosystem decreased in the order of fertilizer > feed > rice seedling > duckling. Fertilizer Cd input was mainly from phosphate fertilizer, duck Cd input was mainly from duck feed, and matured duck feed Cd input was greater than duckling feed Cd input. Cd cycling inner the ecosystem was from duck feces, weeds and insects transforming to rice straws and roots. In the mutual rice-duck ecosystem, heavy metal Cd was amplified along food chain transformation. The migration process of duck feces Cd input was higher than that of duck feed Cd input. For both rice-duck mutual ecosystem and conventional rice system, the order Cd contents of rice organs was root > straw > seed. Compared with conventional rice cultivation, rice-duck mutualism did not increase Cd content and accumulation in rice plants. Cd output of paddy soil under rice-duck mutualism and conventional rice cultivation was not significantly different (P > 0.05). For rice-duck mutual ecosystem, the contents of Cd in brown rice and duck were 0.033 mg·kg^-1 and 0.008 mg·kg^-1, respectively, lower than the limit standard of food Cd content. In the short-term, mutual rice-duck ecosystems provided a safe and non Cd contaminated mode of agricultural production.