Assessing nutrient budgets and N2O emission of newly constructed rice-giant freshwater prawn co-culture on reclaimed land

In recent years, newly constructed rice-giant river prawn (Macrobrachium rosenbergii) co-culture has been widely applied on reclaimed land in southeast China. Raising M. rosenbergii in paddy fields can alter nutrient budgets and have environmental impacts. However, the exact impacts remain unclear. Here, a field experiment was conducted using M. rosenbergii and Macrobrachium nipponense in reclaimed paddy fields to assess and compare the nutrient budgets and N2O emissions of the two ecosystems, henceforth referred to as RS1 and RS2, respectively. Each ecosystem was stocked with 150,000 post-larvae per ha and then compared with a rice monoculture system (RM) to evaluate the environmental impacts of rice-shrimp co-culture. In RS1, the main inputs for N and P were fertilizer and artificial feed, accounting for 59.35% and 34.01% of the total N inputs and 53.15% and 41.43% of the total P inputs, respectively. In RS2 and RM, fertilizers dominated the nutrient budget, accounting for more than 79.28% of the total nutrient input. Nutrient exports were dominated by rice (straw+grain) and commercial shrimp in RS1, accounting for 33.44% and 18.66% of N and 55.56% and 20.02% of P, respectively. The wastewater drainage, NH3 volatilization, and N2O emissions contributed to the remainder nutrient budget. Particularly, the N2O emission fluxes in RS1 (2.21 ± 0.32 kg·ha−1·yr−1) were significantly higher than those in RS2 (1.78 ± 0.24 kg·ha−1·yr−1) and RM (1.56 ± 0.17 kg·ha−1·yr−1) over the one-year experiment. Changes in environmental variables (including NH4+-N, NO2--N, DO, pH, SP, etc.) significantly contributed to the enhanced N2O emissions following the conversion from the reclaimed paddy field to RS1. These results implied that RS1 improved the yields of shrimp and rice grains but at the cost of increasing N2O emissions. Considering the wide distribution of reclaimed paddy fields in southeast China, M. rosenbergii could serve as an ideal alternative species in rice-shrimp co-culture systems to increase nutrient utilization efficiency, although N2O emissions still need to be reduced to mitigate the negative environmental impacts.