Transition of human and livestock waste management in rural Hanoi: a material flow analysis of nitrogen and phosphorus during 1980-2010

Human and livestock waste, i.e., excreta, were intensively used in crop cultivation in Asia over the years. However, rapid economic growth induced changes in excreta management and subsequently in nutrient flow. In northern Vietnam, a large amount of excreta have been traditionally applied to paddy fields, but much of them are currently discharged to ponds for fish farming. This study investigated the historical changes in excreta management and clarified the impact of fish ponds on nutrient flow as a case study in an agricultural community of suburban Hanoi, Vietnam. A material flow model of nitrogen and phosphorus was applied to the study area. The results showed that the crop-livestock system that intensively used excreta for agriculture was transformed between 1980 and 2010 into the crop-livestock-fish system that discharges a significant amount of livestock excreta to fish ponds. Nutrients inputs to ponds were 41.7 kgN/ha and 9.8 kg-P/ha in 2010, which were 7.2-and 6.2-fold higher than those in 1980, respectively. Beside, 41 % of nitrogen and 82 % of phosphorus released to ponds still remained in the sediment or discharged unintentionally after heavy rainfall. The use of pond sediment as a source of nutrients in paddy fields would improve the material cycle.