Human activity controls nitrogen loads in a large sub-tropical delta from 2000 to 2020

Nitrogen (N) is crucial for agricultural yield, but its overuse in fertilisation and presence in uncollected wastewater from urbanization causes eutrophication. The Red River Delta in Vietnam is facing rapidly increasing water quality issues. Here, Material Flow Analysis is applied to quantify N flows in the delta between 2000 and 2020. This novel long-term assessment of changes in rice fertilisation regimes (human excreta, livestock manure and chemical fertilisers), demonstrates dramatic changes in N flows to surface water. The model shows a 41 % increase in rice paddy N use, with chemical fertilisers rising 1.6-fold while manure-derived N declined to 36 %. The total N load into surface water in 2020 increased by 53 % compared to 2000. The “hidden” inflows of domestic wastewater and blackwater into rice fields, which contribute significantly and indirectly to N loads, were identified via the MFA model. This underscores the need for improved fertilisation practices and waste management to mitigate freshwater pollution.