An approach to identify the spatiotemporal patterns of nitrogen flows in food production and consumption systems within watersheds

Highlights: • Integrate substance flow analysis and spatial analysis to quantify nitrogen flows • N losses to the environment increased 8 times since 1949, 7% washed into the lake. • 19% of the region has high N runoff risk, located in eastern and south central areas. • 56% of N lost to the environment, 24% exported, only 5% consumed by local residents. Human has greatly disturbed nitrogen (N) cycles through the food production and consumption (FPC) activities, which caused serious environmental consequences. A few studies address the pathways of N cycles to clarify the interrelationships between human activities and the altered N cycles, especially in terms of the spatial aspects of N flows, which are also influenced by natural factors. Here, we developed an integrated model to quantify the spatiotemporal patterns of N flows within the FPC and applied it to the Chaohu Watershed from 1949 to 2012. We found that total N inputs into the FPC increased by 6-fold during 1949–2012, while N inputs from chemical fertilizer had the largest increase, with an annual growth rate of 8.6%. N generation in manure doubled, but the recycling rate decreased by 50% in the study period. The total N losses increased from 17 to 130 Gg-N/yr between 1949 and 2012. Annually, ~ 7% of total N losses was discharged into surface water annually, which peaked at 9.5 Gg-N/yr during 1998–2003, two times as that in the early 1980s when algae blooms first occurred in Chaohu lake. Spatially, around one-fifth of the catchment area was responsible for all of N losses to surface water. These intensive areas located in the eastern and southern-central regions, which were identified as the critical areas for priority N mitigation. This study provided a method to depict spatiotemporal patterns of regional N flows for identifying the critical sources and areas of N pollution for precision watershed management.