A system dynamics model for managing regional N inputs from human activities

Human activities are the main drivers of alterations of regional N cycles. With increasing population and economic development, human-induced N inputs are expected to continue to increase in the future, especially in many regions of developing countries. Because N sources vary substantially at different temporal and spatial scales and stages of economic development, it is of great importance for environmental managers to be able to simulate the dynamics of N inputs to a specific region of interest. Based on the concept of net anthropogenic N inputs (NANI), a quasi-mass-balance method, a system dynamics model simulating regional N inputs (NANI-SD) is developed and presented here for the first time. The NANI-SD model evaluates how much new N from anthropogenic activities is introduced to the whole basin, providing a simple but effective way to examine human influences on regional N cycles. Our application of the NANI-SD model to the Lake Dianchi basin in China shows that human-induced N inputs will continue to increase under current trends of development. Scenarios focused on lowering population growth rate and banning crop production were not effective in achieving long-term reductions in N inputs because their impacts were compensated by the increases in croplands and food imports, respectively. However, adjusting diet patterns and limiting livestock numbers within the basin were shown to be highly effective in controlling regional N inputs without compromising environmental sustainability of food imported regions. There was a significant trade-off between N self-sufficiency and N inputs to the region, posing the issue of “pollution transfer” as the regions of livestock production providing animal products to the Lake Dianchi basin could suffer from locally intensified levels of N pollution introduced while producing those animal N products. The positive relationship between NANI and the proportion of animal-based protein in food indicates that reducing meat consumption could be an effective way of controlling local N inputs without sacrificing food sovereignty. NANI to the basin could also be reduced by recycling N in human and livestock wastes, but its capacity to reduce NANI is limited and projected to diminish with time.