Potential for high contribution of urban gardens to nutrient export in urban watersheds

Background/Question/Methods

Urban gardens and farms typically use compost as a source of nutrients, often at levels that exceed crop nutrient demands. Although land dedicated to agriculture is a small fraction of urban land use, high input rates coupled with low nutrient use efficiencies suggest that export of nitrogen (N) and phosphorus (P) from this land could be potentially important contributors to urban nutrient budgets. We used the Integrated Valuation of Ecosystem Services and Tradeoffs (InVEST) Nutrient Delivery Ratio model to examine the potential impact of garden density, compost input rates, and nutrient retention efficiency on N and P export from stormwater runoff for a 737-ha urban residential area in Saint Paul, Minnesota.

Results/Conclusions

Although gardens accounted for 0.1-0.5% of land area in our scenarios, compost inputs accounted for as much as 34% of N inputs and 86% of P inputs to the urban landscape. The contribution of gardens to urban nutrient export through stormwater runoff depended greatly on maximum retention efficiency values. If retention efficiency is high, gardens with low compost inputs can be sinks of nutrients in the urban landscape, but gardens become significant contributors to watershed P export if compost inputs are high, or if retention efficiency drops to 75%. These results underscore mass-balance constraints inherent in urban nutrient recycling and highlight the importance of understanding the long-term fate of excess nutrients applied to urban landscapes.