Simulating the fate of compost-derived nutrients in an urban garden

The application of compost to urban vegetable gardens presents an opportunity to recycle nutrients from the urban waste stream back into the human food system. However, many gardeners apply phosphorus (P) in the form of compost at a rate that far exceeds what crops can take up. The fate of this P—whether stored in soil, taken up by plants, or exported through leachate, depends on the dynamics of water, carbon (C), and nitrogen (N) in this agroecosystem. We developed a model representing these four currencies (C, N, P, water) in urban garden soils, that was parameterized and validated using data from four years of data from an experiment in which high or low amounts of labile manure-based compost, or recalcitrant municipal compost, are added to garden plots annually. We used the model to simulate the effects of longer-term (10-year) additions of labile or recalcitrant compost at low, medium, or high levels (based on previously reported survey data for Minneapolis-Saint Paul, Minnesota), tracking the fate of added N and P, as well as calculating net C sequestration. The fraction of compost nutrients recovered over 10 years ranged from 3 to 47% (N) and 4–67% (P) with higher efficiencies associated with lower input rates and for recalcitrant compost. Approximately half of added C was ultimately respired by soil microbes, while C sequestration from crop growth was much lower than soil respiration. This model provides a tool for understanding how management decisions and climate control nutrient recycling and loss via leachate from compost application in urban agroecosystems.