Abstract
Urban agriculture has been expanding rapidly in recent years, and it has the potential to recycle nutrients from local food wastes into new foods through the use of compost as growth medium. Composts typically have low nitrogen: phosphorus (N:P) ratios, and in urban gardens, when composts are typically applied to soils annually to meet the N demand of crops, excess P can build up and be prone to leaching. We measured dissolved P (PO4 3−) and N (NO3 − & NH4 +) losses in the leachate from experimental raised-bed garden plots that received one of two commonly used composts (municipal organics compost derived from food scraps or cow manure derived compost) at three different application levels (15%, 35%, or 50% by volume). PO4 3− concentrations in leachate from garden plots receiving manure composts were high, ranging from 5 to 11 mg/L, depending on the application level. Leachate PO4 3− concentrations from plots receiving municipal organics composts were an order of magnitude lower, ranging from 0.5−1.2 mg/L, while leachate PO4 3− in garden plots receiving no compost was 0.3 mg L−1. Cumulative mass of PO4 3− lost through leachate during the growing season ranged from 1.2 to 4.2 g/m2 for manure compost treatments, compared to 0.12– 0.72 g/m2 for municipal compost treatments, and 0.06 g/m2 for soil with no compost. Leachate accounted for 0–37% and 18–45% of dissolved P and N exported from garden plots, respectively. The high application rate of municipal compost significantly increased crop yield relative to the control treatment. P lost from leachate and removed through harvest only represented 1–10% of total P applied as compost, suggesting that soil build-up was the dominant fate of P in this study. Our results illustrate the potential trade-off in urban agriculture between crop production and recycling P efficiently from urban food waste.