Urban agriculture is rapidly expanding in many cities worldwide. Because compost is generally used as fertilizer, urban agriculture has the potential to recycle nutrients from food waste back into the human food system. However, the ratio of nutrients in compost may not match the nutrient demands of crop production, and imbalanced or excessive application of compost could lead to losses of nutrients through leachate or runoff. In some cases, urban agriculture could be a contributor to the pollution of groundwater or surface water. We measured nutrient recycling efficiency from compost application on urban garden plots, and quantified nutrient loss through leachate, for two different types of compost (municipal organics and cow manure) at three different application levels.
Crop yield was significantly higher in only one compost treatment (high application rate of municipal organics compost) relative to control plots, which received no compost. Leachate nitrate and phosphate concentrations varied across treatments, with highest values measured for manure compost treatments. Leachate accounted for 18-45% of N and 0-37% of P lost/removed from plots. However, total N and P inputs in the form of compost were 1-2 orders of magnitude greater than observed losses, indicating nutrient build-up in soil. Our experiment was representative of common compost application practices in urban agriculture, and our results indicate that annual application of compost could lead to substantial nutrient losses to the environment. There appears to be a tradeoff between recycling urban nutrients through composting coupled with urban agriculture, and minimizing nutrient losses.