Phosphorus recycling and loss from compost‐amended urban gardens: Results from a 7‐year study

AbstractUrban vegetable gardens provide an opportunity to recycle nutrients from food waste back into the human food system through the application of compost. However, a reliance on compost for soil fertility can lead to excess phosphorus (P) inputs that can build up in garden soil and potentially be exported via leachate or runoff. We report the results of a 7‐year experiment in a campus research garden in which replicated raised‐bed garden plots received manure‐based compost or municipal compost that was applied at a higher rate targeted to meet crop nitrogen demand or a lower rate targeted to meet crop P demand. Control plots received either no soil inputs or targeted synthetic fertilizer. Higher input treatments for both types of composts showed steadily increasing concentrations of soil plant‐available P, with a corresponding increase in leachate phosphate concentration. For both higher input compost treatments, approximately 30% of P added as compost was recovered in harvested crops over the 7‐year period, compared to >88% in the lower input compost treatments. In both high‐ and low‐input manure compost treatments, export of P as leachate accounted for approximately 10% of total P input, compared to 4% for the municipal compost. Over the 7‐year study period, P exported as leachate ranged from 0.8 g P/m2 in the no‐input treatments to 6.5 g P/m2 in the higher input manure compost treatments. These results show that tradeoffs are not inevitable as targeted compost applications can lead to high yield and low leachate export.