COS 23-2 - Phosphorus recycling and loss from compost-amended urban gardens: results from a six-year experiment

Abstract: Urban vegetable gardens provide an opportunity to recycle nutrients from food waste back into the human food system, through applications of compost as a soil amendment. However, many sources of compost are high in phosphorus (P) relative to crop demands, and many gardeners apply compost at high rates, resulting in build-up of P in garden soil that can be exported via leachate or runoff, ultimately contributing to eutrophication. We report the results of a 6-year experiment in our campus research garden in which replicated raised-bed garden plots received a labile manure-based compost or a recalcitrant non-manure (“municipal”) compost that was applied at a higher rate to meet crop nitrogen (N) demand or a lower rate to meet crop P demand. Control plots received either no nutrient inputs or synthetic fertilizer (to meet both crop N and P demand). Soils from higher labile and recalcitrant compost input treatments showed steadily increasing concentrations of plant-available P and a sharp increase in years 5 and 6 of leachate PO₄ concentration. For both higher-input manure and municipal compost treatments, approximately 27% of P added as compost was recovered in harvested crops over the 6-year period, compared to >75% in the lower-input compost treatments. In both the higher- and lower-input labile compost treatments, total export of P as leachate accounted for 7% of total P added as compost, compared to 3% for the recalcitrant compost. Over the 6-year study period, P exported as leachate ranged from 0.5 g P/m² in the no-input treatments to 4.6 g P/m² in the higher-input labile compost treatments, suggesting that compost application to urban gardens could collectively be a significant source of P export from urban landscapes. These results highlight both the opportunities and challenges of nutrient recycling in urban ecosystems resulting from compost applications coupled with urban agriculture.