Many cities in the U.S. are making efforts to divert organic wastes from landfills and increase compost production. Composting coupled with urban agriculture has the potential to recycle nitrogen (N) and phosphorus (P) from the waste stream back into the human food system. However, compost may often be applied in excess of crop nutritional demand, especially for P due to the low N: P ratio of most composts. Excess application of compost-derived nutrients can contribute to nutrient losses via sub-surface leachate or runoff, contaminating groundwater and receiving waters downstream. Using an experimental system embedded in a campus-community garden, we characterized nutrient use efficiencies of four different crops in plots receiving different types of compost applications targeted to fulfill either crop N or P demand. We quantified mass nutrient flux losses in the form of leachate and estimated the total percentage of N and P applied recovered in crops and lost via leachate.
Results/Conclusions
We observed no treatment-induced differences in cumulative loads of NO3-, NH4+, and PO43- in the leachate. Overall, NO3- loads (4753 mg N m-2 over the five month-study) were approximately 37 times greater than PO43- loads (130 mg P m-2), attributed to its mobile nature. Significant differences in leachate nutrient concentrations were observed across treatments. For certain compost types, increased PO43- leaching occurred when compost was applied to fulfill crop N demand (i.e. P was applied in excess of crop demand). Crop yield data showed no treatment differences in aboveground plant biomass as anticipated, as the treatments were adjusted to receive adequate nutrients either via compost or synthetic fertilizer application. Fluxes of N leachate represented 8-49% of the magnitude of N, while N export in harvested crops composed up to 95% of added N in certain treatments. P losses in leachate represented up to 2-12% of total P input, while P recovery ranged from 24-176%, suggesting “mining” of P from additional sources in the plots. Overall, these results suggest that targeting compost applications can maintain yield and optimize nutrient recycling efficiency, with minimized impacts on water quality. Nutrient leaching may be a by-product of any agricultural practices relying on compost, but this can be minimized through strategic management practices. This study also illustrates how agroecosystem experimentation can be effectively integrated into a project providing diverse community benefits.