Valorization of farm pond biomass as fertilizer for reducing basin-scale phosphorus losses

Abstract Long-term fertilizer phosphorus (P) inputs are causing phosphorous saturation of agricultural soils globally. The saturation is spreading to the edge-of-the-farm stormwater detention systems (SDSs) from where the legacy P is potentially being released to downstream surface waters. We use baseline biogeochemical and literature data for P-saturated SDSs, to develop and evaluate the biogeochemical and economic feasibility of a P recycling program that targets both low (LIC, sugarcane) and high intensity cropping (HIC, fresh-produce) systems within a watershed. The focus is to close the P cycle loop to rejuvenate P sink function of SDSs. It involves harvesting and composting the SDS's biomass and it's on-farm reuse as an organic fertilizer for crops. Results showed that harvesting-composting can conservatively increase the P retention from 50% to 77% for HIC and almost complete treatment for LIC. Beyond increasing yield and improving soil health, compost use can further increase in-field retention of P (and water). Additional costs incurred in harvesting and composting can be offset by the economic value of compost and the reduction in state expenditure on regional P treatment systems. Treatment costs were $26/kg of P for HIC and $42/kg for LIC, >10 times the current state expenditure of $355–$909/kg P using constructed wetlands. We propose an incentivized, payment for services (PS) program, where producers are paid for P recycling. The PS program considers the intensity of cropping systems and their location along the drainage network from headwaters to the outlet, to achieve basin-scale P load reduction. The LIC SDSs recover regional P by passing the public water through them while recycling is implemented at the HIC. The estimated basin-scale P retention with harvest-compost approach was 854 metric tons, 5 times the P that entered the Everglades Protection Area in 2018, at 88%–93% less cost than the state treatment systems.