RECOVERING NITROGEN FROM FARM-SCALE DRAINAGE: MECHANISM AND ECONOMICS

A two-year study evaluated the nitrogen (N) fluxes, processing, and treatment efficiency (TE) of a 14.85 ha stormwater detention area (SDA) receiving drainage from a vegetable farm in subtropical Florida. The TE was 62% and 89% in years 1 and 2, respectively. Seepage N losses are often ignored in estimating stormwater treatment. Approximately 11% and 20% of the incoming N left the SDA through seepage, reducing the TE to 51% and 67% in years 1 and 2, respectively, indicating the importance of subsurface N losses for downstream water quality. Rainfall variability controlled the timing and volume of the inflow drainage and surface water levels inside the SDA. Variable water levels controlled the aerobic and anoxic conditions inside the SDA, thus controlling the N processing and treatment. Coupled nitrification-denitrification, as a result of frequent wetting-drying cycles, was the main N treatment pathway during year 1. Drought conditions in year 2 led to 89% less surface outflow compared to year 1, resulting in water volume retention being the main process for N retention. The N TE could be increased from 68% to 86% if about two-thirds (63%) of aboveground biomass in the SDA area is harvested annually during the dry season. A payment for environmental services (PES) framework, with the state as buyer and the SDA owner as seller of N treatment services, was evaluated with a 20-year net present worth (NPW) of biomass harvesting for enhanced N treatment. The economic analysis included the benefit from composting the harvested biomass for on-farm use. A positive NPW ($ 835,000) indicated the economic feasibility of the project, predicting an annual benefit of $ 42,000 year(-1) for the 112 ha farm. Scale-up of the PES approach can offer additional N treatment and C sequestration services as well as increased farm productivity.