Assessment of best management practices for nutrient cycling: A case study on an organic farm in a Mediterranean-type climate

The effectiveness of best management practices (BMPs) designed primarily to protect surface water quality was assessed on a farm certified for organic tomato production to consider potential environmental quality and production tradeoffs. The BMPs included winter cover crops typically used in organic farming to cycle nutrients and reduce stormwater runoff; tailwater ponds designed to capture runoff; and tailwater return systems, which recycle runoff back to the field. The study took place at a 44 ha (108 ac) farm in Yolo County, California, over a two-year period. Monitoring throughout the winter rainy season showed cover crops successfully reduced runoff and loads of several constituents during the storm events, when compared to fallow Total discharge was reduced by 44%, total suspended solids was reduced by 83%, ammonium was reduced by 33%, and dissolved organic carbon (DOC) was reduced by 58%. Estimates of leaching losses of DOC in the cover cropped fields, however, were 70% higher than the fallow fields in the winter rainy season and were 30% higher than the fallow fields in the summer irrigation season. During the summer irrigation season, the tailwater pond alone was highly effective in reducing losses of total suspended solids and volatile suspended solids to the neighboring riparian zone by 97% and 89%, respectively. The tailwater pond had no effect on dissolved reactive phosphorous and actually increased concentrations of nitrate-nitrogen (NO3--N) in effluent by 40% and DOC by 20%. As was expected, the NO3--N leaching measured by anion exchange resin bags and nitrous oxide emissions measured by static closed chambers was higher for the tailwater pond than the fallow field. Despite these differences, losses via NO3--N leaching and nitrous oxide emissions accounted for only 24.7 and 0.48 kg N ha(-1) y(-1) (22.0 and 0.40 lb N ac(-1)), respectively, for the entire farm, even including ponds and ditches. When field and plot values were extrapolated to the entire tomato production area to understand the relative potential tradeoffs, results indicate that BMPs could be implemented without an impact on tomato marketable yields; the tailwater pond's higher nitrous oxide emissions would not significantly increase the overall emissions for tomato production given its relatively small size; and using tailwater ponds in combination with cover crops would decrease total suspended solids (TSS) losses compared to cover crops alone, with only minor increases in NO3--N and DOC losses. Adding a tailwater return system to this combination of BMPs could help minimize these NO3--N and DOG losses. Use of cover crops with a tailwater pond and tailwater return system are a combination of BMPS that can thus be recommended for organic production when considering multiple environmental outcomes.