Response of Drainage Water Quality to Fertilizer Applications on a Switchgrass Intercropped Coastal Pine Forest

The objectives of this study were (1) to test the hypothesis that fertilizer applications do not increase nutrient fluxes on a switchgrass/pine forest (IC) when compared to a mature pine forest (MP) and (2) to evaluate post-fertilization (post-fert, 2014-2016) fluxes of nitrogen (N) and phosphorus (P) on IC and compare them to those observed during switchgrass growth prior to fertilization (pre-fert, 2012-2014) and site preparation for switchgrass establishment (site prep, 2009-2012). Nitrogen and P were applied to IC, a paired pure switchgrass site (SG), and MP, each about 25 ha in size, in June 2014, and again in June 2015 for the IC and SG sites only. Nitrogen and P concentrations were measured biweekly and rainfall and drainage outflow were measured continuously. During post-fert, the mean N concentrations and total loads were lower (p < 0.05) in IC than in SG and MP. The mean NO3-N concentration and loads in IC were lower during post-fert than during site prep. The post-fert phosphate concentrations in IC were lower than they were during pre-fert and site prep. Frequent N and P applications in IC did not significantly (alpha = 0.05) increase N and P fluxes, likely due to plant uptake and sorption on the acidic site.