Impact of Alternative Citrus Management Practices on Groundwater Nitrate in the Central Florida Ridge: Ii. Numerical Modeling

Long-term impacts of alternative citrus nitrogen and water management practices implemented at grower cooperator sites on the Central Florida Ridge were modeled using the nitrogen component of the Leaching Estimation and Chemistry Model (LEACHN). A bromide tracer test was conducted at one representative cooperator site to parameterize and validate the model. Following validation, 50-year simulations of the historic management practices at each site were conducted. These simulations confirmed field observations that groundwater nitrate-nitrogen concentrations below mature citrus groves that receive 246 kglha/yr nitrogen (N) as three split applications of dry soluble fertilizer can be expected to exceed the Environmental Protection Agency's (EPA's) Maximum Contaminant Level (MCL) of 10 mglL a majority of the time. Fifty-year simulations of alternative nitrogen and water management practices implemented in the field study predicted that in all cases reducing the rate and increasing the frequency of N application, and improving irrigation management, will increase N uptake by plants and reduce average groundwater nitrate-nitrogen concentrations to within EPA standards. Modeling results support conclusions of the field study that showed that, of the BMPs tested on mature citrus, applying 142 kg Nlhalyr as fertigationlfoliar spray should be the most effective, while applying 180 kg Nlha/year as slow release/dry soluble BMP should be the least effective at reducing groundwater nitratenitrogen concentrations. Modeling results further suggest that, to maintain the average groundwater nitrate-nitrogen concentration below the EPA MCL in this region, the N rate should not exceed 172 kglhalyr if the fertilizer is to be applied in three split applications of dry soluble fertilizer; 208 kglha/yr if the fertilizer is applied in three applications of slow release/dry soluble fertilizer; or 231 kglhalyr if the N is to be applied in 18 split fertigation applications. If 64 kg N Ihalyr is applied as foliar spray, modeling results suggest that an additional 187 kglha/year may be applied in 18 split fertigation applications (for a total N application rate of 251 kglha/yr) while maintaining the average concentration of the leachate below the EPA MCL.