Agroforestry Switchgrass: Productivity and Environmental Quality with Poultry Litter.

An existing unfertilized stand of switchgrass under loblolly pine at a coastal plain site was subdivided into six blocks of five treatments, control and 80 or 160 kg N / ha supplied either as ammonium nitrate or poultry litter (PL).   Runoff sub-plots (1.5 m²) were installed in control and 160 kg N / ha PL plots and in an adjacent stand of unfertilized loblolly pine.  Soil samples were collected prior to fertilization at re-growth, light and soil water were monitored during the growing season, runoff was collected continually and biomass plus tissue samples taken at the end of the growing season.  Fertilization increased biomass despite variability in light and soil water, however, there was no effect of N source.  Loss of nutrients in runoff, particularly phosphorus (P), was not significantly greater for fertilization with PL, and cumulative losses were negligible (e.g., total P << 1 kg / ha, 160 kg N / ha PL) on the highly permeable soils.  Continued monitoring of this system is necessary to determine whether or when the build-up of soil P becomes a source of high loads of P in runoff and it is no longer a benign, alternative for use of PL fertilizer.