POULTRY LITTER AND MANURE CONTRIBUTIONS TO NITRATE LEACHING THROUGH THE VADOSE ZONE

Pastures are commonly fertilized with poultry litter or manure in nort hwestern Arkansas. We hypothesized that this practice may enhance NO3- N movement to groundwater and wanted to ascertain the likelihood of co mmon application rates causing excessive NO3-N leaching. Our objective was to determine the effect of application rate of poultry litter or manure on NO3-N concentration in vadose water under fescue (Festuca ar undinacea Schreb.) plots as a function of depth and time. In August 19 91, poultry litter was applied at 0 (control), 10 (PL10), and 20 Mg ha -1 (PL20) and poultry manure at 17.7 Mg ha-1 (PM20). In June 1992, we applied an additional 4.5 Mg ha-1 litter (PL5) to the PL10 plots and 3 .8 Mg ha-1 manure (PM5) to PM20 plots. Pan lysimeters at the 60-cm dep th and suction-cup lysimeters at 60 and 120 cm sampled soil water. The PL10, PL20, and PM20 treatments produced NO3-N concentrations as high as 13, 54, and 41 mg L-1 at 60 cm. At 120 cm, NO3-N reached 8, 24, an d 37 mg L-1 for the PL10, PL20, and PM20 treatments. In 1992, the PL5 and PM5 treatmentS produced NO3-N concentrations <1 mg L-1 at the 60- and 120-cm depths. The recommended litter application rate in Arkansas is not more than 11.2 Mg ha-1 yr-1, split in two 5.6 Mg ha-1 applicat ions. The PL10 and PL5 rates (similar to a maximum full-year applicati on plus a single split application) did not result in NO3-N concentrat ions above the drinking water standard (10 mg L-1) at the 120-cm depth .