Evaluating management and soil and weather contributions to potential nitrate leaching from a Pennsylvania dairy farm using NLEAP

Developing approaches to reduce nitrate-N (NO3-N) leaching from farms requires an understanding of the factors influencing the potential for NO3-N leaching. An annual screening analysis, NLEAP, was used to determine NO3-N leaching potential (NL) from a 110-ha mixed crop and dairy farm in a central Pennsylvania limestone valley for each of 9 yr. The farmer management goal was to meet the animal and crop needs as much as possible with internal farm resources. Nitrate-N leaching potential was estimated from annual field N balances (NAL) and a yearly soil and weather factor (SWF) based on winter and summer rainfall, soil bulk density, rooting depth, and soil hydrologic group. Primary crops grown were corn (Zea mays L.) and alfalfa-timothy (Medicago sativa L.-Phleum pratense L.). A geographic information system was used to calculate and map SWF, NAL, and NL. The magnitutude and aerial distribution of field SWF was significantly different (Kolomogorov-Smirnov, D-max, p = 0.05) for most year-to-year comparisons. The excess of N inputs over outputs (field NAL) was low for all fields and did not vary significantly during the 9-yr period. The NL was < 25 kg ha(-1) for 75% of the farm and < 50 kg ha(-1) for 85% of the farm for all years of the study. The careful and conservative N management strategy by the farmer, reflected in the consistently low NAL, had an overriding impact on potential NO3-N leaching from the farm.