Fate of nitrogen-15 in a perennial ryegrass seed field and herbaceous riparian area

Intensive management of grass seed fields in the poorly drained soils of the Willamette Valley, Oregon, has prompted concern in the capacity of these landscapes and their associated minimally managed riparian areas to process and retain fertilizer N. Our goal was to determine the extent of N losses and effectiveness of a riparian area and an adjacent perennial ryegrass seed field to retain N. The fate of fertilizer (NH4)-N-15 and (NO3)-N-15 was determined with a N-15 tracer experiment. During the second year of the study, N-15 recovery in the plant and soil (0-30 cm) from the cropping system was 51% for (NH4)-N-15 and 43% for (NO3)-N-15, whereas recovery in the riparian area was only 20% of (NH4)-N-15 and 31% of (NO3)-N-15. Greater cropping system retention of N-15 resulted from both greater uptake by the crop and greater retention of N-15 in the soil. Low recovery of N-15 in the riparian area was possibly the result of two significant spring flood events saturating the surface soil of the riparian area but not the cropping system. The prolonged seasonal saturated conditions significantly reduced riparian plant biomass production and N uptake and increased the potential of N loss through overland flow and denitrification. Results indicate that the cropping system had larger available N pools and a larger potential to retain fertilizer N than the riparian zone. However, both areas were prone to substantial loss of applied N.