Ssj50223 909..919

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 and NO3 was determined with a N tracer experiment. During the second year of the study, N recovery in the plant and soil (0–30 cm) from the cropping system was 51% for NH4 and 43% for NO3 whereas recovery in the riparian area was only 20% of NH4 and 31% of NO3. Greater cropping system retention of N resulted from both greater uptake by the crop and greater retention of N in the soil. Low recovery of N 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. INTENSIVE FORAGE and grass seed production requires high levels of fertilizer N applications. The production of forage and grass seed are often in areas characterized by seasonally cool wet climates such as the Pacific Northwest. Grass seed production is one of the largest agricultural industries in the Willamette Valley, Oregon, making up 55% of the land-use (Nelson, 2003). Grass seed cropping systems in western Oregon typically receive fertilizer applications in the range of 140 to 235 kg N ha yr (Griffith et al., 1997b) that are, on average, 30% greater than extension service recommendations (Horneck and Hart, 1988). Intensive fertilizer N management of these systems, high seasonal rainfall, and an increasing awareness of the economic and environmental consequences of N loss have led to a growing interest in understanding how these systems, and their associated riparian areas, process and retain N. Denitrification, leaching, and overland flow of seasonal precipitation have been shown to be significant pathways of N loss from these systems (Horwath et al., 1998; Wigington et al., 2003). In addition, differences in management (e.g., tillage, monoculture, rotation length, fertilizer applications) and landscape position of grass seed fields and riparian areas will influence the cycling of N into and through the soil and plant pools and determine if these systems will prevent or contribute to NO3 2 contamination of surface and ground water. Competition for N between plants and soil microbes is the primary process controlling the potential for NO3 2 loss through water movement. In unfertilized grassland systems, competition for inorganic N between plants and the soil microbial biomass can be significant because the demands for N are met by (Dell and Rice, 2005) and can exceed supply by mineralization (Woodmansee et al., 1981; Williams et al., 2001). Plant and microbial consumption of NO3 2 was greater in a native prairie compared with a fertilized cultivated soil (DeLuca and Keeney, 1995). Microbial biomass has been shown to control plant N uptake in a N-limited tall grass prairie (Williams et al., 2001), grass swards (Hatch et al., 2000), and perennial ryegrass plots (Hodge et al., 2000). The absence of cultivation can increase the size and activity of the soil microbial biomass (Smith and Paul, 1990; Horwath et al., 1998) and available C (DeLuca and Keeney, 1994). In addition, higher soil C/N ratios often found in unfertilized systems could potentially increase the amount of N immobilized by microbes and incorporated into soil organic matter pools and limit the N available for plant uptake (Hodge et al., 2000). The landscape position of riparian areas exposes them to high, fluctuating water tables further increasing competition for N by reductive soil N processes such as denitrification (Lowrance et al., 1984; Peterjohn and Correll, 1984; Jacobs and Gilliam, 1985; Haycock and Pinay, 1993; Horwath et al., 1998) and dissimilatory NO3 2 reduction to NH4 1 (Davis, 2003). Competition for inorganic N between plants and the soil microbial biomass may be less intense in agricultural systems. High inputs and decreased competition for N in highly managed systems creates the potential for larger N pools susceptible to leakage (Scholefield et al., 1993). Differential processing of NH4 1 versus NO3 2 in the plant–soil systems of grass seed fields and associated herbaceous riparian areas is not well understood. It is often assumed that the fate of soil NO3 2 is limited to immobilization by plants, denitrification, and water movement through mass flow and diffusion (Davidson et al., 1990). Schimel et al. (1989) and Hatch et al. (2000) found greater plant uptake of NO3 2 in grassland soils due to microbial immobilization of NH4 . Microbial preference for NH4 1 has been documented as well as the inhibition of microbial immobilization of NO3 2 in the presence of NH4 1 (Rice and Tiedje, 1989). However, studies have also shown significant microbial uptake of NO3 2 in soil even in the presence of NH4 1 and this has been attributed to microsite depletion of NH4 1 (Jackson et al., 1989; J.H. Davis, USDA–ARS, Corvallis, OR 97331; S.M. Griffith, USDAARS, NFSPRC, Corvallis, OR 97331; W.R. Horwath, Plant and Environmental Sci., Univ. of California, Davis, CA 95616-8627; J.J. Steiner, USDA-ARS, National Program Staff, Beltsville, MD 20705; D.D. Myrold, Crop and Soil Sci., Oregon State Univ., Corvallis, OR 97331-7306. Received 11 July 2005. *Corresponding author (griffits@