GROUNDCOVER MANAGEMENT-SYSTEMS INFLUENCE FUNGICIDE AND NITRATE-N CONCENTRATIONS IN LEACHATE AND RUNOFF FROM A NEW-YORK APPLE ORCHARD

Commercial apple (Mabs domestica Borkh,) orchards in the northeastern United States receive heavy pesticide inputs and are often located on well-drained soils near surface and groundwater resources, Nonpoint-so urce water pollution by agrichemicals has been monitored in agronomic crop systems and simulated using computer models and laboratory soil c olumns, but inadequately studied at field scale in orchards, We monito red the concentrations of agrichemical tracers, nitrate-N, and benomyl fungicide in water samples from two apple orchards under mowed sodgra ss (Mowed-Sod), shredded bark mulch (Bark-Mulch), preemergence residua l herbicides (Resid-Herb), and postemergence herbicide (Post-Herb) gro undcover management systems (GMSs), In one orchard, we evaluated subsu rface spatial patterns and flow rates of a weakly adsorbed blue dye (p esticide analog) and potassium bromide (nitrate analog) under trees af ter six years of Post-Herb and Mowed-Sod treatments, Nitrate and pesti cide tracers leached more rapidly and in higher concentrations under P ost-Herb treatments, apparently via preferential macropore flowpaths s uch as root channels, soil cracks, and macrofauna burrows. At another orchard, we monitored subsurface leaching and surface runoff of benomy l and nitrate-N on a whole-field scale, Peak concentrations of benomyl (up to 29 mg . liter(-1)) and nitrates (up to 20 mg . liter(-1)) were observed in subsoil leachate under Resid-Herb plots during 1993. In 1 994, nitrate concentrations were greater in leachate from all GMSs, wi th upper ranges from 48 to 66 mg . liter(-1), while benomyl concentrat ions were lower in all GMSs compared with the previous summer. In surf ace water runoff during 1993, the highest benomyl concentrations (387 mg . liter(-1)) and most frequent outflows occurred in Resid-Herb plot s, During 1994, benomyl runoff was more frequent in both herbicide GMS s, with concentrations up to 61 mg . liter(-1) observed in the Post-He rb plots, Weather patterns, irrigation intensity, differing soil condi tions under each GMS, and the turfgrass/clover drive lanes affected th e relative frequency and concentrations of benomyl and nitrate leachin g and runoff, Preferential bypass flow appeared to be a major subsurfa ce leaching pathway, and erosion sediment an important factor in surfa ce movement of these agrichemicals. Our studies suggest that nitrate-N and benomyl fungicide may be more prone to leaching or runoff from or chard soils under some herbicide GMSs in comparison with mowed sodgras s or biomass mulch systems.