NUTRIENT BUDGETING UNDER FOUR GROUNDCOVER MANAGEMENT SYSTEMS (GMSs) IN AN APPLE ORCHARD

Overuse of nitrogen fertilizers can pollute surface and ground water, and high leaching losses may occur when nitrogen (N) fertilizer rates are not adjusted to crop demand and N availability in soil. Quantifying nutrient inputs and outputs from orchards is a useful tool to optimize nutrient supply, especially for N.  We conducted an experiment near Ithaca, New York, to develop N budgets based on the N dynamics under four groundcover management systems (GMSs), with and without N fertilization. Three replications of four GMS treatments were randomly assigned to 12 plots and have been maintained since 1992 in 2-m-wide strips within tree rows: Pre-emergence residual herbicide (PreHerb), post-emergence herbicide (PostHerb), mowed-sod (Sod), and hardwood bark mulch (Mulch).  We measured N inputs to the system (N fertilizer, rain and irrigation water), N outputs (harvested fruit, surface runoff and subsurface leaching), and internal fluxes of N (recycling groundcover vegetation, soil mineralization, leaf litter fall and pruned wood). The overall balance for N among GMS treatments for the year with N fertilizer additions was positive, and was greater in the PostHerb and Mulch treatments. In the year without fertilizer, the overall balance for N was negative for the PreHerb and PostHerb treatments, but positive for the Mulch and Sod treatments. Soil mineralization and recycling groundcover biomass accounted for more than 60% of total internal N fluxes in both years, and harvested fruit represented over 80% of N outputs from the system. During the year with N fertilizer additions, N losses averaged 1-2% from surface runoff, and 15-18% from subsurface leaching among the four GMSs. In the year without fertilizer N, surface runoff N losses were twice as great as subsurface N leaching losses from the orchard.