Nitrogen Dynamics and Nutrient Budgets in Four Orchard Groundcover Management Systems

Excessive nitrogen (N) applications can increase surface and water contamination, and leaching losses may occur when N fertilizer rates are too high relative to crop demands and soil N availability. Quantifying nutrient inputs, cycling, and outputs from orchards provides a method to measure surplus of nutrients, particularly N, that may leach or runoff. We conducted a long-term study to develop N budgets based on observed nutrient dynamics under four groundcover management systems (GMSs) with and without N fertilization. Four GMS treatments were randomly assigned to 12 plots and maintained since 1992 in 2-m-wide strips within tree rows: pre-emergence residual herbicide (PreHerb), post-emergence herbicide (Post Herb), mowed-sod (Sod), and hardwood bark mulch (Mulch). We measured system N inputs in fertilizer, mulch biomass, rain, and irrigation water; N outputs in harvested fruit, surface runoff, and subsurface leaching; and internal N cycling from surface vegetation, soil mineralization, leaf fall, and pruned wood. For the year with N fertilizer (2005), the overall N balance was positive (inputs exceeded outputs) in all GMSs but greater in the PostHerb and Mulch treatments. In the year without N fertilizer (2007), the overall N balance was negative for PreHerb and PostHerb and positive for Mulch and Sod treatments. Soil mineralization and recycling groundcover biomass accounted for greater than 60% of internal N fluxes, and harvested fruit represented greater than 70% of N outputs from the system during both years. During the year with N fertilizer, N losses were 1% to 4% and 18% to 22% through surface runoff and subsurface leaching, respectively. During the year without fertilizer, surface runoff N losses were twice the subsurface leaching N losses in all GMSs.