A major scientific challenge for modern agriculture is control of off-site effects on the water resource. In the Valle Central of Costa Rica, coffee plantations may leach fertilizer-derived NO−3to groundwaters, as a result of high fertilization rates (annually ∼270 kg/ha as N), highly permeable and well structured Andisols, and high rates of annual runoff (>1000 mm). The objective of this study was to examine several aspects of the nitrification and NO−3adsorption that control NO−3leaching from these highly productive soils. Monthly collections from four Andisols indicated that soil NO−3varied seasonally, with NO−3accumulating to about 280 kg/ha in the upper meter of soil during the 5-month dry season. Soil NO−3was reduced during wet season months, even though fertilization was confined to the wet season. During these months, soil NO−3averaged about 140 kg/ha as N in the upper meter of soil, apparently reduced by wet season leaching, root uptake, and, possibly, dentrification. Field and laboratory incubations at different soil moisture and temperature regimes demonstrated how soil microflora mineralized N and nitrified NH+4at relatively high rates, even at low water potentials, e.g.,−3can be adsorbed by these allophanous Andisols, especially at low pH (up to about 5 cmol/kg at pH−3to groundwater; however, the effectiveness of adsorption as a protection of groundwater quality is probably limited due to high inputs of fertilizer N and to liming management of coffee soils that maintains relatively high soil pH. Additional research into the coffee N cycle and fertilizer efficiency in coffee is needed to ensure high coffee productivity and to protect aquifer water quality in the Valle Central