NITROGEN LOSS FROM COFFEE AGROECOSYSTEMS IN COSTA-RICA - LEACHING ANDDENITRIFICATION IN THE PRESENCE AND ABSENCE OF SHADE TREES

Coffee (Coffea arabica L.) management in Costa Rica is changing from t raditional agroecosystems, where coffee is grown beneath a tree overst ory, to management systems where shade trees are removed and N fertili zer is applied at high rates (ca. 300 kg N ha(-1) yr(-1)). Although fe rtilization increases coffee bean production, it also increases the po tential for substantial loss of N to groundwater and the atmosphere. W e investigated NO3- leaching and the factors controlling denitrificati on in shaded and unshaded coffee plantations in the Central Valley of Costa Rica; both plantation types were fertilized with 300 kg N ha(-1) yr(-1). Nitrate leaching was quantified using porous ceramic cup lysi meters placed 60 cm below the soil surface. Losses were estimated by m ultiplying the soil water NO3- concentration by the monthly soil water excess, determined as the difference between precipitation and actual evapotranspiration. In addition, a laboratory experiment was conducte d to investigate the influence of NO3-, C, and O-2 availability on N2O production and total denitrification (N2O-N + N-2-N). Annual leaching losses of NO3- were almost three times greater in unshaded plantation s (24 kg NO3- ha(-1) yr(-1)) than those in shaded plantations (9 kg ha (-1) yr(-1)). In contrast, mean total denitrification rates in control soil samples from shaded plantations were 60% higher (732 mu g N2O-N kg(-1) d(-1)) than in unshaded plantations (455 mu g N2O-N kg(-1) d(-1 )). Carbon additions elicited the largest increase In denitrification, generating nearly a threefold Increase (+ C = 8396 mu g N2O-N kg(-1) d(-1); -C = 2985 mu g N2O-N kg(-1) d(-1)) in both plantation types. An aerobic conditions also significantly increased denitrification (+ O-2 = 4331 mu g N2O-N kg(-1) d(-1); -O-2 = 6656 mu g N2O-N kg(-1) d(-1)). In both plantation types, the potential for N loss via NO3- leaching was small compared with that for gaseous N loss.