NITROGEN CYCLING IN COFFEE AGROECOSYSTEMS - NET N MINERALIZATION AND NITRIFICATION IN THE PRESENCE AND ABSENCE OF SHADE TREES

Coffee (Coffea arabica L.) agroecosystems in Costa Rica receive relati vely large inputs of N through fertilization (approximately 30 g N m-2 year-1). However, little is known regarding the rate at which N is mi neralization from organic matter and its subsequent transformation wit hin the soil. We studied the seasonal and spatial variation of N trans formations in coffee plantations with and without shade trees in the C entral Valley of Costa Rica. Net N mineralization and net nitrificatio n, assayed using an in situ incubation procedure, were measured at mon thly intervals for 1 year. Both net N mineralization and net nitrifica tion displayed marked seasonal variation; the significantly lowest rat es occurred during the dry season (January - March). The mean annual r ate of net N mineralization was 14.8 g N m-2 year-1 in shaded and 11.1 g N m-2 year-1 in unshaded plantations; 95% of mineralized N was oxid ized to NO3- in both plantation types. Even though N availability was greater in unshaded plantations, related studies indicate that leachin g losses are less than those from unshaded plantations. In combination , these results suggest that N is cycled more conservatively in shaded plantations than in unshaded plantations.