ELSEVIER Agriculture, Ecosystems and Environment, 48 (1994) 107-113 Agriculture Ecosystems & Environment Nitrogen cycling in coffee agroecosystems: net N mineralization and nitrification in the presence and absence of shade trees Liana I. Babbar, Donald R. Zak* School of Natural Resources and Environment, University of Michigan, Ann Arbor, M148109-1115, USA (Accepted 12 October 1993 ) Abstract Coffee (Coffea arabica L. ) agroecosystems in Costa Rica receive relatively 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 mineralization from organic matter and its subsequent transformation within the soil. We studied the seasonal and spatial variation of N transformations in coffee plantations with and without shade trees in the Central Valley of Costa Rica. Net N mineralization and net nitrification, assayed using an in situ incubation procedure, were measured at monthly intervals for 1 year. Both net N mineralization and net nitrification displayed marked seasonal variation; the significantly lowest rates occurred during the dry season (January - March). The mean annual rate of net N mineralization was 14.8 g N m -2 year -1 in shaded and 11.1 g N m -2 year -l in unshaded plantations; 95% of mineralized N was oxidized to NO j- in both plantation types. Even though N availability was greater in unshaded plantations, related studies indicate that leaching losses are less than those from unshaded plantations. In combi- nation, these results suggest that N is cycled more conservatively in shaded plantations than in unshaded plantations. I. Introduction Coffee agroecosystems are often subsidized by relatively large inputs of nitrogen (N) fertilizer. This practice is common throughout Latin America, particularly in Costa Rica, where ap- plications of approximately 30 g N m -2 year-l are used to increase coffee bean yield (Minis- terio de Agricultura y Ganaderia, 1992). In ad- dition to large inputs from fertilizer, soil N avail- ability within these ecosystems is influenced by the rate at which soil microorganisms release in- organic N during the process of organic matter mineralization. This biologically-mediated pro- cess is regulated by the amount and chemistry of *Corresponding author. organic matter returned to the soil from above- and below-ground plant litter production. How- ever, data regarding rates of net N mineraliza- tion in coffee agroecosystems are absent from the literature. As such, our understanding of the N budget of coffee agroecosystems is incomplete. Because N is the element most limiting the pro- ductivity of these ecosystems (Carvajal, 1984), and because N additions through fertilization are large, it is important to determine if soil N avail- ability meets or exceeds plant demand. If nitri- fication is an important process within coffee agroecosystems, for example, then any N in ex- cess of plant demand has the potential to be lost through leaching or denitrification. Soil temperature, water potential, and sub- strate availability directly control the activities 0167-8809/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0167-8809 (93)00469-H