Nitrogen dynamics (coffee productivity, nitrate leaching and N2O emissions) in Coffea arabica systems in Costa Rica according to edaphic conditions, fertilization and shade management

A synthesis of a five year research program on nitrogen (N) cycling in coffee plantations in Costa Rica is presented. In the main areas where coffee is grown, full sun coffee plantations were compared with coffee agroforestry systems in order to improve N fertilization and shade management to optimize the production of coffee and associated trees while reducing nitrate (NO3 -) leaching to ground water and N2O emissions. Costa Rican coffee soils exhibited: (1) high permeability allowing large water drainage; but (2) different adsorption patterns resulting in different NO3 - dynamics. Acrisols (Ultisols, USDA classification) presented a dominance of positive charges in the subsoil which were responsible for NO3 - adsorption and mitigated NO3 - leaching and groundwater contamination. On the other hand, andisols from the Central Valley did not retain NO3 - so that NO3 - was leached to the aquifers. In a full sun coffee plantation, fertilization experiments showed that increasing the N fertilizer level from 150 to 350 kg N ha-1 yr-1 resulted in a low additional coffee production and a high potential for NO3 - water contamination. In optimum growing conditions for coffee in full sunlight and high fertilization, the inclusion of shade trees (Eucalyptus deglupta or Inga densiflora) increased N accumulation in litter and permanent biomass and slightly reduced water drainage, but also reduced coffee production by 25-33%. In these conditions, the effect of the shade tree on NO3 - leaching varied according to coffee production. Additionally, in a highly fertilized coffee-Inga densiflora association (250 kg N ha-1 y-1), the use of 15N labelled fertilizer showed a low efficiency of N fertilizer for both coffee and tree production. In a full sun coffee plot, where N fertilizer was only provided in coffee pulp (¿150 kg N ha-1 yr-1), coffee production was very low (0.75 t dry matter [DM] ha-1 yr-1). Incorporating a legume tree stratum (Erythrina poeppigiana) increased berry production to 3 t DM ha-1 yr-1 without significant changes in NO3 - leaching. Compared to the highly fertilized agroforestry system (250 kg N ha-1 y-1) previously described, NO3 - leaching in this organically managed agroforestry system was reduced at least by two fold. N2O emissions were always insignificant compared to the losses though NO3 - leaching. However these emissions were increased by high inputs of mineral N fertilizer and to a lesser extent by leguminous shade trees. Coffee producers should be encouraged to adopt environmentally sound N management, reducing N inputs and refining shade management based on a combination of leguminous and timber tree species. Implementing environmentally sound practices for coffee production in Costa Rica should help preserve the ecological image of the country while generating incomes to the farmers. (Résumé d'auteur