Nitrogen dynamics and nitrate water contamination in a Coffea arabica - Eucalyptus deglupta agroforestry system in Southern Costa Rica

The study was carried out from April 2002 until March 2003, in an Ultisol in the low wet Pacific southern zone of Costa Rica (600 masl and 2700 mm annual rainfall). We evaluated the effect of Eucalyptus deglupta, introduced as a shade timber tree in Coffea arabica plantations, on N loss from the system through leaching, run-off, emissions of N2O and coffee harvest. Coffee plots in full sun and under E. deglupta shade received 180 kg N ha-1 as chemical fertilizer. Mineral N losses in surface runoff and N2O emissions were very low (less than 5 kg N ha-1 per year). N export in coffee harvest were 34 and 25 kg ha-1 in full sun and shaded coffee, respectively. Lysimeters located at 0.6, 1.2 and 2 m below the soil surface in full sun coffee, detected mean NO3--N concentrations of 16.5, 9.2 and 2.3 mgl-1 respectively. For the same depths in shaded coffee, mean NO3--N concentrations were 12.6, 6.1 and 1.2 mgl-1 respectively. NO3--N concentrations in soil solution at 2 m soil depth and in spring water were close. Nitrate lixiviation estimated at 2m depth, was about 22 and 12 kg N ha-1 in full sun and shaded coffee, respectively. The strong decrease in NO3--N concentrations with depth and the low N leaching were mainly attributed to nitrate adsorption in soil layers below 80 cm depth. This process might be more common than formerly thought. Nevertheless, the sustainability of this process is not presently known. (Résumé d'auteur