Low nitrogen use efficiency and high nitrate leaching in a highly fertilized Coffea arabica-Inga densiflora agroforestry system: a 15N labeled fertilizer study

In intensive cultivation of coffee (Coffea arabica L.), large N fertilizer inputs are thought to increase nitrate (NO3 (-)) water contamination and greenhouse gas emissions. This study was carried out (1) to evaluate the nitrogen use efficiency of a highly fertilized C. arabica-Inga densiflora agroforestry system on an Andosol and (2) to determine the control mechanisms of N fluxes and losses. Nitrogen pools and fluxes were monitored for one cropping season in a coffee plantation (density 4,722 plants ha(-1), height 2.1 m), shaded by regularly pruned leguminous trees (density 278 trees ha(-1); height 8 m), in the Central Valley of Costa Rica. The fate of N fertilizer (250 kg N ha(-1) year(-1)) was traced by adding N-15-urea at 1.61 kg N-15 ha(-1). The labeled urea was rapidly nitrified or immobilized in soil organic matter with 20.8 % recovered in organic form at the end of the cropping season in the top 2 m of the soil. There was high net N mineralization and nitrification in the top soil (a parts per thousand 200 kg N ha(-1) year(-1) in 0-10 cm) and up to 257 kg NO3 (-)-N ha(-1) were found in the top 2 m of the soil. Only 25.2 % (63 kg N ha(-1)) of the applied fertilizer (N-15 recovery) was taken up by the two plant species (13.5 % in the coffee plants, 9.6 % in the shade trees and 2.1 % in the litter). Total N export in the coffee fruit harvest accounted for 110 kg N ha(-1) but only 17.6 kg N ha(-1) came from the applied fertilizer (7 % of N-15 recovery). During this year of high coffee production, the coffee plant acquired most of its N from mineralized soil N rather than from N fertilizer. High fertilization resulted in a low rate of N-2 fixation by I. densiflora, estimated at 22.7 kg N ha(-1) year(-1) (Ndfa of 16.1 %). As a result of high water drainage (1,745 mm for a total rainfall of 2,678 mm), the main fate of N fertilizer was NO3 (-) leaching (33-55 % of N-15 recovery). The annual NO3 (-)-N leaching at a depth of 120 cm was 157.2 kg N ha(-1) year(-1)(including 82.8 from applied N) and the N2O-N emission was 5.8 kg N ha(-1) year(-1). These results clearly showed that the system was N saturated, leading to low use efficiency of the N fertilizer and significant losses of N, principally through NO3 (-) leaching. This study provided an insight on how to reduce the negative environmental impact of N fertilization in intensive coffee cultivation and increase N use efficiency.