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 15N-urea at 1.61 kg 15N 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 (&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 (15N 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 15N 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 at 22.7 kg N ha-1 year-1 (Ndfa of 16.1 %). As a result of high water drainage (1,745 mmfor a total rainfall of 2,678 mm), the main fate of N fertilizer was NO3 - leaching (33-55 % of 15N 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.