Tropentag, October 5 - 7, 2011 in Bonn "Development on the margin"

N-oxides Fluxes, N₂O Sources, and Soil-profile N₂O Concentrations of Tropical Forests after Chronic N Addition

Georg-August-Universität Göttingen, Soil Science of Tropical and Subtropical Ecosystems, Germany

Abstract

N deposition is projected to increase in tropical region and emissions of climate-relevant N-oxide (NO and N₂O) gases are expected to rise. However, few studies quantify long-term impact of increased N availability on these gases and on the processes responsible for their production. We used N addition experiments to achieve N-enriched conditions in contrasting montane (3-4-yr N addition) and lowland (11-12-yr N addition) forests in Panama. Control and N-addition (receiving 125 kg urea-N ha^-1 yr^-1) treatments were represented by four (40 m × 40 m) replicate plots each. We wanted to 1) quantify changes in surface N-oxide fluxes during N addition in tropical montane and lowland forests and 2) assess the contribution of denitrification and nitrification to the surface N₂O fluxes and deduce which process might be dominant at lower depths.

In the montane forest, N-oxide fluxes from N-addition plots were higher than the control. During the two-year measurement period (2008-2009), a two-fold increase in annual N₂O fluxes was observed while annual NO fluxes decreased from the N addition plots. Nitrification contributed ≥60% to the N₂O flux from both treatment plots while ≤40% was attributed to denitrification. In the lowland forest, N-oxide fluxes from N-addition plots were also higher than the control. Annual N₂O and NO fluxes from the N-addition plots remained comparable. Denitrification appeared to be the dominant process producing N₂O in N-addition plots during both dry and wet seasons. In the control plots, nitrification accounted for 70% of the total flux during the wet season. At both sites, soil-profile N₂O concentrations in the N-addition plots were significantly higher than the control, starting at about 40-cm depth. High water-filled pore space (≥80%) at these depths suggests that denitrification might be the dominant process contributing to the measured N₂O concentrations.

Keywords: Chronic N-addition, denitrification and nitrification, N-oxides, tropical forest

Contact Address: Juvia Sueta, Georg-August-Universität Göttingen, Soil Science of Tropical and Subtropical Ecosystems, Buesgenweg 2, 37077 Goettingen, Germany, e-mail: jsuetagwdg.de