Impact of chronic nitrogen addition on soil trace gas fluxes in a tropical lowland rainforest

Though human activities have more than doubled the input of nitrogen (N) to terrestrial ecosystems worldwide, effects to tropical ecosystems are still poorly researched. The project NITROF investigates effects of N addition on biogeochemistry and productivity of tropical forests (Panama). The lowland site belongs to a manipulative experiment started in 1998 (four randomized blocks, unfertilized control plots vs. +N-plots where 125 kg urea-N ha^-1 yr^-1 is added in four applications during rainy season). Here we report changes in nitrous oxide (N₂O), carbon dioxide (CO₂), methane (CH₄), and nitric oxide (NO) fluxes and in stable trace gas concentrations in soil air which are measured at six depths in 2.5 meter deep pits. In 2006, N oxide emissions were significantly higher from the +N-plots than the control. The fertilizer-induced gaseous N losses amounted to 13.7 ± 3.1 % of the applied N as NO and 3.7 ± 1.4 % as N₂O. The average fluxes of CO₂ and CH₄ did not differ between the +N-plots and the control but for CH₄ there were pronounced plot specific differences with higher uptake rates in the +N-plots measured in most chambers. The rainy season average content of N₂O-N within one m² and 2.5 m depth was significantly higher in the +N-plots than in the control (1.04 ± 0.36 vs. 0.44 ± 0.14 mg N). There was no difference in CO₂-C content between +N-plots and control, CH₄-C content was significantly lower in the +N-plots at -0.2 m depth.