Variations in soil N cycling and trace gas emissions in wet tropical forests

We used a previously described precipitation gradient in a tropical montane ecosystem of Hawai’i to evaluate how changes in mean annual precipitation (MAP) affect the processes resulting in the loss of N via trace gases. We evaluated three Hawaiian forests ranging from 2200 to 4050 mm year⁻¹ MAP with constant temperature, parent material, ecosystem age, and vegetation. In situ fluxes of N₂O and NO, soil inorganic nitrogen pools (NH₄⁺ and NO₃⁻), net nitrification, and net mineralization were quantified four times over 2 years. In addition, we performed ¹⁵N-labeling experiments to partition sources of N₂O between nitrification and denitrification, along with assays of nitrification potential and denitrification enzyme activity (DEA). Mean NO and N₂O emissions were highest at the mesic end of the gradient (8.7±4.6 and 1.1±0.3 ng N cm⁻² h⁻¹, respectively) and total oxidized N emitted decreased with increased MAP. At the wettest site, mean trace gas fluxes were at or below detection limit (≤0.2 ng N cm⁻² h⁻¹). Isotopic labeling showed that with increasing MAP, the source of N₂O changed from predominately nitrification to predominately denitrification. There was an increase in extractible NH₄⁺ and decline in NO₃⁻, while mean net mineralization and nitrification did not change from the mesic to intermediate sites but decreased dramatically at the wettest site. Nitrification potential and DEA were highest at the mesic site and lowest at the wet site. MAP exerts strong control N cycling processes and the magnitude and source of N trace gas flux from soil through soil redox conditions and the supply of electron donors and acceptors.