Spatial and Temporal Variability of Soil CO2 and N2O Fluxes in Tropical Forest Soils: the Influence of Tree Species, Precipitation, and Soil Texture

Abstract

The tropics are the largest natural source of CO2 and N2O to the atmosphere and many tropical forests are changing due to climate change and forest fragmentation. To understand how these changes are affecting ecosystem feedbacks to climate change we need to investigate the plant-soil interactions in tropical forests on both local and regional scale. Locally, tree species do influence soil N2O fluxes and biogeochemistry in diverse forests. I found N2O fluxes close to four out of fifteen tree species consistently elevated above the overall mean and consistently low near two other species. However, only large (20%) changes in species composition would cause an appreciable effect on the landscape-scale forest flux. Experimental sugar additions elevated, whereas root abscission by permanently installed chambers reduced N2O fluxes, suggesting that N2O cycling is mostly driven by heterotrophic (carbon-limited) denitrifiers, and that carbon transport into the soil by trees may present a mechanism for the observed differences. To isolate species effects on soil processes, I investigated tropical monoculture plantation soil gas fluxes. As in the natural forest, flux differences were associated with tree species identity, though species N2O flux rank order on the plantation was unlike the forest, indicating that monoculture plantation plots are not generally informative for species influence on soil processes in diverse forests. Fast tree growth rates and overall lower fluxes of CO2 and N2O from plantation vs. natural forest or agricultural soils suggest that plantations initiated on abandoned farmland may reduce tropical ecosystem greenhouse gas fluxes. Motivated by the finding that tree carbon export may influence soil N2O fluxes, I investigated the effect of stand level growth rates on regional N2O flux variation and found that spatial and temporal N2O flux variability in the Amazon basin is significantly and positively correlated with forest growth rates. Extrapolating this correlation across the Amazon basin yields a mean soil N2O flux of 2.6 kg-N ha-1 -1 , higher than previously estimated. My work, through accounting for the processes that link vegetation carbon dynamics to soil biogeochemistry, indicates that tropical forests may be a bigger contributor to global N2O budget than previously thought.