Effects of an experimental drought on soil emissions of carbon dioxide, methane, nitrous oxide, and nitric oxide in a moist tropical forest

Changes in precipitation in the Amazon Basin resulting from regional deforestation, global warming, and El Niño events may affect emissions of carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O), and nitric oxide (NO) from soils. Changes in soil emissions of radiatively important gases could have feedback implications for regional and global climates. Here we report results of a large‐scale (1 ha) throughfall exclusion experiment conducted in a mature evergreen forest near Santarém, Brazil. The exclusion manipulation lowered annual N₂O emissions by >40% and increased rates of consumption of atmospheric CH₄ by a factor of >4. No treatment effect has yet been detected for NO and CO₂ fluxes. The responses of these microbial processes after three rainy seasons of the exclusion treatment are characteristic of a direct effect of soil aeration on denitrification, methanogenesis, and methanotrophy. An anticipated second phase response, in which drought‐induced plant mortality is followed by increased mineralization of C and N substrates from dead fine roots and by increased foraging of termites on dead coarse roots, has not yet been detected. Analyses of depth profiles of N₂O and CO₂ concentrations with a diffusivity model revealed that the top 25 cm soil is the site of most of the wet season production of N₂O, whereas significant CO₂ production occurs down to 100 cm in both seasons, and small production of CO₂ occurs to at least 1100 cm depth. The diffusivity‐based estimates of CO₂ production as a function of depth were strongly correlated with fine root biomass, indicating that trends in belowground C allocation may be inferred from monitoring and modeling profiles of H₂O and CO₂.