Topography and Tree Species Improve Estimates of Spatial Variation in Soil Greenhouse Gas Fluxes in a Subtropical Forest

Subtropical and tropical forests account for over 50% of soil CO2 production, 47% of N2O fluxes of natural ecosystems, and act as both significant sources and sinks of atmospheric CH4. However, ecosystem-scale estimates of these fluxes typically do not account for uncertainty that arises from environmental heterogeneity over small spatial scales. To assess the effects of small-scale environmental heterogeneity on GHG fluxes in a tropical forest ecosystem, we measured fluxes of CO2, CH4, and N2O across a topographic gradient and at the base of different tree species. We then used Bayesian linear models together with maps of trees and topography to quantify spatial heterogeneity in ecosystem-scale estimates of GHG emissions. The relationship between GHG fluxes and species and topography varied for each gas type. CO2 varied strongly by species but was only weakly related to topographic variation. In contrast, CH4 and N2O, which are more strongly regulated by soil oxygen, had strong relationships with topography but did not vary across species. Assuming spatial homogeneity and average rainfall conditions, we estimated ecosystem soil CO2 emissions to be 28.91 kg CO2-C/ha/day, net CH4 consumption of − 5.15 g CH4-C/ha/day, and net N2O emissions of 1.78 g N2O-N/ha/day. Including variation caused by tree species decreased ecosystem-level estimates of CO2 emissions by 8.03%, whereas including topographic variation decreased net CH4 consumption by 12.98% and increased net N2O emissions by 1.05%. This translates to a net decrease of 8.32% in estimated CO2-equivalent emissions. Our findings show that ignoring small-scale environmental heterogeneity has implications for bottom-up estimates of GHG fluxes in tropical forests. Given the increasing availability of fine-scale topographic models, incorporating this source of variation in estimates of ecosystem soil GHG emissions could improve our understanding of the role tropical forests play in global GHG cycles.

Andrew W. Quebbeman, Duncan N.L. Menge, Jess Zimmerman, & María Uriarte