Nitrous oxide (N2O) and methane (CH4) are potent greenhouse gases (GHGs) with global warming potentials. Previous catchment studies have shown that topography affects N2O and CH4 fluxes in forest soils. However, when considering ecosystem-level GHG balances, researchers often overlook the spatial heterogeneity of tropical forest soil which is often linked with topography.
In a study published in Science of the Total Environment, researchers from Xishuangbanna Tropical Botanical Garden (XTBG) tried to explore topographic controls on N2O and CH4 fluxes in tropical rainforest catchments. They found pronounced topographic controls on N2O emissions and CH4 uptake.
The researchers measured N2O and CH4 fluxes at three hydrologically connected topographic positions (upper hillslope, footslope, and groundwater discharge zone) in the Xishuangbanna rainforest catchment over 1.5 years.
They found that both the emission rates of N2O and CH4 uptake were much higher at the hillslope (where soils were relatively dry) than the footslope and the groundwater discharge zone, particularly during the wet season. In the dry seasons, the N2O emission was weaker and the CH4 uptake became stronger.
“Our results suggest that soil moisture gradient associated with topography is an important driver for the spatial variabilities observed for N2O and CH4 fluxes,” said Dr. ZHOU Wenjun of XTBG.
Based on the annual fluxes of N2O and CH4, the researchers calculated net global warming potential (GWP). They found that N2O emission contributed significantly larger than CH4 at the Xishuangbanna catchment. The soil N2O emission was largely offset the GWP resulting from CH4 uptake, thus turning the catchment into a net source of radiative forcing based on N2O and CH4 which offsets the CO2 sink of this forest.
“With projected climate change in the tropics, weather extremes may interact with these controls in regulating forest GHG fluxes. Future studies should take these factors into account,” said the researchers.
ZHOU Wenjun Ph.D Key Laboratory of Tropical Forest Ecology, Xishuangbanna Tropical Botanical Garden, Chinese Academy of Sciences, Mengla, Yunnan 666303, China