One year of spectroscopic high-frequency measurements of atmospheric CO2, CH4, H2O and δ13C-CO2 at an Australian Savanna site

Abstract

We provide a 1-year dataset of atmospheric surface CO₂, CH₄ and H₂O concentrations and δ¹³C-CO₂ values from an Australian savanna site. These semi-arid ecosystems act as carbon sinks in wet years but the persistence of the sink in dry years is uncertain. The dataset can be used to constrain uncertainties in modelling of greenhouse gas budgets, improve algorithms for satellite measurements and characterize the role of vegetation and soil in modulating atmospheric CO₂ concentrations. We found pronounced seasonal variations in daily mean CO₂ concentrations with an increase (by 5–7 ppmv) after the first rainfall of the wet season in early December with peak concentrations maintained until late January. The CO₂ increase reflected the initiation of rapid microbial respiration from soil and vegetation sources upon initial wetting. As the wet season progressed, daily CO₂ concentrations were variable, but generally decreased back to dry season levels as CO₂ assimilation by photosynthesis increased. Mean daily concentrations of CH₄ increased in the wet season by up to 0.2 ppmv relative to dry season levels as the soil profile became waterlogged after heavy rainfall events. During the dry season there was regular cycling between maximum CO₂/minimum δ¹³C-CO₂ at night and minimum CO₂/maximum δ¹³C-CO₂ during the day. In the wet season diel patterns were less regular in response to variable cloud cover and rainfall. CO₂ isotope data showed that in the wet season, surface CO₂ was predominantly a two-component mixture influenced by C₃ plant assimilation (day) and soil/plant respiration (night), while regional background air from higher altitudes represented an additional CO₂ source in the dry season. Higher wind speeds during the dry season increased vertical mixing compared to the wet season. In addition, night-time advection of high-altitude air during low temperature conditions also promoted mixing in the dry season.