Sensitivity of Terrestrial Ecosystem Respiration to Soil Moisture Under Different Aridity Conditions in Australia

Terrestrial ecosystem respiration (TER) is the second largest CO₂ flux between biosphere and atmosphere after photosynthesis. It is therefore crucial to understand the dynamics and drivers of TER to be able to accurately model the net CO₂ exchange between biosphere and atmosphere under a changing climate. Most studies focus on the temperature dependence of TER. However, precipitation and soil moisture can also have a major impact on TER, especially in arid environments. Disentangling the impacts of temperature and soil water on TER is an important challenge to reduce uncertainties in modelling the carbon cycle and its climate change feedbacks.

Here we use daily nighttime net ecosystem exchange (NEE) data as proxy for TER collected by more than 30 flux tower stations within the OzFlux network over the last 20 years in Australia. These stations cover a broad range of climate conditions enabling us to analyze the dependence of TER on soil moisture under varying aridity conditions. We find that the sensitivity of TER to soil moisture variability is much stronger in semi-arid regions than in arid or humid areas. For the most arid stations, soil respiration is in general limited by the small amount of available litter substrate. Soil respiration fluxes at humid stations, however, are large but show only low or even negative sensitivity to the high soil moisture levels indicating that TER at humid stations is not water-limited. We show that common model approaches assuming a constant TER sensitivity for all soil moisture levels fail in reproducing the observed TER behavior in Australia. Hence a more sophisticated description of TER with respect to its soil moisture dependence is necessary to capture TER dynamics under different climate conditions accurately.