Legacy effects of multiyear summer drought on soil CO2 production, transport and efflux in a sub-alpine grassland

Climate change is expected to lead to an increase in frequency and severity of extreme climatic events like summer drought. Drought and rewetting have strong impacts on soil respiration, which constitutes the largest flux of CO₂ from terrestrial ecosystems to the atmosphere. However, little is known about the role of biotic and abiotic factors in driving CO₂ production and transport across the soil profile and how these processes are affected by repeated drought events. Soil CO₂ transport can be assessed using the flux-gradient approach, a method which assumes that diffusion is the only transport mechanism for CO₂ through soil, with diffusion rates primarily dependent on air-filled pore space. It is therefore generally assumed that the calculated soil CO₂ concentration gradient translates directly into soil CO₂ efflux, however, a discrepancy between measured soil CO₂ efflux and modeled soil CO₂ concentration gradients can indicate presence of non-diffusive transport mechanisms.

In a multiyear drought and rewetting experiment at a mountain meadow in the Austrian Alps, we compared soil CO₂ production, transport and efflux for plots which were exposed to two and twelve subsequent years of experimental summer drought, respectively, versus plots with ambient precipitation and soil moisture. We measured soil respiration using automated chambers and assessed the production and transport of CO₂ using the flux-gradient approach on data obtained with solid-state sensors in three soil depths through the soil profile. We tested the hypothesis that drought-driven reduction in soil respiration will be more intense for the 12-year drought treatment, but the CO₂ pulse induced by rewetting will be higher. We furthermore expected that non-diffusive transport mechanisms would play a crucial role during drought and would be more pronounced in the 12-year drought treatment compared to the 2-year drought treatment. Data analysis is currently in progress, the findings will be presented at the conference.