Seasonal effects of altered precipitation regimes on ecosystem-level CO2 fluxes and their drivers in a grassland from Eastern Australia

We evaluated the impacts of altered precipitation regimes on multiple aspects of the C cycle, including C fluxes, plant and soil microbial communities, and plant-soil interactions in a south-eastern Australian grassland. Our experimental treatments, operated through an automated system, included: (i) reduced and (ii) increased rainfall amount by 50%, (iii) reduced rainfall frequency but no change in amount (i.e., more extreme downpours), (iv) and an extreme summer drought. Temporal dynamics of ecosystem-level CO2 fluxes fluctuated seasonally and were driven by variations in soil water availability, soil temperature and photosynthetically active radiation. Reducing the frequency of rainfall events, but without change in the amount of rainfall, resulted in lower ecosystem-level net CO2 uptake due to relatively greater Reco stimulation after the heavy downpours, particularly during the late summer season. The extreme summer drought downregulated both respiration and photosynthesis. Microbial abundance and activity did not change in response to rainfall manipulation and were not strongly related to precipitation-driven changes in C cycling. In contrast, a greater proportion of live to dead plant biomass, in turn driven by greater water availability, was a main driver of greater respiration and photosynthesis. Our study suggests that grasslands could shift from net C sinks to C neutral or even net sources of C under future scenarios of more variable rainfall regimes, thus reinforcing climate change.