Impacts of altered rainfall regimes on ecosystem diversity, functioning and stoichiometry in a south-eastern Australian grassland: The DRIGrass project
Climate models predict more extreme rainfall patterns in the future, an increase in the frequency of severe drought and longer periods between rainfall events. In this context, we are studying the impacts of altered rainfall regimes on (1) soil microbial community composition and biomass, (2) nutrient availability and (3) several ecosystem processes, including CO2 fluxes and the activity of several soil enzymes associated with the break-up of soil organic matter in an south-eastern Australian grassland. We are particularly interested in how impacts at the soil level determine indirect changes at the plant community level and also in investigating the role of ecosystem stoichiometry in the ecosystem response to altered precipitation regimes. The mean annual precipitation of the study site is approximately 800mm. Treatments (6 replicates per treatment) include: ambient rainfall, reduced and increased rainfall amount by 50%, reduced number of rainfall events without altering the total amount of rainfall (altered frequency) and seasonal droughts. All treatments involve reapplication of water with automated sprinklers. The altered frequency treatment releases all the rainfall accumulated during a three-week period as a single deluge event, whereas the seasonal drought treatment consists in the complete exclusion of ambient rainfall for a nine-week period during summer.
Early results of the project (i.e., only one year of rainfall manipulation; summer 2014) include higher nitrate availability in response to summer drought conditions, which may be due to reduced nutrient uptake and leaching. In contrast, the concentration of exchangeable cations decreased under summer drought conditions, possibly as a consequence of reduced microbial activity and, therefore, organic matter mineralization. Given that base cation availability was the main predictor of plant biomass at the site level, indirect impacts of drought on plant biomass production can be expected via reductions in soil cation availability. Taken together, these responses suggest a potentially high sensitivity of Australian grassland communities to altered patterns of rainfall, particularly to seasonal drought. However, more results (expected by June 2015) are needed in order to better understand the implications of climate change in the ability of Australian grasslands to maintain the rate of supply of those ecosystem services that they currently provide (including carbon sequestration and pasture).