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Effects of climate warming and altered biodiversity on the carbon, water and nitrogen balance of grasslands under drought conditions

Final Activity Report Summary - GRASSLAND (Effects of climate warming and altered biodiversity on the carbon, water and nitrogen balance of grasslands under drought conditions)

Global warming accelerates land surface drying, increasing the incidence of extreme climatic events such as severe droughts with detrimental effects on ecosystem functioning and structure. Within this project we investigated the effects of an imposed extreme drought (24 days) on fully established synthesized grassland communities with three species richness (S) levels (one, three or nine species), grown for 3 years at either ambient air temperatures (unheated) or ambient +3 degrees Celsius (heated). More in detail, we evaluated the effects of an imposed drought period on grassland ecophysiological parameters, percentage of green vegetation cover and biomass production.

Since water supply during these three years was equal in all treatments, heated communities experienced more frequent, short mild droughts, but it was unknown whether this conferred greater resistance for facing prolonged droughts. During the 24-day drought period, soil water availability decreased in a similar way over time in both temperature treatments and was lower at increasing S-levels. Heated communities showed higher stress levels than those at ambient from the beginning of the imposed drought, as indicated by their lower photosynthetic rate, stomatal conductance, and transpiration.

Sensitivity of stomata during the imposed drought was similar in both temperature treatments indicating the absence of stomatal acclimation to lower soil water contents in heated conditions. Although green vegetation cover was significantly reduced during drought in both temperature treatments, the decrease was higher in heated than in unheated communities indicating a lower resistance of heated communities to the drought.

Only 13 days after the water was re-applied to the ecosystems, green vegetation cover of both temperature treatments approached values similar to those observed before the imposed drought, suggesting similar resilience in both treatments. Above-ground biomass was reduced by elevated temperature, consistently in all species richness levels, showing that the drought period did not change the biomass production patterns observed before the imposed drought.

Our results suggest that, regardless of the continuous exposure to elevated temperatures and associated mild droughts, heated communities had not developed clear mechanisms to better cope with extended summer droughts.

In the context of global changes and their impacts on biodiversity and ecosystem functioning, arbuscular mycorrhizal fungi (AMF) and plant communities are known to (i) influence each other, (ii) be involved in several ecosystem processes, including C and N cycle, and (iii) be affected by climate changes. We evaluated the impacts of elevated CO2, temperature on AMF and plant biodiversity, AMF-plant associations, C and N cycle. Grassland communities composed of six plant species were exposed to ambient temperature (Tair) and atmospheric CO2 concentration (380 ppm), while the other half were continuously warmed at 3C above Tair and exposed to elevated CO2 concentration (610 ppm). Communities were planted in soil that was either (i) pasteurised, (ii) pasteurised and subsequently inoculated with AMF.

Ecosystem above-ground biomass significantly increased from July until September, similarly in the two climate scenarios in pasteurised and AMF soil treatments. Species biomass differed significantly and species such as Medicago lupulina and Poa pratensis performed better in presence of AMF, confirming the important role of AMF in community composition. Analysis of root biomass indicated no effect of the climate scenario for pasteurised and AMF. In all the soil treatments, root biomass was higher in the 0-9 cm layer and decreased significantly with soil depth. Higher root biomass was observed in the pasteurised compared to the AMF soil in September. By extending in the soil, AMF allow roots to acquire nutrients (i.e. N and P, particularly) by exploring more soil with their extra-radical mycelium, and their presence may lead to less investment in root length and therefore less root biomass.