The interactive effects of rising atmospheric CO2 and nutrient supply on carbon and water relations of grassland ecosystems

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Prediction of the effects of climate change on grassland requires an understanding of ecosystem responses to rising atmospheric CO2 and feedback responses. This retrospective study of stable carbon and oxygen isotope composition (δ13C and δ18O) in grassland plants investigates the responses of plant carbon and water relations to the recent history of CO2 increase. We will study how nutrient availability influences the regulation of carbon and water exchange of grassland plants under rising atmospheric CO2 and whether this regulation results from changes in photosynthesis (A) or stomatal conductance (gs). Nitrogen limitation constrains the response of assimilation to rising CO2 and could limit the increase of A. C3 plants were shown to increase A and decrease gs under elevated CO2. This influences water use efficiency (WUE). Therefore, we will study the trends in intrinsic and instantaneous WUE of grassland plants during the last century. Instantaneous WUE accounts for changes in atmospheric evaporative demand and thus estimates actual leaf-level WUE. Conversely, intrinsic WUE is a measure of carbon gain per unit water lost under standard evaporative demand. We will analyse δ13C and δ18O of archived hay samples from fertilizer treatments at the Park Grass Experiment in Rothamsted, England, that differ in the amount of applied nitrogen, phosphorus and potassium. This experiment allows study of trends from 1915 to today. To verify the relationship between δ18O of plant organics and regulation of transpiration, we will analyse data from a recently conducted field experiment at Grünschwaige Grassland Research Station. In this 4-year study we sampled plant biomass and water pools influencing δ18O of organics. This work contributes to a better understanding of grassland responses to rising CO2 and will improve vegetation-climate models.