Effects of elevated atmospheric CO2 concentrations on carbon and nitrogen fluxes in a grazed pasture

Abstract : Predicting the response of grazed grasslands to elevated C02 is of central importance in global change research as grasslands represent 20% of the worlds land area and grassland soils are a major sink for carbon (C). Grasslands responses to elevated C02 are strongly controlled by the availability of other nutrients and nitrogen (N) in particular. There have been many previous studies of N cycling in grasslands exposed to elevated C02 but none of these experiments were grazed. In this thesis I present data on C02 effects on N cycling from an experimental system (FACE: Free Air Carbon dioxide Enrichment) that enabled grazing to be included. The thesis focuses on the effects of elevated C02 on the different processes involved in organic matter (OM) returns from the plant to the soil and the consequences for N availability. In Chapter 1, it was shown that e1evated C02 modified N returns by grazing animals by altering the partitioning of N between faeces and urine creating a potential for enhanced N losses at elevated C02. Plant litter decomposition rates were, at the ecosystem scale, not affected by elevated C02 (Chapter 3), but a marked increase in the organic matter fluxes, from roots, led to an accumulation of coarse OM in the soil (Chapter 4). In Chapter 5, using 14C and 15N labelling, I compared short-term (plant mediated) and long-term (soil mediated) effects of elevated C02 on soil OM dynamics and concluded that soil OM accumulation under elevated C02 was not caused by C or N limitation but probably by the availability of other nutrients. The thesis demonstrates that the inclusion of grazing animals can strongly modify N cycling under elevated C02. As most grasslands are grazed, the prediction of grassland responses to elevated CO2 must be derived from systems in which animais are an integral part.
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Vincent Allard. Effects of elevated atmospheric CO2 concentrations on carbon and nitrogen fluxes in a grazed pasture. Biologie végétale. Institut National Polytechnique de Lorraine, 2004. Français. ⟨NNT : 2003INPL013N⟩. ⟨tel-01749757⟩

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