Browsing by Author "Allard, Vincent"
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- ItemEffects of elevated atmospheric CO2 concentrations on carbon and nitrogen fluxes in a grazed pasture : a thesis presented in partial fulfilment of the requirements for the degree of Ph. D. in Plant Science at Massey University and the degree of Docteur en Sciences, speciality Sciences Agronomiques at the Institut National Polytechnique de Lorraine(Massey University, 2003) Allard, VincentPredicting the response of grazed grasslands to elevated CO2 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 CO2 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 CO2 but none of these experiments were grazed. In this thesis I present data on CO2 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 CO2 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 elevated CO2 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 CO2. Plant litter decomposition rates were, at the ecosystem scale, not affected by elevated CO2 (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 CO2 on soil OM dynamics and concluded that soil OM accumulation under elevated CO2 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 CO2. As most grasslands are grazed, the prediction of grassland responses to elevated CO2 must be derived from systems in which animals are an integral part.