suivi de l'azote dans une hêtraie tempérée avant et après traçage isotopique 15N-NO3 (Morvan, France)

Abstract : In forest ecosystems, nitrogen cycling is complex because it involves many processes closely related to the turnover of microbial metabolites and to plant uptake (Dannenmann et al., 2009). In the Morvan area (France) an experimental site had been dedicated to compare the effect of different trees species on the main biogeochemical cycles. All experimental plots had been equipped to monitor the different nutrient fluxes in the whole soil-plant system. According to nitrogen, previous studies have shown that nitrification was strong in the upper mineral soil in beech, pine and Douglas fir, intermediate in oak and low in Nordmann fir and spruce (Zeller et al., 2007). However, despite a strong mineralization and nitrification in the beech stand, nitrates are not found in the drainage water contrary to those observed in Douglas (Andrianarisoa, 2009). To understand the dynamics of nitrate on this plot, an isotope tracing was performed with 15N (application of 15N-NO3 100 mg / m²). In the upper mineral soil [0-15cm], applied 15N was assimilated immediately by soil microorganisms. One day after tracer application about 45% of 15N had been incorporated into the microbial biomass. But this percentage of applied 15N into the microbial biomass decreased rapidly with time. A week after tracing, microorganisms contain only 15% of the 15N and 7% after two months. In addition, the amount of 15N decreased with depth, indicating that the vertical transfer was slow and that microorganisms acted as efficient competitors for nitrate. The 15N that is not recovered in the microbial biomass can be found in the aerial plant biomass or stabilized as soil organic N in aggregates of different sizes. At least some 15N was probably transferred vertically in the soil profile. Tree uptake of applied 15N was rapid and high, one month after tracing, 40% of 15N was in the leaves and branches of beech trees and this amount remained fairly constant during the whole growing season. Spring uptake of nitrate seems to be an important source for structural compounds of the leaves. Nitrate leaching is low under this species; however 15N data are not yet available. Microorganisms play a major role in the retention and conversion of nitrates (organization process). Much of the nitrates are converted into immobile organic nitrogen (probably amino acids, amino sugars and proteins), which partly explains the poor drainage of nitrates in such a beech soil.
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Master thesis
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Léa Bédel. suivi de l'azote dans une hêtraie tempérée avant et après traçage isotopique 15N-NO3 (Morvan, France). Sciences de la Terre. 2011. ⟨hal-01880569⟩

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