Variations of the natural isotopic composition (15N) of mineral nitrogen from calcareous soils, studied during incubation experiment and on the field

This study was intended to follow the variations of isotope composition of mineral nitrogen formed during incubation of calcareous soils (rendzine), humidity and temperature conditions approximating natural ones. We compared the isotope composition with that of mineral nitrogen formed in a lysimeter cut out from a natural soil with the same pedological features. During the incubation experiments, two steps were recognized. A step with production of nitrates and nitrites depleted in 15N and a second step where the isotope composition leans towards an equilibrium value. During the first step, the 15N depletion for the sum NO3 + NO2 correlates with a 15N enrichment for ammonium. On the whole, the total mineral nitrogen isotope composition is approximately constant. This first step corresponds also to a great speed of nitrification, to the presence of nitrite and to a large evolution (production or use) of ammonium. On the contrary, nitrites have disappeared and ammonium is in dynamic equilibrium (constant concentration) when the delta 15N of nitrates is stabilized. In another set of experiments, with the same conditions, small quantity of a nitrogen substrate (vegetal proteins or amino acids) easily mineralizable, was added to the soil: the 15N depletion of NO3 + NO2 formed during the first step is much greater than with the mere soil. It is concluded that the initial step corresponds to the fast mineralization of a very labile organic component which could be, for the mere soil, the microbial biomass destroyed during air-drying of sample before incubations: this could correspond to the 'flush effect'. On the contrary, the step when nitrates becomes constant could correspond to the mineralization (slower and more regular) of an another organic pool, perhaps part of humified organic matter