Denitrification Nitrification abundance KEY WORDSNitrogen isotope fractionation Nitrogen-15 natural SUMMARY A few principles relative to the presentation and use of nitrogen stable isotopic data are briefly reviewed. Some classical relationships between the isotope composition of a substrate undergoing a single-step unidirectional reaction, are introduced.They are illustrated through controlled experiments on denitrification in a soil, and through nitrification by pure cultures of Nitrosomonas europaea. In the latter case, the isotope fractionation is calculated from the isotopic composition of the residual substrate, then of the product and the result is shown to be statistically the same for the two procedures.The isotopic enrichment factor for denitrification is -29.4 + 2.49'0o at 20~ and -24.6 + 0.9%o at 30~ for nitrification this factor is -34.7 • 2.5%0 under the experimental conditions employed.
It is commonly predicted that the intensity of primary production and soil carbon (C) content are positively linked. Paradoxically, many long-term field observations show that although plant litter is incorporated to soil in large quantities, soil C content does not necessarily increase. These results suggest that a negative relationship between C input and soil C conservation exists. Here, we demonstrate in controlled conditions that the supply of fresh C may accelerate the decomposition of soil C and induce a negative C balance. We show that soil C losses increase when soil microbes are nutrient limited. Results highlight the need for a better understanding of microbial mechanisms involved in the complex relationship between C input and soil C sequestration. We conclude that energy available to soil microbes and microbial competition are important determinants of soil C decomposition.
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