Computer Science
Scientific paper
Apr 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003eaeja....14751p&link_type=abstract
EGS - AGU - EUG Joint Assembly, Abstracts from the meeting held in Nice, France, 6 - 11 April 2003, abstract #14751
Computer Science
Scientific paper
Tropical rain forest soils are the largest natural source of N2O to the atmosphere. Uncertainty in the signature of this source limits the utility of isotopes in constraining the global N2O budget. Differentiating the relative contribution of nitrification and denitrification to the emitted N2O using stable isotopes has been difficult due to the lack of enrichment factors values for each process measured in situ. We have devised a method for measuring enrichment factors using soil incubation experiments. We selected three Amazon rain forest soils: (1) Clay and (2) Sandy from Santarem, Pará State, and (3) Sandy from Nova Vida Farm, Rondonia State, Brazil. The enrichment factor values for nitrification and denitrification are: -97.8±4.2 and -9.9±3.8 per mil for clay Santarem soil, -86.8±4.3 and -45.2±4.5 per mil for sandy Santarem soil and-112.6±3.8 and -10.4±3.5 per mil for Nova Vida Farm soils, respectively. Our results show that enrichment factors for both processes differ with soil texture and location. The enrichment factors for nitrification are significantly smaller than the range reported in the literature (-66 to -42 per mil). Also, the enrichment factors for the Santarem soils (clay and sandy) differ significantly implying that soil texture (which will affect the soil air filled pore space at a given water content) is influencing the bacteria isotopic discrimination. However, the enrichment factors for the Santarem clay sand Nova Vida sandy soils do not differ by much. This suggests that the enrichment factors not only can be affected by texture but also by the microbial fauna present in these soils. We also determined the measurement of the N2O positional dependence. N2O is a linear molecule with two nitrogen atoms. The 15N isotope can be located in either the central nitrogen (alpha position) or in the terminal nitrogen (beta position). The isotopomer site preference (15N alpha - 15N beta) can be used to differentiate processes of production and consumption of N2O as a potential method to determine the contributions of nitrification and denitrification. We measured the isotopomer composition of the incubated soils and calculated the site preference of each process for each soils. The site preference for nitrification and denitrification are: -114.5 and 56.6 per mil for clay Santarem soil, -75.2 and 11.8 per mil for sandy Santarem soil and -209.7 and 28.8 per mil for Nova Vida Farm soils, respectively. To our knowledge these are the first N2O isotopomer characterizations for nitrification and denitrification in soils. The results show that nitrifying bacteria population has 15N site preference fingerprints smaller by up to 200 per mil than denitrifying bacteria. This data set strongly suggests that N2O isotopomers can be used in concert with traditional N2O stable isotope measurements as constraints to differentiate microbial processes producing N2O. We can conclude that nitrifiers produce N2O with a smaller site preference values and more negative del 15N beta than do denitrifiers. These results show a new proxy to differentiate N2O formation processes in soil and will contribute to produce interpretations of the site preference isotopomeric N2O values found in the troposphere.
Boering K.
Cerri Carlos C.
de Camargo P.
Garcia Daniel
Moreira Manuel
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