Measurement of nitrous oxide and di-nitrogen emissions from agricultural soils

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Nitrous oxide can be produced during nitrification, denitrification, dissimilatory reduction of NO3− to NH4+ and chemo-denitrification. Since soils are a mosaic of aerobic and anaerobic zones, it is likely that multiple processes are contributing simultaneously to N2O production in a soil profile. The N2O produced by all processes may mix to form one pool before being reduced to N2 by denitrification. Reliable methods are needed for measuring the fluxes of N2O and N2 simultaneously from agricultural soils. The C2H2 inhibition and 15N gas-flux methods are suitable for use in undisturbed soils in the field. The main disadvantage of C2H2 is that as well as blocking N2O reductase, it also blocks nitrification and dissimilatory reduction of NO3− to NH4+. Potentially the15 N gas-flux method can give reliable measurements of the fluxes of N2O and N2 when all N transformation processes proceed naturally. The analysis of 15N in N2 and N2O is now fully automated by continuous-flow isotope-ratio mass spectrometry for 12-ml gas samples contained in septum-capped vials. Depending on the methodology, the limit of detection ranges from 4 to 11 g N ha−1day−1 for N2 and 4 to 15 g N ha−1day−1 for N2O. By measuring the 15N content and distribution of 15N atoms in the N2O molecules, information can also be obtained to help diagnose the sources of N2O and the processes producing it. Only a limited number of field studies have been done using the 15N gas-flux method on agricultural soils. The measured flux rates and mole fractions of N2O have been highly variable. In rain-fed agricultural soils, soil temperature and water-filled pore space change with the weather and so are difficult to modify. Soil organic C, NO3− and pH should be amenable to more control. The effect of organic C depends on the degree of anaerobiosis generated as a result of its metabolism. If conditions for denitrification are not limiting, split applications of organic C will produce more N2O than a single application because of the time lag in the synthesis of N2O reductase. Increasing the NO3− concentration above the Km value for NO3− reductase, or decreasing soil pH from 7 to 5, will have little effect on denitrification rate but will increase the mole fraction of N2O. The effect of NO3− concentration on the mole fraction of N2O is enhanced at low pH. Manipulating the interaction between NO3− supply and soil pH offers the best hope for minimising N2O and N2 fluxes.

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