MEASUREMENT OF NITROUS-OXIDE AND DI-NITROGEN EMISSIONS FROM AGRICULTURAL SOILS

Nitrous oxide can be produced during nitrification, denitrification, d issimilatory reduction of NO3- to NH4+ and chemo-denitrification. Sinc e 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 o ne pool before being reduced to N-2 by denitrification. Reliable metho ds are needed for measuring the fluxes of N2O and N-2 simultaneously f rom agricultural soils. The C2H2 inhibition and N-15 gas-flux methods are suitable for use in undisturbed soils in the field. The main disad vantage of C2H2 is that as well as blocking N2O reductase, it also blo cks nitrification and dissimilatory reduction of NO3- to NH4+. Potenti ally the N-15 gas-flux method can give reliable measurements of the fl uxes of N2O and N-2 when all N transformation processes proceed natura lly. The analysis of N-15 in N-2 and N2O is now fully automated by con tinuous-flow isotope-ratio mass spectrometry for 12-ml gas samples con tained in septum-capped vials. Depending on the methodology, the limit of detection ranges from 4 to Il g N ha(-1)day(-1) for N-2 and 4 to 1 5 g N ha(-1)day(-1) for N2O. By measuring the 15N content and distribu tion of N-15 atoms in the N2O molecules, information can also be obtai ned to help diagnose the sources of N2O and the processes producing it . Only a limited number of field studies have been done using the N-15 gas-flux method on agricultural soils. The measured flux rates and mo le fractions of N2O have been highly variable. In rainfed agricultural soils, soil temperature and water-filled pore space change with the w eather and so are difficult to modify. Soil organic C, NO3- and pH sho uld be amenable to more control The effect of organic C depends on the degree of anaerobiosis generated as a result of its metabolism. If co nditions for denitrification are not limiting, split applications of o rganic C will produce more N2O than a single application because of th e time lag in the synthesis of N2O reductase. Increasing the NO3- conc entration above the K-m value for NO3- reductase, or decreasing soil p H from 7 to 5, will have little effect on denitrification rate but wil l increase the mole fraction of N2O. The effect of NO3- concentration on the mole fraction of N2O is enhanced at low pH. Manipulating the in teraction between NO3- supply and soil pH offers the best hope for min imising N2O and N-2 fluxes.