Tuesday, July 26, 2011

Van Groenigen et al., 2005

Van Groenigen JW, Zwart KB, Harris D, van Kessel C. 2005. Vertical gradients of δ15N and δ18O in soil atmospheric N2O – temporal dynamics in a sandy soil. Rapid Communications in Mass Spectrometry 19: 1289-1295.

These authors examined the production and consumption of N2O in soil profiles using stable-isotope analysis. Enzymatic processes in the microbial nitrification and denitrification pathways all strongly fractionate isotopes of both N and O, with products of reactions significantly depleted in the heavier isotopes. This allows identification of regions of soil producing or consuming N2O by distinguishing between local production and diffusion through the soil layer.

Similar to my own studies, gas concentrations were measured using diffusion wells, in this case probes buried in the soil and sampled by syringe; gas samples were measured by GC-IRMS. Internationally-recognized isotope samples for N2O are not available, so standards for analysis were prepared from commonly available laboratory chemicals and abiotic chemical reactions to completion to avoid fractionation (e.g. reduction of N2O to N2 over copper at 600ºC).

The largest concentrations of N2O were found at the deepest sampling position, 90cm. There was a negative logarithmic relationship between the δ15N value of N2O in the soil and depth, consistent with a relatively large single source of N2O at 90 cm and consumption shallower in the soil combined with vertical-upwards diffusion of N2O; reduction of N2O to N2 enriches the remaining N2O pool for 15N.