Scale-dependent linkages between nitrate isotopes and denitrification in surface soils: implications for isotope measurements and models

摘要

Natural abundance nitrate (NO3 (-)) isotopes represent a powerful tool for assessing denitrification, yet the scale and context dependence of relationships between isotopes and denitrification have received little attention, especially in surface soils. We measured the NO3 (-) isotope compositions in soil extractions and lysimeter water from a semi-arid meadow and lawn during snowmelt, along with the denitrification potential, bulk O-2, and a proxy for anaerobic microsites. Denitrification potential varied by three orders of magnitude and the slope of delta O-18/delta N-15 in soil-extracted NO3 (-) from all samples measured 1.04 +/- 0.12 (R (2) = 0.64, p 0.0001), consistent with fractionation from denitrification. However, delta N-15 of extracted NO3 (-) was often lower than bulk soil delta N-15 (by up to 24 aEuro degrees), indicative of fractionation during nitrification that was partially overprinted by denitrification. Mean NO3 (-) isotopes in lysimeter water differed from soil extractions by up to 19 aEuro degrees in delta O-18 and 12 aEuro degrees in delta N-15, indicating distinct biogeochemical processing in relatively mobile water versus soil microsites. This implies that NO3 (-) isotopes in streams, which are predominantly fed by mobile water, do not fully reflect terrestrial soil N cycling. Relationships between potential denitrification and delta N-15 of extracted NO3 (-) showed a strong threshold effect culminating in a null relationship at high denitrification rates. Our observations of (1) competing fractionation from nitrification and denitrification in redox-heterogeneous surface soils, (2) large NO3 (-) isotopic differences between relatively immobile and mobile water pools, (3) and the spatial dependence of delta O-18/delta N-15 relationships suggest caution in using NO3 (-) isotopes to infer site or watershed-scale patterns in denitrification.