Print PageA novel method that precisely measures the soil gas ratio of nitrogen to argon can potentially be used to measure denitrification. The direct measurement of denitrification in unsaturated soils by measuring an increase in N2 gas has been prohibitive because it is difficult to measure a small increase in nitrogen over the large background concentration of N2 gas (78%). Our method allows: (1) a small injection volume (<1 mL), (2) a short time for sample analysis (<5 min), and (3) high precision (~0.05%). We developed a capillary inlet that allows the direct injection of soil gases into a quadrapole mass spectrometer to measure the 14/40 or N+/Ar+ ratio. An increase in this ratio is due to denitrification or abiotic processes. Vadose zone gas and groundwater samples were collected at an agricultural riparian buffer and analyzed for the N+/Ar+ (gas) or N2/Ar (water) ratio, N2O, CH4, CO2, O2 (water), NO3, (water) and NH4 (water).
Results/Conclusions
Positive N+/Ar+ gradients from groundwater towards the soil surface are observed and are used to estimate diffusional N2 fluxes from the soil surface in the range of 0.42-17.25 mmoles N2-N m-2 day-1. The increase in the N+/Ar+ ratio is in the range of 0.09-0.9 %. N2O-N fluxes are in the range of 0-0.035 mmoles m-2 day-1. Negative gradients from groundwater towards the soil surface are also observed and are likely caused by solubility differences in groundwater due to seasonal temperature changes. These fluxes range from 0 to -14.25 mmoles m-2 day-1. Interacting biological and physical processes are likely effecting the N+/Ar+ ratio. Our method is precise enough to measure small significant changes in the N+/Ar+ ratio of soil gases. Further research is needed to determine and account for the influences of different physical soil processes on the N+/Ar+ ratio in order to use this method to measure soil denitrification.
