Emerging groundwater carries evidence of biogenic N2 production rates and processes in an agricultural watershed
Watersheds usually export much less reactive nitrogen than they receive from net anthropogenic nitrogen inputs (NANI). The remaining NANI may accumulate in groundwater, soil, biomass, or detritus, but these storage pools may eventually saturate. On the other hand, microbial conversion of NANI to N2 effectively removes reactive N from the ecosystem. It is difficult to measure biogenic N2 production at large spatial scales due to its spatial and temporal variability and the large background of N2 in the atmosphere. However, we were able to infer rates and processes of biogenic N2 production by analyzing the groundwater emerging into the streams draining a 4.8 km2 watershed where agriculture was the dominant N source. We sampled emerging groundwater with piezometers inserted to 0.5 m depth mid-channel in the stream bed and measured the rate of groundwater emergence by dilution of Br tracers added to the stream water. We used membrane inlet mass spectrometry to measure N2, oxygen, and argon concentrations in groundwater and estimated biogenic N2 by comparisons to argon concentration.
Concentrations of nitrate and biogenic N2 in emerging groundwater varied markedly along the stream channel: nitrate ranged from near 0 to 1.5 mM and biogenic N2 ranged from 0.05 to 0.59 mM. Large spatial differences often occurred over tens of meters of channel length with no clear relation to changes in the adjacent land cover. Biogenic N2 increased as nitrate decreased but by less than equimolar amounts, suggesting that groundwater flow paths to the stream differed in initial nitrate concentration as well as in biogenic N2 production. Concentrations of dissolved iron, oxygen, nitrate, and biogenic N2 followed expected oxidation-reduction relationships. As oxygen declined, nitrate declined and biogenic N2 increased. Iron concentrations generally remained below 0.01 mM but ranged up to 0.6 mM when nitrate dropped below 0.01. Sulfate concentrations increased with decreases in nitrate, suggesting that nitrate reduction might be coupled to sulfide oxidation in the aquifer. However, the stoichiometry suggested that this coupled process could not account for all the nitrate reduction. Based on comparison to other studies, our estimates of biogenic N2 delivered by emerging groundwater could account for most of the NANI that is not exported from the watershed as reactive N.