Patchiness in Microbial Nitrogen Transformations in Groundwater in a Riparian Forest

We measured microbial N transformations in 15 cm diam. by 40 cm intact horizontal sections of aquifer material (mesocosms), taken from a riparian forest in Rhode Island, USA, incubated under ambient conditions. The mesocosms allowed us to measure these transformations on the same scale as hydrologic tracer methods (Br − /NO − 3 ratios) that measure net NO − 3 removal. Our objective was to reconcile discrepancies between hydrologic tracer and microbial measurements in previous studies where laboratory‐based microbial NO − 3 consumption measurements were much lower than in situ hydrologic measurements of net NO − 3 removal. We hypothesized that small “patches” of organic matter in the aquifer matrix, which are easily missed when sampling for microbial measurements, are “hotspots” of NO − 3 removal and are responsible for these discrepancies. Mesocosms were subjected to three treatments [Br − only, Br − + 15 NO − 3 Br − + 15 NO − 3 + dissolved organic carbon (DOC)]. Solution (NH + 4 , NO − 3 , dissolved organic N) and gaseous (N 2 O, 15 N 2 O, and 15 N 2 ) inputs and outputs to the mesocosms were measured over a 132‐d incubation, followed by destructive sampling for the presence of patches and residual 15 N in aquifer matrix and patch material. Total (gross) NO − 3 consumption by denitrification and immobilization was greater than net removal of NO − 3 measured by Br − /NO − 3 ratios. Net NO − 3 consumption was only observed in mesocosms that contained “patches” of organic matter and was not increased by addition of DOC, suggesting that these patches, which represent <1% of aquifer weight, are critical to groundwater NO − 3 removal in riparian forests.