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-/NO3- ratios) that measure net NO3- removal, Our objective was to reconcile discrepancies between hydrologic tracer and microbial measurements in previous studies where laboratory-based microbial NO3- consumption measurements mere much lower than in situ hydrologic measurements of net NO3- 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 NO3- removal and are responsible for these discrepancies. Mesocosms were subjected to three treatments [Br- only, Br- + (NO3-)-N-15, Br- + (NO3-)-N-15 + dissolved organic carbon (DOC)]. Solution (NH4+, NO3-, dissolved organic N) and gaseous (N2O, (N2O)-N-15, and N-15(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 N-15 in aquifer matrix and patch material, Total (gross) NO3- consumption by denitrification and immobilization was greater than net removal of NO3- measured by Br-/NO3- ratios. Net NO3- consumption was only observed in mesocosms that contained "patches" of organic matter and was not increased by addition of DOG, suggesting that these patches, which represent <1% of aquifer weight, are critical to groundwater NO3- removal in riparian forests.