Patchiness in microbial nitrogen transformations in groundwater in a riparian forest

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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 were 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- + 15NO3/-, Br- + 15NO3/- + dissolved organic carbon (DOC)]. Solution (NH4/-, NO3/-, dissolved organic N) and gaseous (N2O, 15N2O, and 15N2) inputs and outputs to the mesocosms were measured over a 132-d incubation, followed by destructive sampling for the presence of patches and residual 15N 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 DOC, suggesting that these patches, which represent <1% of aquifer weight, are critical to groundwater NO3/- removal in riparian forests.

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Journal of Environmental Quality