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Inst. of Ecosystem Studies, Box AB, Millbrook, NY 12545;
Univ. of Rhode Island, Dep. of Natural Resources Science, Kingston, RI 02881;
USDA-ARS, Ft. Collins, CO 80521.
* Corresponding author (capg{at}vm.marist.edu).
ABSTRACT
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– + 15NO–3 Br– + 15NO–3 + dissolved organic carbon (DOC)]. Solution (NH+4, NO–3, 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) 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.
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