Patchiness in Microbial Nitrogen Transformations in Groundwater in a Riparian Forest

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~-ratios) that measure net NO~-removal. Our objective was to reconcile discrepancies between hydrologic tracer and microbial measurements in previous studies where laboratory-based microbial NO~-consumption measurements were much lower than in situ hydrologic measurements of net NO~-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~-removal and are responsible for these discrepancies. Mesocosms were subjected to three treatments lBr-only, Br-+ ~sNO~, Br-+ uNO~-+ dissolved organic carbon (DOC)]. Solution (NI~, NO~-, dissolved organic and gaseous (N20, ~SN20, and UN2) inputs and outputs to the mesocosms were measured over a 132-d incubation, followed by destructive sampling for the presence of patches and residual lSN in aquifer matrix and patch material. Total (gross) NO~ consumption by denitrification and immobilization was greater than net removal of NO~-measured by Br-/NO~ ratios. Net NO~-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~removal in riparian forests.

Spatio-temporal analysis of factors controlling nitrate dynamics and potential denitrification hot spots and hot moments in groundwater of an alluvial floodplain

“…Therefore, OC dynamics can be interpreted as a key factor explaining spatio-temporal variations in DEA and OC sources need to be thoroughly investigated. Indeed carbon supply is often considered as the major limiting factor for denitrification (Starr and Gillham, 1993;Jacinthe et al, 1998;Devito et al, 2000;Rivett et al, 2008;Zarnetske et al, 2011;Peter et al, 2012). However, bacterial richness, which was the only variable describing the bacterial community, was not related to DEA in this study.…”

Denitrification and indirect N2O emissions in groundwater: Hydrologic and biogeochemical influences

“…We observed N 2 O-N production at a wide range of DO suggesting that N 2 O-N production in comparatively aerobic conditions in groundwater might take place in microsites. Aerobic denitrification in soils (~80% air saturation) was reported by Carter et al (1995), but in groundwater denitrification actually is more likely to occur under locally anaerobic conditions within microsites in particulate organic matter (Hammersley and Howes, 2002), heterogeneous organic rich patches of sediments (Jacinthe et al, 1998) or biofilms (Seiler and Vomberg, 2005). Denitrification may cease with the formation of NOx where oxygen levels are more intermediate or variable (Brady and Weil, 2002) (Fig 1 and 2).…”

Transport and Fate of Nitrate in a Glacial Outwash Aquifer in Relation to Ground Water Age, Land Use Practices, and Redox Processes

“…When ground waters of increasing age are analyzed, the older waters will appear to have less NO 3 -simply because they received less at the time of recharge. Also, conditions favoring denitrification can be patchy in some environments (Addy et al, 1999;Gold et al, 1998;Jacinthe et al, 1998) …”