Dissolved organic matter induced denitrification in subsoils and aquifers?

Abstract

Nitrate inputs into aquifers may be attenuated by denitrification. This study aims to evaluate the relevance of dissolved organic matter (DOM) as a substrate for denitrifiers. Seepage water and groundwater were sampled using suction plates, suction cups and multilevel wells and were analyzed for dissolved organic carbon (DOC) and for NO 3 −. DOM was fractionated and subjected to batch incubations. The investigated soils were three well-drained Plaggic Anthrosols, one Gleyic Podzol and one Eutric Gleysol. Concentrations of DOM ranged from 9 to 32 mg C l −1 at 90 cm depth. Fluxes of DOM at this depth were between 60 and 90 kg DOC ha −1 year −1. Concentrations decreased to 4–7 mg C l −1 at depths of 3–5.6 m at the well-drained sites. Corresponding DOC fluxes at these depths were between 10 and 20 kg C ha −1 year −1. No change in DOC concentrations and fluxes with increasing depth was observed at the poorly drained sites. Because DOM fractions did not change along the flowpath, retention by sorption was probably negligible at these sites. Decreasing NO 3 − concentrations, which indicate denitrification, were observed at the poorly drained sites and 9–13 m below the groundwater table. During incubation studies, DOC and NO 3 − concentrations changed little (mean decrease: <1 mg C l −1, 0.9 mg N l −1). We conclude that DOM leached from soils does not contribute significantly to the natural attenuation of NO 3 − leached to aquifers because (a) the zones where DOM concentrations and NO 3 − concentrations decrease are separated spatially and (b) the bioavailability of leached DOM is low.