The dynamics of N₂O near the groundwater table and the transfer of N₂O into the unsaturated zone

A case study from a sandy aquifer in Germany

authored by

M. Deurer, C. von der Heide, J. Böttcher, W. H.M. Duijnisveld, D. Weymann, R. Well

Abstract

The research area was the Fuhrberger Feld aquifer (FFA) in northern Germany. It is situated about 30km northeast of the city of Hannover and covers about 300km². Six multilevel sampling wells along a representative strip under predominantly arable land along a groundwater flow-line were sampled from the groundwater table down to a depth of 10m below the soil surface. We measured N₂O, CO₂, NO₃^-, SO₄^2-, DOC, pH, redox potentials and O₂ concentrations. N₂O accumulated at four out of six wells close to the groundwater table. About 20% of N₂O that occurred between the groundwater table and 7-8m below it resided in the top 0.4m. An exchange zone for N₂O at the interface between the saturated and the unsaturated zone extended 0.55 ± 0.22m below the groundwater table and acted as a source and sink for N₂O. N₂O below the exchange zone cannot be transferred from the groundwater to the atmosphere. The upward fluxes from the exchange zone into the unsaturated zone at the six wells ranged between 0.0009 and 0.3kg N₂O ha^{- 1} year^{- 1}. The yearly downward fluxes into the exchange zones had about the same order of magnitude as the upward fluxes. The upward and downward fluxes of N₂O at the (fluctuating) water table did cancel out each other, but this does not yet imply, that the N₂O fluxes at the soil surface also cancel out each other. N₂O-N:NO₃-N ratios were highly variable ranging from 0.0002 to 0.0417. A multiple regression for the monthly N₂O amounts in the exchange zone could explain 66% of the yearly variation. The significant variables were NO₃^-, CO₂, pH, and O₂. Therefore, a combination of the land use (NO₃^-), the geochemical boundary conditions (pH) and the type of denitrification reaction (O₂ and CO₂ indicate the importance of a heterotrophic denitrification process) governed the N₂O dynamics in the surface groundwater of the FFA and its transfer into the unsaturated zone.

Organisation(s)

Institute of Soil Science
Section Soil Biophysics

External Organisation(s)

Plant & Food Research New Zealand
Federal Institute for Geosciences and Natural Resources (BGR)
University of Göttingen

Type

Article

Journal

CATENA

Volume

72

Pages

362-373

No. of pages

12

ISSN

0341-8162

Publication date

31.01.2008

Publication status

Published

Peer reviewed

Yes

ASJC Scopus subject areas

Earth-Surface Processes

Sustainable Development Goals

SDG 13 - Climate Action, SDG 15 - Life on Land

Electronic version(s)

https://doi.org/10.1016/j.catena.2007.07.013 (Access: Unknown)