Natural Dynamics and Control of Soil Gas Oxygen in Wetlands: Fibre-Optic Oxygen Measurements at Two Field Sites in the Doñana National Park (Spain)

Oxygen measurements of soil air were carried out to determine daily variations and seasonality of oxygen in soil air and their dependence on ambient parameters such as groundwater chemistry, hydrodynamics and physico-chemical sediment characteristics. Two field sites in the Donana National Park were equipped with fibre-optic oxygen- and temperature sensors down to 6 m below the groundsurface, and oxygen was sampled from November 2010 until July 2012. One of the sites lies in the saltmarshes and the other in the dune belt, so providing coverage of contrasting hydrogeochemical conditions. Sediments from both locations were analyzed for physico-chemical parameters. Furthermore, groundwater levels were measured and samples taken for hydrochemical analysis. Soil gas in the salt marsh ranged from anaerobic conditions during the wet season to aerobic conditions in the dry period, which is related to the anaerobic groundwater and the high amount of electron donors in the sediment. The location in the dune belt yielded aerobic conditions throughout the year due to the minor amount of organic carbon in the sediments and the high oxygen content of the groundwater. The oxygen time series exhibits daily oscillations related to microbiological activity in the root zone of the Juniperus Sabina tree.