Relationship between groundwater levels and oxygen availability in fen peat soils
Introduction
Low availability of O2 is crucial for the development and persistence of organic soils. Oxygen limitation in wetland soils impedes the microbial mineralization of organic substrates (D’Angelo and Reddy, 1999, Moore and Dalva, 1997). Hydrologically intact peatlands feature water levels near the surface, which maintain permanent anoxic conditions already in few centimeters depth. However, the predicted more frequent and more severe summer droughts in Central Europe (Kovats et al., 2014) may cause marked drops in water levels and hence increase the oxygen availability also in greater soil depths.
As the peat consists of tightly-bound organic carbon, aeration of formerly anoxic peat layers promotes the activity of O2-dependent enzymes which catalyze the oxidative breakdown of phenolic compounds, the phenol oxidases. Hence, the concentration of phenolic compounds decreases upon aeration. This will in turn offset the phenolics’ suppressive effect on the activity of hydrolytic enzymes, which conduct the breakdown of a large variety of organic compounds (Freeman et al., 2001, Freeman et al., 2004). In this way, O2 releases an ‘enzymatic latch’ on peat decomposition, and temporary oxygenation can propel the decomposition of soil organic carbon in peatlands even if the oxygen is no more present (Brouns et al., 2014). Therefore, constrained soil aeration is a crucial requirement for sustained carbon storage in peatlands.
Organic soils distinguish from mineral ones by their low density, heterogeneous pore structure, high water holding capacity and a marked swelling ability. These properties, which are attributable to the particulate organic material, promote the binding of residual water and result in a specific response of gas diffusivity to changes in soil moisture (Grover and Baldock, 2013, Iiyama et al., 2012, Oleszczuk and Brandyk, 2008, Weiss et al., 1998).
Soil oxygen is mainly provided diffusively through the pores in the unsaturated zone. The diffusion of gases in soils depends on the proportion of the air-filled pore space and is negatively related to the water content (Refsgaard et al., 1991). Besides the fraction of water bound within the soil pores, also the unbound water moving freely in the matrix can affect the overall oxygen balance. Surface water which is saturated with O2, e.g. run-off rain water or water from fast-running streams, presents a convective oxygen input to adjacent soil regions. In turn, O2-depleted surface water, e.g. in slowly running drainage ditches within peatlands, may attenuate O2 concentrations and expel oxygen from the soil (Reddy and DeLaune, 2008, pp. 199–200).
Within the ground water level, all soil pores are entirely filled by water. Above the water table extends a nearly saturated zone where the water is held in pores or associated to soil particles. This layer is predominately anoxic and can exhibit variable thickness (Fenton et al., 2006), depending on the hydro-physical properties of the soil matrix. Water retention there is linked to bulk density, porosity and pore structure as well as organic matter content and composition (da Rocha Campos et al., 2011, Iiyama et al., 2012, Walczak and Rovdan, 2002, Weiss et al., 1998).
The O2 saturation in the peat soil layer above the water table does not decrease gradually with depth, but declines sharply from nearly atmospheric to almost anoxic levels within few millimeters of vertical distance (Askaer et al., 2010). This sharp decrease resembles the steep oxygen gradient which is commonly observed in inundated or saturated soils and sediments (Lloyd et al., 1998, Lüdemann et al., 2000, Rahalkar et al., 2009). The thin transition layer between hypoxic and oxic soil zones marks the lower margin of the surface peat layer that is particularly vulnerable to aerobic degradation.
External factors modulate the water retention and the gas exchange characteristics of soils. Plant rooting was shown to enhance soil water retention (Głab and Szewczyk, 2014, Leung et al., 2015), but on the other hand root growth loosens the peat structure and thus enhances gas diffusivity (Cannavo and Michel, 2013). Furthermore, radial oxygen loss from roots of wetland plants (Armstrong et al., 1992, Inoue and Tsuchiya, 2008) can also contribute to better oxygen supply in wetland soils.
Land use can affect the structure and texture of peat soils and hence alter their gas diffusivity. Tillage and drainage promote the disintegration of organic particles and the compaction of the soil (Głab and Szewczyk, 2014, Huang et al., 2006). A higher decomposition state of the peat with a more amorphous structure and smaller pores was shown to raise the capacity for residual moisture at unsaturated conditions but also to decrease the saturated water content (Gnatowski et al., 2010, Grover and Baldock, 2013). Consequently, the interplay of rooting and peat decomposition should modulate the extension of the hypoxic layer above the groundwater table.
The goal of this field study was to clarify the relation between groundwater levels and oxygen availability in fen peat soils under different land use. For this purpose, O2 was measured in 5 cm and 20 cm depth on five different sites within the same peatland over periods of two or three years. It was hypothesized that with a given offset, peat oxygen availability is directly linked to the position of groundwater table. In connection to this, the critical groundwater level for the shift between hypoxic and oxic conditions differs between sites depending on soil properties and land use. This study shall uncover the currently missing link between oxygen availability in peat soils as a function of groundwater levels.
Section snippets
Study sites
The study was conducted in the Pfrunger-Burgweiler Ried (Baden-Württemberg, Germany, 47°54′ N, 9°24′ E, 610 m ASL), a peatland complex of approximately 2600 ha. Rewetting measures in this area had started in 2002. At the time of this study (2013–2015), an area of 1700 ha surrounding the abandoned core zone was under agricultural use, partitioned in zones of extensive and intensive pasture, grassland and cropping (Kapfer et al., 2005). The peatland largely consists of fens with Fibric, Hemic and
Soil properties
All of the examined soil parameters bulk density (BD), organic carbon content (Corg), porosity (POR), water holding capacity (WHC) and ammonium nitrogen (NH4-N) differed significantly between the five study sites (Table 1). Sites I and III showed higher BD than site V. Peat on sites I and II had higher Corg than on all the other sites. Soil porosity was lowest for site I, intermediate for sites II, III and IV, and highest for restored site V. In contrast, site I had significantly lower WHC than
Dynamics of groundwater levels and O2 saturation
The studied peat soils exhibited mostly either hypoxic or nearly atmospheric O2 levels (Fig. 2). This finding demonstrates a near-binary distribution of soil oxygen availability in wetland soils and a rare occurrence of intermediate levels of oxygen saturation.
The predominantly bimodal distribution of O2 saturation values indicates that the transition between hypoxic and oxic conditions took place fast within a short vertical distance. Sharp gradients between anoxic and oxic zones situated in
Conclusions
Peat on the extensive pasture (site IV) and the restored sedge fen (site V) was subjected to oxic conditions in 5 cm depth already at groundwater levels much closer to the soil surface compared to the intensively and the extensively managed grasslands (sites II and III). Especially on the restored sedge fen (site V) the transition from hypoxia to nearly atmospheric O2 saturation occurred within a narrow GWL range (c.f. Fig. 3).
In the frame of extensification and wetland restoration, groundwater
Acknowledgements
We like to thank all colleagues from the Institute for Systematic Botany and Ecology, Ulm University, especially Beatrice Weiss, Pia Burkhardt, Katrin Baumeister, Kerem Caglar, Hans Malchus, Carina Stöcker, and Martin Werth. We also thank the ‘Stiftung Naturschutz Pfrunger-Burgweiler Ried’, namely Mr. Bernd Reißmüller and Mrs. Sabine Behr as well as the farmers Mr. König and Mr. Haberkorn for their support. This study was part of a project assessing the greenhouse gas emissions from peatlands
References (38)
- et al.
Soil heterogeneity effects on O2 distribution and CH4 emissions from wetlands: in situ and mesocosm studies with planar O2 optodes and membrane inlet mass spectrometry
Soil Biol. Biochem.
(2010) - et al.
Short period of oxygenation releases latch on peat decomposition
Sci. Total Environ.
(2014) - et al.
Peat particle size effects on spatial root distribution, and changes on hydraulic and aeration properties
Sci. Hortic.
(2013) - et al.
Regulators of heterotrophic microbial potentials in wetland soils
Soil Biol. Biochem.
(1999) - et al.
A regulatory role for phenol oxidase during decomposition in peatlands
Soil Biol. Biochem.
(2004) - et al.
Influence of simulated traffic and roots of turfgrass species on soil pore characteristics
Geoderma
(2014) - et al.
Hydraulic properties of fen peat soils in Poland
Geoderma
(2010) Pressurised ventilation in floating-leaved aquatic macrophytes
Aquat. Bot.
(1996)- et al.
The link between peat hydrology and decomposition: beyond von Post
J. Hydrol.
(2013) - et al.
Effects of a compact layer on soil O2 diffusion
Geoderma
(2006)
Soil O2 profile affected by gas diffusivity and water retention in a drained peat layer
Soils Found.
A conceptual model of volume-change controls on the hydrology of cutover peats
J. Hydrol.
Effects of plant roots on soil-water retention and induced suction in vegetated soil
Eng. Geol.
Methanogenesis and CO2 exchange in an ombrotrophic peat bog
Atmos. Environ.
The role of nutrient availability in regulating root architecture
Curr. Opin. Plant Biol.
Methane and carbon dioxide exchange potentials of peat soils in aerobic and anaerobic laboratory incubations
Soil Biol. Biochem.
Effect of soil organic carbon on soil water retention
Geoderma
A model for oxygen transport and consumption in the unsaturated zone
J. Hydrol.
Root growth, aerenchyma development, and oxygen transport in rice genotypes subjected to drought and waterlogging
Environ. Exp. Bot.
Cited by (0)
- 1
Both authors contributed equally to this study.