Elsevier

Ecological Engineering

Volume 120, September 2018, Pages 85-93
Ecological Engineering

Relationship between groundwater levels and oxygen availability in fen peat soils

https://doi.org/10.1016/j.ecoleng.2018.05.033Get rights and content

Highlights

  • Ground water levels cannot be directly translated into oxygen availability.

  • The hypoxic zone is of different extent above groundwater levels (GWL).

  • Full oxygen saturation never occurred in 20 cm soil depth, even at GWL of −80 cm.

  • The offset between groundwater level and oxygen availability is site specific.

  • Higher groundwater levels are needed for anoxic conditions on extensified sites.

Abstract

Groundwater levels (GWL) are a major controlling factor for aeration and organic matter turnover in wetland soils but little is known about this relationship under field conditions. This study tested how the O2 availability in fen peat soils is related to groundwater levels. The study encompassed five sites over a wide range of land use intensity. Ground water levels and soil oxygen saturation in 5 cm and 20 cm depth were measured biweekly in three replicates per site over periods of 2–3 years.

The O2 levels were not linearly proportional to the GWL, but changed sharply from anoxic to nearly atmospheric levels depending on the positions of water table. Binary logistic regression analyses (LRA) were calculated for the individual sites in order to predict the threshold GWL for defined probabilities of hypoxic or oxic conditions in 5 cm depth. The GWLs for 95% probability of oxic conditions were markedly lower for the managed grasslands (−116 cm and −89 cm to surface level, respectively) than for the unmanaged pasture and the sedge fen (−60 cm and −38 cm). Hypoxic conditions required GWLs close to the surface (7 cm and −2 cm for the pasture and the restored site, respectively) while in 5 cm soil depth managed grasslands remained hypoxic even at GWLs of −8 cm and −28 cm. In 20 cm soil depth, full oxygen saturation never occurred even at GWL as low as 80 cm. Threshold GWL required for 95% probability of oxic conditions was higher with increasing porosity and rooting density. The offset between GWL and oxic conditions can be used for hydrological wetland management, especially for restoration efforts.

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

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Both authors contributed equally to this study.

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