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The role of macropores in the hydrological functioning and carbon balance of peatlands

Prof Andy Baird (SoG), Prof Pippa Chapman (SoG), Prof Joseph Holden (SoG)

Contact email: a.j.baird@leeds.ac.uk

Project summary

This project will investigate how water flow and aquatic carbon (C) transport in peatlands are affected by peat structure. The hydrological regime or 'behaviour' of peatlands is fundamental to their functioning as ecosystems, and novel field measurements and laboratory experiments will be used to determine the role of micropores and macropores in water and C transport in undisturbed, drained, eroded, and restored peatlands. In these different settings, measurements will be made of the bulk permeability of the peat and also of the dynamics of the water table within it. The dissolved organic carbon (DOC) levels in different sized pores in the study peatlands will be measured, and experiments undertaken to elucidate how DOC is transported within peat and how different pore structures affect this transport. Overall, data from the investigations will enable us to better inform peatland managers of how drainage and restoration affect the hydrological and C transport function of peatlands.

Background and rationale

Macropores and water flow in peatlands

Macropores are ubiquitous features of peatlands worldwide (e.g. Baird, 1997; Chason and Siegel, 1986; Holden et al., 2001), and may contribute 60-90% of overall water flow through peat soils (e.g. Baird, 1997; Carey et al., 2007; Chason & Siegel, 1986; Holden, 2009a; Holden, 2009b; Wallage & Holden, 2011). There have been reports of increased macroporosity and macropore flow in drained and eroded peatlands due to desiccation cracking (Holden et al., 2006; Mayfield & Pearson, 1972), with cracks often evident on the sides of drainage ditches and gullies (Fig. 1). It has also been commonly reported that subsidence of peat occurs in drained systems (e.g. Leifeld et al. 2011; Williamson et al., 2017) and it is possible that some of this is associated with peat compression and macropore closure. Therefore it is not clear whether artificial peatland drainage causes a significant increase or decrease of macroporosity and macropore flow.

Many peatlands that were formerly ditched, gullied or subject to peat extraction, are now being 'restored'. Dams or bunds are commonly used to hold back water within ditches (e.g. Parry et al., 2014; Price et al., 2005; Shantz & Price, 2006). As it re-wets, a peat soil may expand, causing desiccation cracks to close, thus shutting down macropore flow. However, much depends on how the peat swells. If the peat surface rises to accommodate the swelling, macropores may remain open. These alternatives have not been investigated and we know little about the effects of peatland restoration on macropore flow and overall hydrological functioning.

Macropores and dissolved carbon transport in peatlands

Aquatic fluxes of carbon (C) from peatlands are dominated by dissolved organic carbon (DOC) which can be equivalent to 10-40 % of the net ecosystem C balance (Dinsmore et al., 2010; Nilsson et al., 2008; Roulet et al., 2007). DOC can be problematic for water supply companies because chlorination of DOC-rich waters at treatment plants can produce potential carcinogenic by-products, and one of the key drivers for peatland restoration in water supply areas has been improvements in raw water quality. In previous University of Leeds work, we showed that natural pipes were an important hydrological pathway in blanket peatlands, accounting for 10-13% of streamflow and 21% of aquatic C losses (Holden et al., 2012). We also showed that drainage was associated with larger densities of pipes (Holden, 2005). However, there has been no work on the role of macropores in the transport of C compounds through peatlands or on how transport characteristics may change as a result of peatland drainage/rewetting. Given the potential hydrological importance of macropores outlined above, and the likelihood that they are far more numerous than pipes, and transport a greater proportion of flow in peatlands than pipes, further work is urgently needed. Most peatland C cycle research has focused on factors controlling DOC production, such as water-table position, temperature, and vegetation type, rather than factors that control the transport of DOC through the peat to surface waters. For example, laboratory studies investigating DOC concentrations in samples of peat are usually run without letting water percolate through the samples (e.g. Clark et al., 2011). Discrepancies between field and laboratory results may be due to neglecting preferential macropore flow in laboratory studies. Improved understanding of the role of peatland pore structure on C transport would provide a significant advance (Rezanezhad et al., 2016) in the understanding of the overall peatland C balance.

Different methods of soil solution sampling yield water from different sized pores. Tension-free samplers collect gravitational water flowing within macropores (Blodau and Moore, 2001), while samplers to which suction is applied can extract less-mobile water held by capillary forces in meso and micropores (Nambu et al., 2005). Measuring the DOC concentrations in water from different pore sizes combined with improved measurements of flow rates through different pore size pathways, and the contributions of different pore sizes to the transport of DOC, will greatly improve our understanding of DOC transport in peatlands.

The proposed project

A combined field and laboratory-experimental approach will be used to fill the knowledge gaps identified in the section above (see also Methods below). Fieldwork for the project will take place on well-established peatland sites in the North Pennines and Upper Conwy (North Wales) that have been used for related studies by researchers at the University of Leeds. Permissions to take peat samples from these sites for laboratory work have been granted in the past, and site owners and managers have expressed interest in the proposed study.

Figure 1: Top left: freshly-exposed peat face in new drainage ditch. Top right: crack macropores on the side of an old drainage ditch. Lower left: self-logging pressure transducer used for measuring water-table dynamics. Lower right: manipulation of water-table position in peat cores, from which the soil solution can be extracted (via rhizon samplers placed at different depths) for DOC analysis.

Key research questions

The study will:

(i) Test the hypothesis that macroporosity and the proportion of total flow that occurs as macropore flow is as follows in order of importance: old drained peatland = gullied > newly drained > re-wetted > undisturbed.

(ii) Test the hypothesis that macropore flow is greater near the edge of a ditch than further away and, if this difference occurs, determine the spatial rate of change in macropore flow away from ditch edges.

(iii) Determine how pore size distributions and structure influence DOC concentration and transport.

Methods

Weather conditions at the study sites will be measured using portable automatic weather stations available from the University of Leeds. Water tables will be measured using automatically-logged dipwells (Fig. 1), and soil moisture dynamics will be monitored using high-resolution soil-water probes (equipment again available at Leeds). The 'bulk' permeability of the peat (i.e. combined permeability of macropores and micropores) will be measured using tracers and breakthrough tests (conventional piezometer-based measurements may not work well in highly macroporous peats). DOC will be measured from samples of peat pore water collected in-situ but also in the laboratory using large blocks of peat removed from the field sites. In both proposed study areas – the North Pennines and Upper Conwy – we have excellent working relationships with the site owners and managers.

Requirements

BSc (Hons) and MSc (Merit or Distinction, attained or predicted) in physical geography, environmental science, or related discipline. Applicants with an MSc involving environmental hydrology or soil dynamics are particularly encouraged to apply.

Training

The successful candidate will develop a range of research skills, including experimental design, field sampling, chemical analysis, statistical analysis and data interpretation, and giving presentations. Training will be provided in field/laboratory health and safety procedures and the use of field and analytical equipment. Data analysis skills will be acquired through the use of training datasets in sessions with the project supervisors and through formal statistics training courses (see below). The candidate will be part of a large group of peatland researchers at Leeds who can provide informal training in a range of topics – knowledge exchange and mutual support are central to the group's ethos, as reflected in a recent paper published by group members in the journal Mires and Peat (see: http://mires-and-peat.net/pages/volumes/map19/map1912.php). The successful PhD student will also have access to an extensive range of training workshops provided by the University, with topics including statistical analysis, managing your degree, and preparing for your viva.

Research context

The successful candidate will join a vibrant research environment at the School of Geography which includes the River Basins Processes and Management research cluster and the informal 'Peat Club' which both meet weekly.

Potential for high impact outputs

Although this work will have a UK focus, the approach, methods, and findings will be of international relevance. The work in this project will provide a fundamental advance in our understanding of how macropores affect the hydrological functioning and C budget of peatlands and should lead to papers in high-profile hydrology and general geophysics journals such as Hydrological Processes, Water Resources Research, and Geophysical Research Letters. The supervisory team has an excellent record of publishing in the named journals, and in a wide range of similar journals.

References

Baird AJ 1997, Hyd. Proc. 11: 287295; Blodau C & Moore TR 2002, Soil Sci. 167:98109; Carey SK et al. 2007, Hyd. Proc. 21:25602571;

Chason DB & Siegel DI 1986, Soil Sci. 142:9199;

Clark JM et al. 2011, Eur. J. Soil Sci. 62:267–284;

Dinsmore KJ et al. 2011, J. Geophy. Res. Biogeo.116:G03041;

Holden J 2005, J. Geophy. Res., 110:F01002;

Holden J 2009a, J. Hydrol. 364:342348;

Holden J 2009b, Earth Surf. Proc. & Landf. 34:345351;

Holden J et al. 2001, Hyd. Proc. 15:289303;

Holden J et al. 2006, J. Env. Qual. 35:17641778;

Holden J et al. 2012, Glob. Change Biol. 18:35683580;

Leifeld J et al. 2001, Soil Use & Manag. 27: 170-176;

Mayfield B & Pearson MC 1972, East Mid. Geog. 12: 245251;

Nambu KM et al. 2005, Geoderma 127:263269;

Nilsson M et al. 2008, Glob. Change Biol. 14:23172332;

Parry LE et al. 2014, J. Env. Man. 133:193205;

Price JS et al. 2005, Hyd. Proc. 19:201214;

Rezanezhad F et al. 2016, Chem. Geol. 429:7584;

Roulet NT et al. 2007, Glob. Change Biol. 13:397411;

Shantz MA & Price JS 2006, J. Hydrol. 331:543553;

Wallage, ZE & Holden J 2011, Soil Use & Manag. 27:247254;

Williamson J et al. 2017, J. Env. Manag. 188: 278-286.

Related undergraduate subjects:

  • Earth system science
  • Environmental science
  • Geography
  • Hydrology
  • Natural resource management
  • Physical geography
  • Soil science