Introduction
Background and Rationale
Peatlands and Horticultural Peat Extraction
Horticultural peat extraction is an expanding industry in Canada with potentially detrimental consequences for downstream water quality. Peatlands suitable for extraction are waterlogged ecosystems with large stocks of poorly decomposed organic material, called peat, and typically have low pH and low oxygen conditions which inhibit decomposition and favour organic material accumulation (Landry & Rochefort, 2012; Williams & Crawford, 1983). Peatlands are primary source areas for runoff to the larger landscape and provide receiving aquatic ecosystems with dissolved organic carbon, inorganic and organic nitrogen, phosphorus, and other biologically significant major ions (Devito et al., 2017; Strack et al., 2015; Waldron et al., 2009).
However, to remove peat for horticultural purposes, the physicochemical environment is changed by installing ditches, altering the water table, and removing living vegetation. These changes can influence the concentration and mobility of nutrients stored in peat, which are transported by water moving through the disturbed peatland and offsite. Increased concentrations of these compounds in outflow water can alter water quality for agricultural and human consumption in downstream water bodies, and increase the risk of eutrophication (Figure1.) (Niedermeier & Robinson, 2009; Worrall et al., 2007). Understanding the changes to the physicochemical environment when the peatland is altered can help estimate the potential leaching risk of nutrients available in peat surface soils.
Figure 1. Algal bloom in shallow creek
The Peat Extraction Process
Peat extraction is a multi-year process that occurs in three stages: opening the peatland, extraction of peat, and restoration once all desirable peat has been removed. When opened, vegetation is removed, and ditches are installed to drain the peatland and lower the water table (Landry & Rochefort, 2012). This allows operators to vacuum thin layers of dried peat from the surface of the peatland (Figure 3.). These ecosystem modifications allow for easier extraction, but have unintended consequences for peat nutrient availability.
Removing the vegetation increases the peat surface temperature by inhibiting shade, raises the water table by decreasing evapotranspiration, and decreases the nutrients removed from the system via roots and rhizoid uptake (Walbridge & Lockaby, 1994). Lowering the water table alters the ground and surface water flow paths, changes the peat bulk density, and increases the aeration status at the peat surface (Price et al., 2003). The additional oxygen, increased nutrient availability, and elevated temperatures encourage decomposition (Williams & Crawford, 1983). As decomposition accelerates, nutrient mineralization and nitrification increase, and available nitrogen and phosphorus are leached from the peatland via subsurface drainage (Nieminen et al., 2017; Strack et al., 2008; Wells & Williams, 1996). However, peat has excellent water storage capacity, and although water tables are depressed, the soil moisture content of the surface peat can remain elevated which may inhibit oxygen and subsequent microbial decomposition (Price et al., 2003).
Once all suitable peat is extracted, ditches are backfilled, moss propagules and straw are added to revegetate the area, and the restored site is amended with phosphorus fertilizer (Rochefort et al., 2003). Consequent rewetting facilitates phosphorus mobility and increases concentrations in runoff (Niedermeier & Robinson, 2009). However, the presence of iron, aluminum, and calcium can influence the solubility of phosphate and change its availability and mobility (Carlyle & Hill, 2001).
Multiple studies have identified increased nutrient levels in outflow water from disturbed peatlands (Niedermeier & Robinson, 2009; Nieminen et al., 2017; Tuukkanen et al., 2017). However, there are fewer studies addressing the linkage between the physicochemical conditions in the peat soil with nutrient availability. The current literature presents conflicting results and does not look at all peat extraction phases within one research area. Munir et al. (2017) found increased temperature and lowered water tables elevated the available nitrogen concentrations of peat in a drained peatland however; a similar study by Harris et al. (2020) found no significant difference in nutrient availability between a drained and undisturbed peatlands.
Figure 2. Perimeter ditch at a newly opened peatland
Figure 3. Heavy equipment extracting dried peat from an an active extracted peatland
Figure 4. Restored peatland showing vegetative regrowth
Research Objectives, Hypotheses, & Expectations
Peat extraction activities (ditching and vegetation removal) result in changes to in situ physicochemical processes in peat soils, such as soil moisture, aeration, and temperature. In this study, we will look at the surface peat in peat fields undergoing different extraction activities to understand the potential availability of chemical compounds able to leach into surface water and be transported off site.
To answer this question, we ask:
1. How do extraction activities impact the physicochemical conditions of the peat?
2. Are the observed physicochemical changes enough to influence the availability of nitrogen and phosphorus?
Ditching
We expect that ditching the peatland will lower the water table, increase oxygen in peat soils, and accelerate aerobic decomposition. Therefore, we hypothesize that ditched areas will have high oxygen, low soil moisture, and support chemical compounds mobile in aerobic conditions.
Alternatively, peat structure can retain moisture for long periods of time and may remain anaerobic even when the water table is lowered. Therefore, we hypothesize that peat in ditched areas will have low oxygen content, high soil moisture, and support chemical compounds mobile in anaerobic conditions despite lowered water tables.
Vegetation Removal
We expect that removing vegetation will decrease shade and increase the surface and near-surface soil temperatures and accelerate decomposition. Absence of nutrient uptake by plants will allow for more available nutrients for microbial activity. Therefore, we hypothesize that peat in areas without vegetation will be warmer and have higher concentrations of available nutrients.
Alternatively, vegetation removal increases the duration of ice in subsurface peat, decreasing the soil temperature and slowing microbial activity. Therefore, we hypothesize that peat in areas without vegetation will have lower concentrations of available nutrients and have cooler temperatures.