Introduction
Background and Rationale
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).
Risk of Eutrophication
Understanding nutrient dynamics in disturbed peatlands is essential for predicting downstream water quality. Eutrophication is often observed in areas where increased nutrient exports are released to downstream water bodies, such as near agricultural fields and wastewater treatment plants (Conley et al., 2009) (Figure 1.). 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. 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, and help predict the impact, if any, to downstream aquatic ecosystems.
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) (Figure 2.). 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) (Figure 4.). 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).
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
Peat extraction activities (ditching and vegetation removal) result in changes to in situ physicochemical processes in peat soils, such as soil moisture, aeration, temperature, and water pH and electrical conductivity. In this study, we will look at the surface peat and water expressed from peat fields undergoing different extraction activities to understand the potential availability of nitrogen compounds able to leach into surface water and be transported off site.
To answer this question, we ask:
1. Does the extraction phase alter the peat physicochemical properties and in situ available nitrogen in the peat field?
2. Compared to undisturbed peatlands, are nitrogen concentrations higher at extracted and restored outflows, potentially leading to undesirable downstream water quality?
Extracted Peatlands
Hypothesis: Extracted peatlands will have higher rates and concentrations of available nitrogen compared to natural peatlands.
We expect that ditching the peatland will lower the water table and increase oxygen in peat soils. Removing vegetation will decrease shade, increasing the surface and near-surface soil temperatures, and the absence of nutrient uptake by plants will allow for more available nutrients for microbial activity. Higher oxygen availability, increased temperatures, and more available nutrients will lead to accelerated decomposition and higher rates and concentrations of nitrogen freely available in the peat. Therefore, we predict that extracted areas will have higher aeration, lower soil moisture, higher soil temperature, and higher concentrations of nitrate compared to the undisturbed peatland.
Restored Peatlands
Hypothesis: Restored peatlands will have similar rates and concentrations of available nitrogen to natural peatlands.
We expect that blocking ditches will raise the water table and decrease oxygen in peat soils. Establishing vegetation will increase shade, decreasing the surface and near-surface soil temperatures, and the ready uptake of nutrients by plants will decrease the available nutrients for microbial activity. Lower oxygen availability, decreased temperatures, and fewer available nutrients will lead to slowed decomposition and lower rates and concentrations of nitrogen freely available in the peat. Therefore, we predict that restored areas will have lower aeration, higher soil moisture, lower soil temperature, and lower concentrations of nitrate compared to the undisturbed peatland.