Soil properties following borrow pit reclamation with insufficient topsoil amended with peat and biochar

Disturbed sites, such as borrow pits and oil and natural gas well sites, require reclamation to restore and sustain levels of productivity similar to those prior to the disturbance. However, salvaged topsoil at many sites is often insufficient to meet the 80% topsoil replacement depth (TRD80) required for successful reclamation in western Canada. This 5‐year study evaluated soil responses to 50% topsoil replacement depth without organic amendment (TRD50) or amended with either peat (PTRD50) or biochar (BTRD50), relative to the TRD80 treatment (Control), following borrow pit reclamation at a disturbed boreal site near Cold Lake, Alberta, Canada. Amendments were applied once at rates calculated to raise the soil organic carbon (SOC) content in the TRD50 soil to a level equal to that in the TRD80 treatment. Results showed a 143%, 87%, and 116% increase in total Kjeldahl nitrogen concentration in the PTRD50 relative to the TRD80, TRD50, and BTRD50 treatments, respectively, while soil potassium (K) concentration was significantly greater for BTRD50 than PTRD50. Peat and biochar significantly increased SOC concentrations by 83% and 88%, respectively, relative to the mean of TRD80 and TRD50 treatments. Our results show that peat and biochar can improve soil properties of disturbed boreal sites reclaimed with insufficient salvaged topsoil to a level suitable for successful reclamation. This has important implications on the reclamation of a multitude of disturbed sites, in Canada and globally, that have insufficient volumes of salvaged topsoil needed for successful reclamation.

reclamation and restoration (Macdonald et al., 2015).Wellsite preparation activities, such as vegetation removal, stripping of soil for salvage, and stockpiling alter soil physical and chemical properties (Bekele et al., 2015;Lupardus et al., 2019;Rowland et al., 2009).Reclamation of these disturbed sites depends primarily on the amount and quality of topsoil returned to the site following decommissioning of the wellsite (Larney et al., 2003(Larney et al., , 2005;;Powter et al., 2012).Suboptimal topsoil replacement depth (TRD) has been shown to result in low nutrient availability and productivity, slowing the reclamation process.Bowen et al. (2005) reported significantly lower total nitrogen (N) and soil organic carbon (SOC) concentrations in control plots (no topsoil replacement) than plots reclaimed with 40 cm and 60 cm topsoil depth in Wyoming, USA.Larney et al. (2005) studied the effect of varying TRD on soil properties at oil well sites in central Alberta.
They observed an increase in SOC and nitrate-N concentrations for the 150% TRD (i.e., 150% of the pre-disturbance topsoil depth) relative to the 50% TRD and 0% TRD (i.e., no topsoil replacement) treatment.
Topsoil replacement is a critical reclamation step that involves spreading salvaged topsoil on disturbed land to provide a growing medium for plants during the reclamation process (Rowland et al., 2009;Strohmayer, 1999).Recommended reclamation practices, such as topsoil salvaging and stockpiling, usually provide optimal topsoil with good physical and chemical properties necessary to ensure reclamation success (Merino-Martín et al., 2017).Topsoil properties for sustained productivity post-reclamation include high water retention, nutrient availability, resistance to surface erosion, and the ability to support plant root development (Rowland et al., 2009).Current Alberta regulations require an optimal TRD of at least 80% of the original topsoil depth or a variance of no less than 20% from the adjacent undisturbed land (ESRD, 2013).In many cases, lack of available topsoil for reclamation is the result of little or no topsoil salvage.However, in other cases, the topsoil would have been sourced from the sites (borrow pits) for use in the construction of well pads and access roads (Alberta Environment & Parks, 2018).A consequence of these borrow activities is the creation of disturbed sites (borrow pits) which in turn require reclamation.
Due to the scarcity of topsoil for reclamation and since disturbed soils are typically low in organic matter, approaches such as the use of organic amendments have been implemented to reconstruct and improve the quality of disturbed topsoil and enhance revegetation (Larney & Angers, 2012).Organic amendments play a significant role in reclaiming degraded soils and fostering the development of healthy ecosystems (Bradshaw, 2000).Various organic amendments, such as alfalfa hay, compost, biosolids, and cattle manure have been used during reclamation to improve soil physical and chemical properties of disturbed soils, including SOC (Larney et al., 2005), available nutrient supply (Zvomuya et al., 2007), and water retention (Rezanezhad et al., 2016) of disturbed well sites (Curtis & Claassen, 2009).Peat is commonly used in the reclamation of disturbed upland forests due to its abundance in such ecosystems (MacKenzie et al., 2012;MacKenzie & Naeth, 2011).For example, peatlands account for approximately 64% of Canada's Athabasca oil sands region in the boreal forest region of Alberta (Alberta Environment & Water, 2012;Rooney et al., 2012).
Peat is, therefore, easily and economically (i.e., low transportation costs) accessible for use in the reclamation of disturbed sites in the area.Peat improves organic carbon content, nutrient availability (Hemstock et al., 2010;Quideau et al., 2017), and water retention properties (Rezanezhad et al., 2016;Rowland et al., 2009) of soil, all of which are essential for early vegetation growth and development.Ojekanmi and Chang (2014) reported a 5-35 g kg À1 increase in SOC and an increase in water holding capacity after mixing mineral soil with 10%-50% peat by weight.Recently, biochar has gained popularity as an organic amendment for reclamation due to its water retention properties and sustained nutrient supply (Dietrich & MacKenzie, 2018;Mukherjee et al., 2014;Page-Dumroese et al., 2016).Biochar can be prepared close to or on-site using tree wood chip residues from site preparation as raw materials, thereby reducing transportation costs (Rowland et al., 2009).
There is a dearth of information on the reclamation of disturbed sites (borrow pits or well sites) with suboptimal TRD and organic amendments.Most of the few published studies focused on cropland or were conducted under controlled environments, with little focus on forest ecosystems.For example, previous studies on the application of biochar have focused predominantly on agricultural land (Enders et al., 2012;Laird et al., 2010;Yang et al., 2017), with a few studies conducted on forest sites (Page- Dumroese et al., 2018;Thomas & Gale, 2015).Therefore, there is a need to investigate the impact of TRD and organic amendments (peat and biochar) on the productivity of borrow pit reclaimed using insufficient topsoil in the boreal zone.
The objective of this experiment was, therefore, to evaluate the changes in topsoil chemical properties as a function of TRD and amendment type.

| Study site
The study was conducted in a disturbed borrow pit situated in the Central Mixedwood region of the Canadian boreal forest, in Cold Lake, Alberta, Canada (54 36 0 22.26 00 N, 110 29 0 28.24 00 W).The site comprised a borrow pit that was established in 2010, spanning a land area of 2.7 ha.The pit was specifically created to facilitate the construction of access roads associated with nearby oil activities.Prior to the construction of the borrow pit, a pre-disturbance assessment revealed that the study area was dominated by tree species that included trembling aspen (Populus tremuloides Michx.), white spruce (Picea glauca), and basalm poplar (Populus balsamifera L.), along with understory shrub species such as green alder (Alnus viridis), and low bush cranberry (Viburnum edule).The soil at the site was an Orthic Gray Luvisols of the Athabasca soil series, underlain by glacial till parent material and devoid of living vegetation at the time of reclamation.
The surface soil texture ranged from sandy loam to sandy clay loam, overlying a subsoil composed mostly of medium-sized gravel with some presence of fine-sized gravel.The soil had an angular blocky to subangular blocky structure and a bulk density of 1500 kg m À3 .Drainage ranged from moderate to well-drained.The slope was classified as nearly level (0.5%-2.5%) to very gentle (2.5%-5%).
In early 2014, following 4 years of operation, the borrow pit was decommissioned and rendered inactive with insufficient topsoil for reclamation.Reclamation of the site was completed in November 2014 using topsoil that had been stockpiled at the site for 4 year (that is, since 2010).The original (pre-disturbance) topsoil depth at the site was 28 cm.

| Plot establishment and experimental layout
Study plots were established on a 0.5 ha section of the borrow pit in November 2014.Topsoil replacement was completed during that time when temperatures averaging À9.6 C and snow depth of 38.5 cm ensured minimal soil structure disruption.The plots were laid out in a randomized complete block design with three blocks representing three replicates.Each block was divided into four 20 m Â 20 m plots (n = 12) with 2 m buffers.The treatments were (1) the recommended optimal topsoil depth (TRD80) as the control, (2) the unamended suboptimal topsoil depth (TRD50), (3) the 50% TRD amended with peat (PTRD50), and (4) the 50% TRD amended with biochar (BTRD50).The 80% TRD corresponded to a topsoil layer thickness of 22 cm, based on the original topsoil depth of 28 cm, while the 50% TRD corresponded to a 14-cm topsoil layer thickness.

| Amendment application
Peat and biochar were applied to the respective plots in May 2015.One-year stockpiled peat was sourced from another nearby oil well pad in Cold Lake.Prior to application, amendments were analyzed for their initial properties (Table 1).Biochar sourced from Biochar Now, Colorado, USA, had been prepared from raw pine tree logs using slow pyrolysis at temperatures ranging from 550 to 650 C for 6-12 h.The amendments were applied uniformly as needed to the plots using a tractor (Massey Ferguson 1525), a manure spreader (520 New Holland), and a Buhler Farm King 705 disc to incorporate the amendments into the topsoil.Peat was applied at 20 kg m À2 and biochar at 4.75 kg m À2 to bring the SOC concentration in each plot to the same level as measured in the TRD80 plots.

| Soil sampling
Soil samples were collected from the 0-to 22-cm layer in the TRD80 plots and 0-to-14-cm in the TRD50 plots prior to amendment application in 2014 for baseline characterization (Table 1).Thereafter (2015-2019), soil samples were collected from the 0-to-15-cm depth every August.At each sampling time, 12 soil cores (5 cm in diameter) were randomly collected from each plot using a hand auger and composited.Soil cores for bulk density determination in the topsoil (TS) horizon were collected from each plot with a 313 cm 3 corer.Subsoil samples were taken using a shovel and a hand auger from the bottom of the topsoil layer to the 1 m depth in the northwestern corner of each plot.The subsoil was divided into upper subsoil (USS, bottom of topsoil horizon to 50 cm) and lower subsoil (LSS, 50-to 100-cm layer).

| Laboratory analysis
Biochar samples were shipped to Loring Laboratories Ltd. (Calgary, Alberta) for proximate and ultimate analysis.Fixed carbon, volatile matter, ash, and volumetric water content were determined using standard ASTM methods D5357 (ASTM, 2014), D3175, D3174, and D3173 (ASTM, 2011a(ASTM, , 2011b(ASTM, , 2011c)).Soil samples were analyzed by Bureau Veritas Laboratories (Calgary, Alberta).Bulk density was determined using the core method (Blake & Hartge, 1986).Soil organic C concentration was determined by the dry combustion method with a LECO TruMac ® CNS analyzer (LECO Corporation, USA).Soil organic carbon stock (SOC stock, Mg C ha À1 ) in the 0-to 0.15-m soil layer was calculated using the equation: where SOC is soil organic carbon concentration in g kg À1 , BD is soil bulk density (kg m À3 ), D is the sampling depth (0.15 m), and 10 4 (m 2 ha À1 ) is the factor for converting m 2 to ha.
T A B L E 1 Initial properties of biochar, peat and unamended soil.Total Kjeldahl nitrogen (TKN) concentration was determined according to EPA method 351.2 (USEPA, 1993).Available P and K concentrations in the soil were determined using the modified Kelowna extraction method (Qian et al., 1994) in which the two nutrients were extracted using 0.25 M ammonium acetate, 0.015 M ammonium fluoride, and 0.025 M glacial acetic acid (Kelowna extraction), followed by analysis using a Varian Vista Pro ICP-OES spectrometer (Varian Inc., Palo Alto, CA, USA).Available S was extracted using a 2:1 ratio (vol/mass) of 0.01 M CaCl 2 and air-dry ground soil (Houba et al., 2000) and measured using a Varian Vista Pro ICP-OES spectrometer.
Electrical conductivity (EC), water-extractable cations, and soil pH were measured in soil samples taken in 2015, 2017, and 2019.
Soil pH and EC were measured with a pH/EC meter in a 1:2 (vol/vol) soil: water suspension (Janzen, 1993;Rhoades, 1996).Watersoluble calcium (Ca), magnesium (Mg), and sodium (Na) were extracted according to EPA Method 200.7 (USEPA, 1994) followed by ICP-OES analysis.Sodium adsorption ratio was calculated from water-extractable cations using the equation: SAR , where concentrations were milliequivalents/liter.Moisture content at saturation (σ s , the weight of water required to saturate the pore space expressed as a percentage of the weight of the dry soil) was determined using the saturated paste method (Rhoades, 1996).

| Statistical analysis
Analysis of variance (ANOVA) was carried out using the generalized linear mixed model procedure (PROC GLIMMIX) of SAS 9.4 (SAS Institute, 2013).Data for all soil properties followed a normal distribution, except for available S, which followed a lognormal distribution, and σ s , which followed a beta distribution.Treatment, year, and horizon were modeled as fixed effects, with year as a repeated measures factor, while block was a random effect.Based on the Akaike Information Criterion (Littell et al., 1996), the compound symmetry (CS) covariance structure was selected for repeated measures ANOVA of TKN, EC, SAR, pH and soluble cations data while the heterogeneous compound symmetry (CSH) covariance structure was the most suitable for SOC, available P, and S. Treatment means were compared using the Tukey multiple comparison procedure at α = 0.05.

| Weather conditions
Cumulative precipitation during the growing season (May-October) ranged from 304 to 366 mm annually (Figure 1).The 30-year (1981-2010) mean cumulative growing season (May-October) precipitation for Cold Lake is 308 mm.The highest mean growing season temperature of 14.5 C was recorded in 2015, while the lowest mean air temperature was 11.5 C in 2019, which was slightly below the long-term (1981-2010) growing season mean temperature of 12.2 C (Figure 1) (Environment Canada, 2020).

| Bulk density, SOC and TKN
Bulk density differed significantly among treatments but did not vary with year (Table 2).Bulk density was significantly higher for the TRD80 (control) treatment than the peat (PTRD50) and biochar (BTRD50) treatments but did not differ significantly between the TRD80 and the TRD50 treatments (Table 2).The addition of peat or biochar to the 50% TRD significantly reduced bulk density by 27% and 15%, respectively, relative to the control.Mean bulk density ranged from 0.95 g cm À3 for the PTRD50 to 1.3 g cm À3 for the TRD80 treatment (Table 2).
As expected, across years, peat and biochar application significantly increased SOC concentration and SOC stock relative to unamended soil.Soil organic C concentration in soils amended with peat and biochar increased by 83% and 88%, respectively, relative to the mean of the TRD50 and the TRD80 treatment, but there was no significant difference in SOC concentration between the two amendments (peat and biochar) (Table 2).Soil organic C concentration did not differ significantly between the TRD80 and the TRD50 treatment.
Soil organic C concentration varied significantly during the 5-year study, but there were no significant temporal trends.Soil organic C concentration was greater by 43% in 2016 and 49% in 2018 relative to 2019.
There was a significant treatment effect on TKN concentration (Table 2).Averaged across years, the TKN concentration was 143%, 87%, and 116% significantly higher for the PTRD50 treatment than the TRD80, TRD50, and BTRD50 treatments, respectively.Although TKN concentration was lowest for the TRD80 treatment, it was not significantly different from the TRD50 and the BTRD50 treatments (Table 2).Averaged across treatments, N varied significantly with time over the 5 years, with no consistent temporal trends (Table 2).The TKN concentration was greatest in 2016, the year following topsoil Growing season (May-October) precipitation and mean growing season temperatures during 2015-2019 at the borrow pit at Cold Lake, Alberta, Canada.
and amendment placement, compared with all the other years and was significantly greater in 2016 and 2018 than in 2019.

| Available phosphorus, potassium, and sulfur
Averaged across years, available P (modified Kelowna-extractable P, MKP) concentration did not vary significantly among treatments but with the main effect of year was significant (Table 2).The MKP concentration was significantly higher in 2016 than in all the other years except 2019, for which the MKP concentration did not differ significantly from that in 2016.
Available K (modified Kelowna-extractable K, MKK) concentration differed significantly among treatments and years (Table 2).The BTRD50 treatment produced significantly greater MKK concentration than the PTRD50 treatment.However, MKK concentration did not differ significantly between TRD80 and TRD50.The MKK concentration was significantly greater in 2019 than in 2017 but did not differ significantly between 2019 or 2017 and the other 3 years.Available S was not significantly affected by treatment across all years but varied with year.Across all treatments, available S concentration was significantly greater in 2016 than in all the other years.
3.4 | EC, sodium adsorption ratio, waterextractable cations, and soil pH EC was not significantly affected by treatment, but there was a significant horizon Â year interaction for EC (Table 3).The EC was significantly greater in the LSS horizon than the TS and USS horizons in 2019, whereas the horizon effect was not significant in 2015 and 2017 (Figure 2a).There was a significant horizon by year interaction for SAR (Table 3).While there was no horizon effect on SAR in 2015, SAR in 2019 was significantly higher in the USS horizon than in the TS and LSS horizons.By comparison, SAR in 2017 was significantly greater in the USS than the TS horizon but did not differ significantly between the USS and the LSS horizons (Figure 2b).The SAR in the USS horizon increased by 43% in 2019 relative to 2015, while there was no significant change in SAR in the TS and LSS horizons over time (Figure 2b).
Water-extractable Ca, Mg, and Na concentrations did not vary significantly with TRD and organic amendment treatment (Table 3).Waterextractable Na concentration was significantly lower in the TS horizon than in the USS and LSS horizons.There was a significant horizon Â year interaction for water-extractable Ca and Mg.In the last year of monitoring (2019), water-extractable Ca and Mg concentrations in the LS horizon were significantly higher than in the TS and USS horizons, while there was no significant difference between horizons in 2015 and 2017.
Water-extractable Ca and Mg concentrations increased in the LSS horizon and decreased in the USS horizon over time, but the difference between the two horizons was only significant in 2019 (Figure 2c,d).
The TRD and organic amendment treatments had no significant effect on soil pH, but there was a significant treatment Â horizon interaction (Table 3).Soil pH was significantly lower for the PTRD50 than the BTRD50 treatment in the TS horizon, but there was no significant treatment effect in the USS and LSS horizons (Figure 3a).The PTRD50 treatment had a lower soil pH, which, however, did not differ significantly from those for the TRD80 and TRD50 treatments in the TS horizon.
T A B L E 2 Topsoil replacement depth and organic amendment effects on soil bulk density and nutrient concentrations.Note: Means within a column followed by the same letter are not significantly different at α = 0.05 according to the Tukey multiple comparison procedure.Abbreviations: BTRD50, 50% topsoil replacement depth plus biochar; PTRD50, 50% topsoil replacement depth plus peat; SOC, soil organic carbon; SOC stock, soil organic carbon stock; TKN, total Kjeldahl nitrogen; TRD50, 50% topsoil replacement depth; TRD80, 80% topsoil replacement depth.

| Soil water content at saturation
There were significant treatment Â horizon and horizon Â year interactions for σ s (Table 3).The σ s was significantly higher for the PTRD50 and BTRD50 than the TRD50 and TRD80 treatments in the TS horizon but did not differ significantly among treatments in the USS and LSS horizons (Figure 3b).The σ s did not differ significantly between horizons for the TRD50 and TRD80 treatments (Figure 3b).Peat and biochar addition to the 50% TRD treatment, on the other hand, significantly increased σ s in the TS horizon by 48% and 32%, respectively, relative to the TRD80 treatment.Across all treatments, σ s increased significantly from 2015 through 2017 to 2019 in the TS horizon but did not change significantly with year in the USS and LSS horizons (Figure 4).

| Bulk density, SOC and TKN
This study is one of the first to examine the reclamation of disturbed boreal sites using suboptimal TRD in conjunction with organic amendments.Findings from the study indicated that, while reduced TRD had no effect on soil nutrient availability or soil quality 5 years following reclamation, adding peat and biochar improved soil nutrient availability.The decrease in bulk density with amendment application reflects the increase in the total porosity of amended soils (Forsch et al., 2021;Page-Dumroese et al., 2016).Although bulk density was greater for TRD80 (1.3 g cm À3 ) than peat and biochar treatments, it was still lower than the pre-disturbance bulk density of 1.5 g cm À3 in the Ae horizon of the Orthic Gray Luvisolic soil at the site and remained within acceptable limits to support plant establishment and growth (Dlusskiy et al., 2017).Our results corroborate those of Forsch et al. (2021), who reported significantly lower bulk density (0.89 g cm À3 ) in plots reclaimed with peat mineral mix relative to the unamended control plots.Similarly, Ojekanmi and Chang (2014) reported a decrease in bulk density from 1.4 to 0.7 Mg m À3 and a concomitant increase in SOC in peat-amended soils.Githinji (2014) observed a reduction in bulk density from 1.33 to 0.89 g cm À3 when biochar application rate was increased from 25% to 50% (vol/vol).
These and our results indicate that increased total porosity of amended soils results in lower soil bulk density.Although our soils had a bulk density below the pre-disturbance levels, our results demonstrate the effectiveness of peat and biochar in reducing bulk Note: Means within a column followed by the same letter are not significantly different at α = 0.05 according to the Tukey multiple comparison procedure.Abbreviations: σ s , moisture content at saturation; BTRD50, 50% topsoil replacement depth plus biochar; EC, electrical conductivity; lower subsoil, 50-100 cm; PTRD50, 50% topsoil replacement depth plus peat; SAR, sodium adsorption ratio; TRD50, 50% topsoil replacement depth; TRD80, 80% topsoil replacement depth; upper subsoil, bottom of topsoil horizon to 50 cm.
density in disturbed and compacted soils to facilitate successful revegetation.
Depletion of SOC stocks is one of the primary indicators of anthropogenic soil disturbance in forest ecosystems (Stavi, 2018).
SOC stock, which represents the SOC concentration of a soil profile per unit area, is essential for microbial processes required for soil aggregation and fertility in degraded lands.Since the SOC stock is dependent on SOC concentration, the factors that affect SOC concentration will also have an impact on SOC stocks in soils.Generally, temporal changes in SOC concentration and SOC stocks in the soil are dynamic, varying in response to plant biomass and litter accumulation (Reeder et al., 1998).Unlike the unamended treatments, the suboptimal TRD (TRD50) amended with peat and biochar showed greater SOC concentrations and SOC stocks across all years.While organic amendments were applied at rates to raise SOC in the 50% TRD equivalent to the TRD80, the greater SOC concentration and SOC stock observed in the peat and biochar treatments reflects greater organic C additions from the vegetation biomass.Organic amendments have previously been shown to increase SOC concentrations and build SOC stocks in reclamation studies.For example, Ojekanmi and Chang (2014) observed a 5-35 g kg À1 increase in SOC when peat and mineral soil were mixed in proportions ranging from 10% to 50% peat by weight.
Similarly, Hemstock et al. (2010) reported a higher concentration of SOC in fibric peat-amended reclaimed soils than at undisturbed sites.
Their findings indicated that the higher C/N ratio of fibric peat resulted in higher SOC levels compared with other peat amendments.
Biochar's initially high C content, which is resistant to decomposition, enables disturbed soils to increase long-term SOC concentration and SOC stock reserves over time (Lehmann et al., 2003).Schultz et al. ( 2017) reported a 62.2-95.5 μg g À1 increase in SOC concentration following biochar application to sodic soils during reclamation.
These and our results indicate that amending suboptimal disturbed topsoil with peat or biochar can increase SOC stocks in reclaimed soils relative to the mandated TRD80, thus enhancing the restoration and rebuilding of a healthy ecosystem.Both peat and biochar are recalcitrant and therefore resist rapid microbial decomposition, supplying C and other nutrients slowly and thus sustaining soil productivity for extended periods (Baldock & Smernik, 2002;Enders et al., 2012;Hemstock et al., 2010).
Higher TKN concentrations observed in the peat-amended plots reflect the higher TKN additions from peat (18,000 mg TKN kg À1 ) in these plots compared to those that received biochar (2300 mg TKN kg À1 ).Ojekanmi and Chang (2014) reported an increase of up to 0.56% in total N concentration as the peat application rate increased during reclamation in the oil sands region of Alberta, Canada.
Similarly, Quideau et al. ( 2017) observed a sevenfold increase in total N concentration in soils amended with peat relative to those receiving forest-floor mix.They attributed the increase to N addition from the peat.
Peat has been shown to be a good source of nutrients, including N, required for plant growth during the reclamation of disturbed land (Quideau et al., 2017).Total Kjeldahl N concentrations in our study were highest in 2016 and lowest in 2019.The higher TKN concentrations in 2016 could be due to increased nutrient transformations, including mineralization and accumulation of N in the early years following topsoil and amendment placement (Carpenter & Fernandez, 2000).Litter accumulation from the vegetation in the plots may have contributed to N accumulation in the topsoil over time (Bowen et al., 2005;Reeder et al., 1998).
As N is the most limiting nutrient in disturbed boreal forest soils where native organic matter is lacking (Bradshaw, 1987), N availability is correlated with the amount of organic matter available for the mineralization process.Organic amendments containing recalcitrant organic C, such as peat, may gradually increase the supply of mineralizable N, enhancing the restoration of disturbed sites (Jamro et al., 2014).Our results are consistent with those of Moss et al. (1989), who reported a significant increase in TKN concentration in plots amended with sludge and sawdust relative to unamended plots after 5 years.Although the change in TKN occurred over a short duration (5 years) in our study, it was likely enhanced by the accelerated vegetation litter decomposition following reclamation.

| Available phosphorus, potassium, and sulfur
Reducing TRD from the mandatory 80% (TRD80) to 50% (TRD50) with or without organic amendment had no significant effect on  increase in compost rate in the 0-to 30-cm soil layer at reclaimed sites in Alberta.Although they used different amendments in their study, the use of high-P organic amendments could improve P availability in the soil.While peat and biochar treatments improved P availability across all years, the low initial P concentrations in these amendments were insufficient to produce a significant treatment effect in our study.Peat typically has low P concentration, and P availability in amended soil is improved by addition of P-based fertilizers in reclaimed soils (Rowland et al., 2009).Rowland et al. (2009) reported greater P availability in reclaimed soils that received fertilized peatmineral mix than those amended with unfertilized peat-mineral mix.
The higher MKK concentration in the BTRD50 than the PTRD50 treatment reflects the higher initial K concentration in biochar compared to peat.Additionally, the biochar pyrolysis process results in higher K concentration in the final biochar product, which increases soil K availability in the amended soil (Page- Dumroese et al., 2016).
Our findings corroborate those of Dumroese et al. (2018), who reported 4-10 times higher soluble K concentrations in soils amended with various biochars compared with peat-amended soils.Similarly, other studies have also shown increased concentrations of available K with biochar application, indicating that biochar can act as a source of K and thus improve K availability in disturbed soil (Altland & Locke, 2013;Wang et al., 2018;Zhang et al., 2014).Additionally, Dietrich and MacKenzie (2018) reported a significant increase in soil K availability in a peat-mineral mix cover soil amended with biochar in the Athabasca oil sands region of Alberta.Other studies demonstrated that biochar can be used as a source of K in K-deficient soils (Bilias et al., 2023;Dumroese et al., 2018).The lack of an organic amendment effect on available S was expected since the two amendments had similar initial S concentrations, which were low relative to initial soil S concentrations.In comparison, Dietrich and MacKenzie (2018) observed an increase in available S concentration in soils amended with a peat-mineral mix with or without biochar amendment relative to a forest-floor mineral mix without biochar.

| EC, sodium adsorption ratio, soil pH, and water-extractable cations
Organic amendment application did not significantly affect EC; however, EC increased with depth over time.The increase in EC in the LSS horizon relative to the USS in 2019 reflects the leaching of soluble salts from the overlying soil layer to this horizon over time (Rhoades, 1996).Wang et al. (2014)  Peat amendment has been shown to lower soil pH due to its acidic nature (Calver et al., 2019;Cao, 2019) and could explain the lower soil pH in plots amended with peat relative to biochar amended plots in our study.Our results corroborate those of Calver et al.
(2019) who reported significantly lower pH values in soils amended with sphagnum peat and sphagnum woody peat (4.1 and 5.0) than those amended with woody peat and woody moss herbaceous peat cover in six reclaimed sites in Athabasca Oil Sands region, Alberta.In contrast, biochar has been reported to increase soil pH due to its liming properties (Bednik et al., 2020;Ippolito et al., 2012;Ippolito et al., 2014).Another mechanism by which biochar increases soil pH is through chemical reactions between its oxygen-containing functional groups and H + ions in the soil, which results in the formation of OH groups, lowering the H + concentration and raising the soil pH (Chintala et al., 2014).The soil pH across all treatments averaged 7.2, which supports optimum growth of most plants (Sheoran et al., 2010).
Organic amendments improved σ s compared to the unamended plots.Greater σ s observed in the PTRD50 reflects the large pore volume and a high specific surface area of peat, which increased water retention capacity to a greater extent than biochar (Petelina et al., 2014;Rezanezhad et al., 2016).In situ and laboratory studies showed that increasing the ratio of peat to mineral soil from 1:3 to 3:1 increased field capacity and plant available water in reclaimed soils in the Oil Sands Region of Alberta (Moskal et al., 2001).Petelina et al. (2014) reported eight times greater water holding capacity in peatamended than in biochar-amended reclaimed soils in field trials evaluating amendments for land reclamation near Lake Athabasca in Alberta, Canada.Although σ s was significantly greater for peat than biochar, biochar increased water retention relative to the control in our study and has been shown to increase water retention properties of mineral soils due to its high pore volume and high specific area (Page- Dumroese et al., 2016Dumroese et al., , 2018;;Zimmerman et al., 2011).Ramlow et al. (2018) reported a 26% increase in soil moisture retention in soils amended with wood biochar compared to the unamended control.
Additionally, Mukherjee et al. (2014) reported a 63% increase in plantavailable water holding capacity in an oakwood biochar-amended silty loamy soil in Ohio, USA.Hardie et al. (2014) reported an increase in total porosity and saturated water content in soils amended with green waste biochar.Over time, the topsoil horizon in our study had a higher σ s than the deeper horizons, indicating the importance of sufficient pores capable of storing moisture in the topsoil relative to the subsoil (Dhar et al., 2022).Our findings indicate that amending disturbed soils with organic amendments can improve water retention and saturation capacity compared to unamended soils.

| CONCLUSION
Decreasing TRD from 80% to 50% with no amendment had minimal effects on soil chemical properties following borrow pit reclamation.Although some treatments produced higher analyte concentrations in the soil, their effects varied from year to year, with no consistent temporal trends.However, amending the suboptimal topsoil with peat and biochar significantly improved TKN, SOC, and σ s in the topsoil compared with the TRD80 and TRD50 treatments.
There was an increase in EC in the LSS horizon and SAR in the USS horizon after 5 years, indicating some level of leaching of salts down the profile over time.Nonetheless, the EC and SAR values were low and comparable to the pre-disturbance levels, therefore, not expected to have adverse effects on future vegetation establishment.Despite the topsoil depth limitation, our findings indicate that reclamation with suboptimal (50%) TRD with or without organic amendments can produce acceptable soil quality indicator levels.Importantly, the addition of peat and biochar improves soil quality relative to unamended topsoil during reclamation and provides a viable option for the reclamation of disturbed sites with limited or no access to salvaged soil volumes needed to achieve the optimal TRD80.Since soil nutrient status evolves over time, especially in boreal ecosystems, continued monitoring will provide valuable insights into the long-term effects of TRD and these organic amendments on soil properties and function.
The accumulation and decomposition of accumulated plant biomass and litter across all treatment plots explains the variation in SOC concentration and SOC stock observed 5 years following reclamation in our study.Our findings corroborate those of Bowen et al. (2005) who reported an increase in SOC concentration in reclaimed soils due to vegetation biomass accumulation over 24 year following reclamation of mined land in Wyoming, USA.Similarly, Reeder et al. (2001) attributed increases in SOC to the accumulation of vegetation biomass over time.

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I G U R E 2 Horizon by year interaction effect on (a) electrical conductivity (EC), (b) sodium adsorption ratio (SAR), (c) water-extractable Mg, and (d) water-extractable Ca following reclamation of the borrow pit.Error bars represent standard errors of the mean.Bars with the same letters are not significantly different at α = 0.05 according to the Tukey multiple comparison procedure.LSS, lower subsoil horizon; TS, topsoil horizon; USS, upper subsoil horizon.

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I G U R E 3 Effect of TRD and organic amendment treatments on (a) soil pH and (b) moisture saturation percent in the TS, USS, and LSS horizons averaged over time (2015-2019) following reclamation of a borrow pit near Cold Lake, Alberta.Error bars represent standard errors of the mean.Bars with different letters are significantly different according to the Tukey multiple comparison procedure.LSS, lower subsoil (50-100 cm); TRD, topsoil replacement depth; TS, topsoil horizon; USS, upper subsoil (bottom of topsoil horizon to 50 cm).F I G U R E 4 Moisture saturation percentage in the TS, USS, and LSS horizons over time (2015-2019) following reclamation of a borrow pit near Cold Lake, Alberta.Error bars represent standard errors of the mean.Bars with different letters are significantly different according to the Tukey multiple comparison procedure.LSS, lower subsoil; TS, topsoil horizon; USS, upper subsoil.
T A B L E 3 Changes in selected chemical properties with topsoil replacement depth and organic amendment in the reclaimed borrow pit.
Merrill et al. (2021)ease in EC in the 0-100 cm soil layer with time elapsed following reclamation.Similarly,Merrill et al. (2021)reported a decrease in EC 28 year after reclamation of a mine site in North Dakota, USA.While organic amendments had no significant effect on EC in our study, Page-