Paper mill wastes and biochar improve physiochemical properties and reduce heavy metals leaching risks in podzolic soils

Background: The incorporation of industrial wastes, such as wood ash and paper sludge, as soil amendments is vital for both environmental sustainability and agroecosystem productivity. Herein, we evaluated the effects of wood ash and paper sludge alone and in combination with biochar on the physicochemical properties and heavy metal leaching risks in podzolic soils. Methods: The treatments included limestone (control), wood ash, paper sludge, wood ash+paper sludge, limestone+biochar, wood ash+biochar, paper sludge+biochar and wood ash+paper sludge+biochar, arranged in a 4 × 2 factorial design with three replicates. The Hydrus-1D model was employed to simulate the water movement under these soil amendments using leaching colums. Results: Overall, wood ash, paper sludge and biochar application significantly increased the pH of amended soil compared to control. Paper sludge amended treatments alone or in combination with biochar significantly decreased bulk density (8%–17%) and increased the total porosity (14%–25%). While biochar addition to wood ash and paper sludge significantly reduced the concentrations of Cd (by 6.42%), Co (by 10.95%), Cu (by 11.76%), Pb (by 30%) and Ni (by 3.75%) in the collected leachates. The treatment paper sludge + biochar was found to be the most effective treatment to retain the heavy metals, with maximum plant available water (0.28 cm3 cm−3) and field capacity (0.36 cm3 cm−3) compared to control treatment. The predictions from Hydrus-1D showed that paper mill wastes with biochar has a significant potential to increase the volumetric moisture contents of amended podzolic soil, with the simulated leaching times and saturation levels closely aligning with the measured values. Conclusion: paper sludge + biochar treatment showed improved soil physicochemical properties and displayed lower heavy metals than allowed limits to be used in soil. Further, experiments are needed to assess the effects of papermill waste products on podzolic soil properties under variable field conditions.


Introduction
Crop growth and food production in boreal agroecosystems, including Newfoundland and Labrador (NL), Canada, are limited by short growing seasons, extreme weather, and poor soil conditions (Altdorff et al 2019, Nadeem et al 2019).Most soils in boreal regions are podzols, characterized by high acidity, coarse texture, and poor inherent fertility due to heavy leaching of nutrients (Maheswaran 2019).Constant applications of liming materials and fertilizers are required to achieve optimal pH and fertility in these soils.As such, the application of organic amendments such as manures, composts, and wastes byproducts from agriculture (Benito et al 2005), fisheries (Laos et al 1998), treated sewage (Gubišová et al 2020), or paper mills (Foley and Cooperband 2002) can go a long way in reducing the reliance on conventional limestone and fertilizers, which contribute to high input costs.
Pulp and paper mills produce substantial volumes of solid and liquid wastes mainly in the form of wood ash and sludge.For instance, Corner Brook Pulp and Paper ltd (CBPPL), NL, Canada, produces 10,000 Mg of wood ash and 50,000 Mg of paper sludge annually as waste byproducts.Typically, paper mill wastes (PMW) are disposed of in landfills, causing substantial economic burden for the paper industry and potential environmental risks.Due to their high carbon contents and pH, these waste products can be valuable amendments for liming or conditioning of poor soils, including podzols commonly found in boreal agroecosystems (Demeyer et al 2001, Rashid et al 2006).The recycling of PMW could improve soil quality, reduce disposal costs, and mitigates environmental risks at landfill sites (Farhain et al 2022).
On one hand, wood ash is reactive in soil, increases soil pH and lowers the exchangeable soil aluminium (Al 3+ ) (Jala and Goyal 2006, Maheswaran 2019, Johansen et al 2021).Studies have shown that wood ash positively impacts soil physicochemical properties (Cherian and Siddiqua 2019), leading to improvement in soil structure, bulk density, total porosity, and water retention (Rautaray et al 2003, Sahu et al 2017).Also, the calcium (Ca) content in wood ash can readily replace sodium (Na) at exchange sites and promote soil flocculation, aggregate stability, and water movement (Jala and Goyal 2006).On the other hand, paper sludge has substantial organic content, which typically ranges from 40% to 50% by weight (Amini and Movahedi Naeini 2013)).Paper sludge can improve soil physical, chemical, and biological properties, including water content at field capacity (Baziramakenga andSimard 2001, Farhain et al 2022) and plant available water in sandy soils (Fierro et al 1999).
Wood ash and paper sludge also contain elements, including limited amounts of essential plant nutrients, as well as heavy metals like arsenic (As), cadmium (Cd), chromium (Cr), cobalt (Co), lead (Pb), molybdenum (Mo) and nickel (Ni) (Camberato et al 2006, Maheswaran 2019, Farhain et al 2022).The main source of heavy metals in these waste products is the oil used during combustion, and contaminant levels can vary depending on the fiber source and the treatment process (Camberato et al 2006).Heavy metals are non-degradable (Pitman 2006, Farhain 2021) and can pose a threat to aquatic life (Singh et al 2011), disrupt the food chain, and contribute to various human health issues (Gupta et al 2019).To mitigate the toxic effects resulting from heavy metals contamination, it is essential to control the soluble and exchangeable fractions of metals in soils amended with wood ash and paper sludge.
Different strategies can be applied to minimize the negative effects of heavy metal contamination in agroecosystems.These include chemical oxidation, soil washing and bioremediation (Zhao 2020).Application of biochar, a carbonaceous material produced from the pyrolysis of organic feedstock, has been shown to limit the mobility and bioavailability of heavy metals (Karami et al 2011) and reduce the uptake of heavy metals by plants and leaching to groundwater (Maheswaran 2019).Biochar has a porous structure (Saha et al 2020), active functional groups (Zhang et al 2017), high pH (Vermooten et al 2019, Saha et al 2020) and high cation exchange capacity (CEC) (Park et al 2011, Zhang et al 2017) which increase its affinity for heavy metals adsorption.Some recent studies showed that biochar has a great potential to improve soil quality and physicochemical properties (Jien and Wang 2013, Moragues-Saitua et al 2017, Saha et al 2020) and remediate contaminated soils (He et al 2019, Wang et al 2021, Yuan et al 2021).When considering land application of PMW for food production, it is important to investigate potential mitigation and control strategies for preventing or minimizing the negative impacts of heavy metals.Hydrologic models can be used to investigate the effects of different organic amendments on soil quality by simulating different scenarios.This approach can improve our understanding on how these amendments alter the hydraulic properties such as water retention or water movement under real field conditions (Pal et al 2014, Iqbal et al 2020, Saha 2020).Hydrus-1D software package is a public domain hydrological model for simulating water, heat, and solute movement in unsaturated, partially saturated, and fully saturated soil profiles (Šimůnek et al 2016).The Hydrus-1D model has been used to accurately predict moisture contents using soil columns (30-60 cm depth) (Pal et al 2014) or solute transportation at various soil depths (Mo'allim et al 2018).
To our understanding, there are no studies that have assessed the effect of PMW combined with biochar on the physicochemical properties of boreal podzolic soils or heavy metal leaching potential.We hypothesize that the addition of biochar to PMW could reduce heavy metal mobility and augment the positive effects on soil properties.To test these hypotheses, we designed experiments focused on answering the following research questions: (i) to what extent do wood ash, paper sludge, and biochar change physicochemical properties of podzolic soil?(ii) what are the effects of these amendments on soil water retention curves (SWRCs)?(iii) what will be the effects of biochar on the retention of heavy metals from wood ash and paper sludge amended soil?(iv) can Hydrus-1D be employed to predict the water flow accurately in podzolic soil amended with wood ash, paper sludge and biochar?
The findings of this study are important for informing future studies under field conditions, which ultimately will be critical for developing guidelines and policy for using these waste products.Overall, amending agricultural soil with wood ash, paper sludge, together with biochar, could be a sustainable and environmentally friendly approach for recycling nutrients, enhancing soil quality, promoting crop productivity, and reducing environmental risks of alternative disposal strategies.

Materials and methods
2.1.Basic properties of soil, wood ash, and paper sludge Thirty samples of wood ash and paper sludge were collected intermittently (twice daily over a 15-day period) from CBPPL to prepare separate composite samples of both wastes.Each composite waste sample was air dried at room temperature and sieved using 2 mm mesh before mixing with soil.The paper sludge was ground using a Wiley Mill (Arthur H. Thomas Co, USA) before sieving.Bulk soil was collected at 0-15 cm depth from an agricultural field newly converted from forest land near the Wooddale Agriculture and Forestry Development Centre, Grand Falls-Windsor (49°01' 30' N and 55°33' 30' W), NL, Canada.Bulk soil was air dried at room temperature and sieved using a 4 mm sieve prior to use in experiments.
Basic chemical properties (calcium carbonate equivalency − CCE, macronutrients, micronutrients, heavy metals, total carbon and total nitrogen, mineral nitrogen) of soil, wood ash, and paper sludge were assessed using standard protocols.For this, samples were sent out to Agriculture and Food Laboratory, University of Guelph, Guelph, ON, Canada for analysis.The biochar for this experiment was purchased from Air Terra Inc. (Alberta, Canada), prepared from yellow pine wood using slow pyrolysis (500 °C, 30 min).Basic properties of biochar are given in table 3. Powder limestone used in this study was purchased from NCL Contractors ltd (Cormack, NL, Canada).

Experimental design and treatments
This research study comprised of eight treatments with two factors.Factor one was PMW at four levels (limestone (No PMW), wood ash, paper sludge, wood ash + paper sludge) and the second factor was biochar at two levels (0 Mg ha −1 and 20 Mg ha −1 ).The experimental design was completely randomized design (CRD) in factorial arrangement with three replicates.The application rates of limestone, wood ash, paper sludge and biochar in the different treatment combinations, where limestone was considered as a control treatment as it has been widely used as a liming material to ameliorate the podzolic soil in NL.

Determination of application rates of limestone, wood ash, paper sludge and biochar
Application rates of limestone, wood ash, and paper sludge were determined based on the amendment calcium carbonate equivalence (CCE) and limestone requirement of the soil to achieve a target pH of 6.3 (table 1).To optimize wood ash and paper sludge rates, a preliminary test was performed using incremental amounts (25%-100%) of the theoretical rate calculated using equation (1).

Moisture content Application rate
Area LR CCE 100 % 1 Where, CCE is the calcium carbonate equivalence and LR is the limestone requirement (Mg ha −1 ).Three replicates of soil amended with 25, 50, 75, 100, and 125% of the recommended rate were set up and soil pH was measured once every 24 h until a constant pH was achieved.The pH stabilized at 5 d, and the rates producing 6.3 ± 0.2 were selected for the study (table 1).Wood ash and paper sludge were applied at 17.30 Mg ha −1 and 55 Mg ha −1 , respectively.Blends were formulated as 4:1 mixture of wood ash and paper sludge (13.8 + 11 Mg ha −1 , respectively).Biochar was applied at 20 Mg ha −1 (Moragues-Saitua et al 2017), Treatments with biochar were included as a preemptive measure to reduce bioavailability and mobility of potential heavy metal and organic contaminants.
All the treatment mixtures were prepared by adding the soil with their respective amendment rates (table 2) and thoroughly mixed.The moisture factor for each treatment mixture was estimated by oven drying three samples from each treatment.All the treatment mixtures were stored in plastic polyethylene bags to keep the moisture contents constant throughout the study period at the Boreal Ecosystems Research Facility (BERF), Grenfell Campus, Memorial, Memorial University of Newfoundland before conducting different lab experiments.Sub samples were obtained from these stored mixtures when measuring physicochemical properties and carrying out the leaching column experiment.

pH and electrical conductivity
A portable pH/EC/TDS/temperature meter (HANNA-HI9813-6 with CAL Check, ON, Canada) was used to measure pH and EC for all treatments using 1:2 soil to deionized water ratios (15 g air dried sample: 30 ml deionized water) according to Brady and Weil (2008).

Cation exchange capacity and soil organic matter
For cation exchange capacity (CEC) and soil organic matter (SOM) measurement, samples from all treatment mixtures were sent to the Soil, Plant and Feed Laboratory of the Government of NL, St. John's, NL, Canada, where rapid method of exchangeable bases used for CEC measurement (Hajek et al 1972) and the function of loss on ignition method was used to measure the SOM (Donald and Harnish 1993).

Bulk density
Disturbed dry bulk density was measured by dividing the dry mass of the sample over the stainless-steel core (5 cm diameter × 5 cm height) volume (88.6 cm 3 ) (Kim et al 2017, Farhain et al 2022).Each core was filled using the respective air-dried sample mixture to 1/3 of the core height at a time and tapped 3-4 times to settle the filled treatment mixture by removing extra macro pores.The procedure was repeated until the entire core was filled and air-dried mass was estimated by subtracting the empty core mass by the total mass.The dry mass was calculated by dividing the filled mass by the respective moisture factor.
2.5.Soil water retention curve 2.5.1.Total porosity A sandbox apparatus (Acc.To ISO 11274, art no.0801, Eijkelkamp, Giesbeek, Netherland) was used to record the porosity (0 kPa) of all treatments by the saturation method.Stainless steel cores used for bulk density measurement were refilled with treatment mixtures and saturated through capillarity in the sand box for three days.The total porosity was calculated as saturation weight by using equation (2) at 0 kPa (Saha et al 2020,  ) were used to record the data from 10 kPa to 700 kPa.Samples were packed in small plastic rings having volume of 21.53 cm 3 on their respective bulk density (similar to bulk density of sand box samples).All samples were saturated in a plastic tub, fully saturated samples were placed in the pressure plate extractor and 10 kPa pressure was applied.When the water discharge stopped from outlet tube, samples were weighted, and gravimetric moisture content was calculated.The same procedure was followed to record data subsequently for 30,40,50,100,200,300,400,500,600, and 700 kPa pressure levels.The field capacity was calculated at 30 kPa by using equation (3) (Glab et al 2016, Saha et al 2020, Farhain et al 2022).
Where, FC−field capacity, W d −drained sample weight at 30 kPa, W o −dried sample weight and V t −the total sample volume (plastic ring volume).

Plant available water
Calculated gravimetric moisture contents were converted to volumetric moisture contents (VMC: θ) by multiplying the respective bulk density.The estimated VMC from 0 kPa to 700 kPa and pressure (ψ) were fitted to the van Genuchten model (equation ( 4)) (van Genuchten 1980).The best fit parameters (α and n) for the van Genuchten model were obtained by using the solver function of MS excel using measured data.Then, VMC values from 800 kPa to permanent wilting point at 1500 kPa were estimated by using the van Genuchten equation and the estimated best fit parameters (0 kPa to 700 kPa).The plant available water was calculated as the difference between VMC at field capacity and permanent wilting point as given in equation (5) (Glab et al 2016, Saha et al 2020, Farhain et al 2022).
Where, PAW−plant available water, θ FC − VMC at field capacity, and θ PWP − VMC at permanent wilting point.

Leaching experiment
A leaching column experiment was conducted to evaluate the mobility and leaching potential of the heavy metals from wood ash, paper sludge and biochar amended soil.This experiment was organized under laboratory conditions at room temperature at the BERF, Grenfell Campus, Memorial University of Newfoundland.The above-mentioned treatments (table 2) with three replicates were arranged under CRD design in a custom built leaching column setup.The transparent polymethyl methacrylate pipes having a length of 30 cm and radius of 2.25 cm were used in this experiment.Treatment mixtures were filled up to 25 cm depth and a 5 cm head space was kept for adding water.The soil columns for each treatment were filled with amended soil at their respective bulk density (used in sand box and pressure plate experiments) by making different layers to ensuring a uniform density as much as possible in the entire column up to 25 cm depth.A total of 24 leaching columns were prepared (8 treatments × 3 replications).Four leaching events were carried out and a one-week interval was maintained between leaching events.Each column was flushed with 254 mL of deionized water for each leaching event with the total amount of 1016 mL deionized water for 4 leaching events.These 4 leaching events were equivalent to 586.19 mm rainfall corresponding to total average rainfall during the growing season (May-Nov) in the study area (soils were collected) calculated using 30 years of data (https://climate.weather.gc.ca, accessed on 15-Dec-2021).In the leaching column experiment, the flux rate of 3.66 cm h −1 was maintained by adding 63.55 mL of deionized water per 15 min based on the average seasonal rainfall, leaching column surface area and leaching experiment time.

Heavy metals measurement in leachate
After each leaching event, the leachates were collected in plastic bottles and concentrated HNO 3 (Catalog No. A509P212, Fisher Scientific, Ottawa, Ontario, Canada) was added to bring the leachate pH to 2, and to facilitate mineral digestion for heavy metal analysis.All the leachate samples were analyzed for As, Cd, Cr, Cu, Pb, Mo and Ni using Inductively Coupled Plasma Mass Spectrometry (Thermo Scientific CAP Q ICP-MS) according to established methods in our research program (Zaeem et al 2021).The concentration of each heavy metal leached out was calculated by using the leachate volume of respective leaching event.

Simulation study of the leaching column experiment
The Hydrus-1D model (Saha 2020) was used for the simulation of VMC from 0-4 h by keeping the leaching column depth of 25 cm and a flux rate of −3.66 cm h −1 ('-' sign represents downward movement of water in Hydrus-1D), as used in heavy metals leaching experiment for all treatments.In simulated leaching columns, the VMC for different treatments were predicted on basis of water movement from surface to bottom as described by modified Richard equation (equation ( 6)) (Zheng et al 2017, Iqbal et al 2020, Saha 2020).
Where, h−Water pressure head (cm), θ−Volumetric water content (cm 3 cm −3 ), t−Time (hours), z−maximum soil depth (cm), x−Spatial coordinate (cm), k−Unsaturated hydraulic conductivity (cm day −1 ), S − Source/ Sink term of the flow equation.The Hydrus-1D model results from time to start leachate was compared with actual time to start leachate from respective leaching column experiment units.

Statistical analysis
To quantify the effects of different combinations of PMW and biochar on physicochemical properties and heavy metals leaching ability from amended podzolic soil, two-way analyses of variance (ANOVA) were performed.Normality of the data set was checked by running Shapiro-Wilkes test.Fisher's least significant difference (LSD) was used to compare the treatments means at alpha = 0.05.Statistix 10.0 version (Analytical software, Tallahassee FL 32317, USA) was used for statistical analysis and graphical visualization was done through MS excel 2016.Coefficient of determination (R 2 ) and root mean square error (RMSE) (equation ( 7)) were computed to check the accuracy of the van Genuchten model predicted values for SWRCs and RMSE and Relative Error (RE) (equation ( 8)) were used to evaluate the precision of Hydrus-1D predicted values by using measured values of leaching column experiment quantitatively.
Where, E t −the experimental value at the time t, S t −the simulated value at the same time, and n −number of observations.

Properties of soil, wood ash, paper sludge and biochar used in experiments
The basic physicochemical properties of soil, wood ash, paper sludge, and biochar are given in table 3.According to the USDA soil classification system, soil is classified as loam texture with sand = 50% (±2), clay = 17.38% (±1.15) and silt = 32.62%(±1.15).

Heavy metals
Heavy metals concentration in wood ash and paper sludge composite samples and their comparison with Canadian Council and Ministers of Environment (CCME) compost category A and B guidelines and biosolids limits are given in table 4.
We observed that measured heavy metals concentrations in both wood ash and paper sludge are below the compost category A except for Mo and below the limits developed for the biosolids application and compost category B (table 4).Therefore, wood ash and paper sludge can be used in agriculture soils under these two categories with extra precautions when deemed necessary by the end users.

pH
The PMW and biochar application significantly (p < 0.001 & p < 0.05, respectively) increased the pH of amended soil; however, their interaction did not show significant (p = 0.207) effects on soil pH (table 5).All the treatments had a pH in the desired range (6.0 − 6.5), where wood ash was found to be the most responsive to increase the pH.As such, the highest pH was observed in wood ash + paper sludge (6.5) and the lowest (6.0) in control (limestone) treatment.

Electrical conductivity
The PMW and biochar application significantly (p < 0.001 & p < 0.001, respectively) increased the EC of amended soils, but their interaction did not show significant (p = 0.815) effects on EC of amended soil (table 5).
Treatments that had wood ash showed higher EC in comparison to paper sludge treatments.The treatment wood ash + biochar showed the highest EC (0.23 dS m −1 ) followed by the lowest of 0.05 dS m −1 for control (limestone) among all treatments.

Cation exchange capacity
The PMW individually showed a significant effect (p < 0.001) in increasing the CEC of amended soil, but biochar addition as a factor reduced the CEC of amended soil, though its effect was not significant (p = 0.619).
The interaction effect was not significant (p = 0.815) to change the CEC of amended soil (table 5).Treatments comprising wood ash have higher CEC compared to paper sludge treatments similar to the effects on EC.The CEC increased in wood ash treatment by 84.2% compared to control (limestone), which was the highest among all treatments and the lowest was found in control (limestone) that is found to be statistically at par with paper sludge treatment.

Soil organic matter
The PMW and biochar showed a significant (p < 0.001 & p < 0.05) effect to increase the SOM of amended soil, where paper sludge application showed the highest increase of 88.93% compared to the control (limestone), while biochar addition significantly increased (p < 0.05) the SOM only by 9.30% compared to the non-amended biochar treatments.The PMW and biochar interaction did not show a significant (p = 0.849) effect on SOM contents (table 5).Paper sludge amended treatments showed higher SOM contents compared to wood ash combinations as expected.The highest SOM contents were observed in paper sludge + biochar (3.55%) among all treatment combinations where, it increased by 57.1% compared to control (limestone), which showed the lowest increased percentage of 2.26.

Bulk density
The PMW and biochar showed a significant (p < 0.001 & p < 0.001) effect to reduce the bulk density of amended soil.The biochar addition significantly (p < 0.001) reduced the bulk density by 6.45% compared to treatments without biochar.The PMW and biochar interaction effect was non-significant (p = 0.068) on the bulk density (table 5).Overall, paper sludge treatments have significantly lowered bulk densities compared to wood ash and control (limestone) treatments.Control (limestone) treatment has the highest bulk density (1.29 g cm −3 ) and paper sludge with combination of biochar (paper sludge + biochar) had the lowest bulk density (1.07 g cm −3 ) which was 17.1% lower than the control (limestone).

Total porosity
The PMW and biochar showed significant (p < 0.001 & p < 0.001) effect on total porosity of amended soil and their interaction did not show a significant (p = 0.3524) effect on changing the total porosity (table 5).However, paper sludge treatment alone and with combination of biochar showed higher total porosity compared to wood ash and limestone treatments.A combination (paper sludge + biochar) showed the highest total porosity (0.68 cm 3 cm −3 ), which was increased by 25.9% compared to control and the lowest total porosity (0.54 cm 3 cm −3 ) was observed in limestone.

Field capacity
The application of PMW and biochar significantly increased the field capacity level of amended soil (p < 0.001 & p < 0.05, respectively) as shown in table 5. Specifically, paper sludge-amended treatments increased the field capacity by 10.34% compared to limestone, while biochar application increased it by 14.28% compared to unamended treatments.Moreover, the interaction effect between PMW and biochar (p < 0.05) was significant, resulting in a further increase in the field capacity level (table 5).Notably, the maximum field capacity was recorded in the paper sludge + biochar treatment (0.36 cm 3 cm −3 ), which was 24.1% greater than control (limestone) (figure 1).Conversely, the field capacity in the wood ash treatment decreased by 3.4% compared to the control (limestone), which was found to be statistically at par with the wood ash + biochar and paper sludge treatment (figure 1).

Plant available water
Both PMW and biochar significantly (p < 0.001 & p < 0.05, respectively) increased the plant available water of the amended soil (table 5).The paper sludge treatment had the highest effect, increasing the plant available water by 14.28% compared to the control (limestone).The biochar amended treatments exhibited a 15% higher plant available water compared to treatments without biochar.Additionally, the interaction between PMW and biochar had a significant (p < 0.05) effect on plant available water of the amended soil (table 5).The paper sludge + biochar treatment significantly increased the plant available water by 40% compared to the control (limestone).On the other hand, the lowest plant available water (0.20 cm 3 cm −3 ) was observed in the control (limestone) (figure 2), which was statistically comparable to treatments involving limestone + biochar, wood ash, and wood ash + paper sludge treatments (figure 1).

Soil water retention curves
The developed SWRCs illustrated in figure 2 elucidated important hydrological parameters (including total porosity, field capacity, plant available water, permanent wilting point) for treatments amended with wood ash, paper sludge and biochar (figure 3).The comparison between measured and predicted values for all treatments spanned from saturation to 700 kPa.We observed that wood ash, paper sludge and biochar applications significantly (p<0.05)increased the water retention characteristics of amended soil (figure 3).The predicted values obtained by using the van Genuchten model from 0 kPa to 700 kPa were very close to the measured values with high R 2 and very low RMSE values (table 6).

Heavy metals concentration in leachate
The PMW showed a significant effect on Cd, Mo and Ni concentrations, while no significant effect was observed for concentrations of As, Cr, Co, Cu and Pb in the leachates collected (table 7).The biochar application significantly reduced the concentrations of Cd (by 6.42%), Co (by 10.95%), Cu (by 11.76%), Pb (by 30%) and Ni (by 3.75%) in the collected leachates.The biochar application also reduced the concentration of As (by 2.83%), Cr (by 2.73%) and Mo (by 0.91%), though these differences were not significant from biochar unamended treatments (table 7).The interaction between PMW and biochar significantly reduced the concentration of Pb and Ni, yet did not show significance in affecting the levels of As, Cd, Cr, Co in the collected leachate (table 7).Among all treatments, wood ash treatment exhibited the highest concentration of Cd and Mo, while paper sludge treatment showed the highest concentration of As, Cu and Pb.Additionally, the combination of wood ash + paper sludge had the highest concentration of Cr and Co in the collected leachates.The treatment paper sludge + biochar showed lowest concentration (5.18 μg L −1 ) of Pb (figure 4) and treatment wood ash + paper sludge and + biochar showed lowest concentration (6.88 μg L −1 ) of Ni (figure 5) among all treatments.In the first leaching event, heavy metals concentrations in the collected leachate were notably low, with no significant difference observed between treatments.However, during the subsequent second and third leaching events, concentrations of Cd, Cu, Pb and Ni increased, revealing clear differences among the various treatments by the  third leaching event.The application of water resulted in a concentration pulse in the leachates during the experiment, followed by subsequent decreases, as expected (a clear breakthrough).In the leaching experiment, we observed a consistent increase in Mo and Ni concentrations with each leaching event, indicating a prolonged period needed to reach peak concentrations.

Summary of heavy metals in leachate and their comparison with different water quality guidelines
According to the observed results, the addition of biochar enhanced the retention of heavy metals within the soil profile during different leaching events.The cumulative concentrations of all heavy metals in the leachate from biochar-amended treatments were significantly lower than treatments without biochar, except for As, Cr and Mo.However, even in the biochar-amended treatments, concentrations of As, Cr and Mo were lower compared to the unamended treatments.The total leached heavy metal concentration was compared with different water quality guidelines established by CCME (1993CCME ( −2019)), such as the Canadian Drinking Water Quality Guidelines, and Water Quality Guidelines for the Protection of Agriculture (Irrigation and livestock).Our findings indicate that the total leached concentration of each heavy metal remained below the specific quality standards for Figure 3.Effect of wood ash, paper sludge and biochar on porosity (cm 3 cm −3 ), FC-Field capacity (cm 3 cm −3 ), DW-Drainable water (cm 3 cm −3 ), PWP-Permanent wilting point (cm 3 cm −3 ) and PAW-Plant available water (cm 3 cm −3 ); LS-Limestone; WA-Wood ash; PS-Paper sludge; WAPS-Wood ash + paper sludge; LSBC-Limestone + Biochar; WABC-Wood ash + Biochar; PSBC-Paper sludge + Biochar; WA+PSBC-Wood ash + paper sludge + Biochar.
Treatment θ s (cm 3 cm −3 ) θ r (cm    drinking water and quality limits for the protection of agriculture production (table 8).This suggests that leaching of heavy metals from the soil amended with wood ash and paper sludge used in our study is unlikely to cause groundwater contamination.

Simulated moisture contents
The simulated results using Hydrus-1D showed that the application of wood ash, paper sludge and biochar to podzolic soil significantly increased the VMC from 0-25 cm depth with a constant flux of -3.66 cm h −1 .The maximum simulated VMC, observed in the paper sludge + biochar treatment (0.49 cm 3 cm −3 ), was 14% greater than the control (limestone), while the wood ash treatment had the lowest simulated VMC, similar to that of limestone (table 9).

Effect of paper mill wastes and biochar on time to leachate
The impact of PMW effect was statistically significant (p < 0.05), whereas the addition of biochar showed not significant effect (p = 0.180) on the time to start leachate (figure 6).The paper sludge treatment from PMW showed the longest time to obtain leachate and biochar addition further prolonged this time compared to treatments biochar.The interaction between PMW and biochar interaction did not show a significant (p = 0.1805) effect on leaching time (figure 6), although the combination of paper sludge + biochar showed the longest duration before leachate initiation.All the treatments have very low RMSE and RE % values which shows higher accuracy of simulated values (table 10).

Discussion
This experiment helped to determine the optimum application rates of wood ash, paper sludge and biochar during field application to enhance the physicochemical properties and reduce the heavy metals leaching potential of amended podzolic soil.Wood ash amended treatments increased the pH and EC followed by paper sludge and limestone (table 5).Biochar addition enhanced the pH and EC significantly compared to no biochar amended treatments (table 5).Cherian and Siddiqua (2019) reported that wood ash from pulp and paper industry has the potential to increase the pH of amended soil due to its buffering capacity.Higher amounts of calcium carbonates, hydroxides and other calcium comprising minerals were attributed to this effect (Moilanen andIssakainen 2000, Scheepers andBen 2016).A significant number of oxides, carbonates, hydroxides and silicates are present in boiler wood ash which is responsible for the induced liming effect on forest land (Moilanen and Issakainen 2000).These compounds present in wood ash undergo a series of chemical reactions where it reacts with water molecules and segregate to generate OH -, HCO 3 -, Ca 2+ , K + and Na + .The OH -or HCO 3 -nullifies the H + and increases the OH -in soil system which increases the soil pH (Cherian and Siddiqua 2019).The results of this study are in line with the findings of Bang-Andreasen et al (2017) who conducted experiments with varying wood ash application rates (0-167 Mg ha −1 ) to measure the pH induced effect on soil bacterial number and community composition and concluded that wood ash application increased the pH and EC of amended podzolic soil compared to control.Authors noted that amended soil pH increased from acidic through neutral at 22 t ha −1 to alkaline at 167 t ha −1 .Additionally, they observed bacterial population dramatically increased up to a wood ash dose of 22 t ha −1 followed by significant decrease at 167 t ha −1 .Similar results were found by Javed (2021) through greenhouse experiments conducted to measure the effect of wood ash, paper sludge and biochar on growth, quality and heavy metals uptake behavior in annual rye grass and kale and reported that wood ash and paper sludge application increase the pH of amended soil by 0.6 and 0.5 respectively.
In an incubation study, where loamy sand soil was amended with wood ash and commercial limestone on a CCE basis showed that pH of amended soil was higher than the values reported from agricultural limestone treatment (Muse and Mitchell 1995).Wood ash application rate has a positive correlation with pH and EC (Farhain et al 2022).The change in pH and EC are also responsible for the soil nutrients bioavailability because of pH dependent soil chemical equilibria (Demeyer et al 2001).Wood ash is highly reactive and have potential to alter several physicochemical properties of forest soils (Saarsalmi andLevula 2007, Karltun et al 2008).Therefore, wood ash addition leads to increase in pH, EC and the concentration of different nutrient elements such as K, S, B, Mg, Ca, Si and P (Pitman 2006, Augusto et al 2008).In the conducted study, we observed that the addition of biochar improved the pH and EC of the amended soil and similar findings regarding the effect of biochar were reported by Jien and Wang (2013) and Saha et al (2020).Paper sludge alone and with combination of biochar increased the pH by 0.4 units and increased the EC by 0.16 units alone and 0.15 unit as combination compared to limestone (table 5).This is attributed to its higher CCE, pH and interaction of minerals with soil particles (table 3).These results are in line with the findings of Torkashvand et al (2010) who conducted a pot experiment to investigate the effect of paper sludge as a soil amendment in an acid soil and observed that paper sludge amendment at 0, 0.5,1, 2 and 4% (weight basis) rates increased the pH of the amended soil due to high CCE (58.4%) and pH (13.2) of paper sludge.Furthermore, the authors reported that the increase in pH was directly proportional to the application rate which occurred concomitant with an increase in the EC compared to the control treatment.Similar results have been reported by Johansen et al (2021), where they observed significant rise in pH and EC of wood ash amended organic soil.Paper sludge application to agricultural land has been recognized as liming substance with high potential because of its neutral to alkaline pH and carbonates (Turner et al 2022).Other studies have stated the same results for paper sludge addition to raise the soil pH (Shipitalo andBonta 2008, Méndez et al 2009).
The PMW showed a significant effect on CEC of amended soil while biochar addition did not show any significant difference from biochar unamended treatments (table 5), additionally wood ash and wood ash + paper sludge have higher CEC values compared to limestone treatments and paper sludge treatments (table 5) because of higher ratio of exchangeable bases (table 3).These results are in line with findings of Gómez-Rey et al (2012), who reported that wood ash from eucalyptus bark used in pulp and paper mill have higher exchangeable bases (Ca, Mg and K) which significantly increased the CEC (0.69 − 1.43 cmol + kg −1 ) compared to other treatments in topsoil.Chantigny et al (2000) conducted field trials following application of paper sludge at 100 Mg ha −1 to measure its effect on soil active carbon pools and enzyme activities in amended soils and noticed that SOM increased in silty clay loam and loam soil up to 15 cm depth and the observed effect was still significant after 3 years of application.Similarly, an increase in SOM was observed at 16 Mg ha −1 during the application season of paper sludge (Simard et al 1998).Wood ash, paper sludge and biochar used in this experiment have bulk density of <1 g cm −3 .We observed PMW have significant effect on porosity and bulk density, where paper sludge treatment increased the porosity by reducing bulk density of amended soil, similarly biochar addition in conducted study significantly increased the porosity by lowering the bulk density.Our results showed similarity with Chow et al (2003) findings, where they conducted field research and applied paper sludge at 160 Mg ha −1 to loam texture podzol and observed that bulk density decreased by 17% and improved total porosity and hydraulic conductivity three times compared to control (without paper sludge).Similar results reported by Wanniarachchi et al (2019) and Saha et al (2020) where they conducted some lab experiments to measure the effect of biochar on podzolic soil hydraulic properties and found that granular biochar significantly reduced the bulk density and improved the total porosity of podzolic soil.Wood ash treatment did not show any effect on total porosity and plant available water in our study, although the field capacity decreased compared to the control (table 5).The wood ash application rate was very low, and it has higher bulk density, thus it did not significantly change the bulk density, porosity and plant available water compared to control (limestone) (table 5).Both factors i.e.PMW and biochar significantly changed the shape of SWRCs (figure 2).The developed SWRCs showed the ability of wood ash, paper sludge and biochar to increase the VMC at field capacity and plant available water (figure 1).We observed in the conducted study that wood ash decreased the mesoporosity, mean pore diameter and effected the total porosity of amended plots and their SWRCs.These results are in line with findings of Moragues-Saitua et al (2017) on Pinus radiata, where wood ash was applied at 4.5 Mg ha −1 and revealed that after 15 months there was no significant difference observed between wood ash amended plots and control (without wood ash) with respect to mean pore diameter and total porosity of loamy texture soil.Total plant available water always depends upon organic matter, pore sizes, soil structure and aggregate stability (Amini and Movahedi Naeini 2013).The paper sludge application increased the VMC at field capacity and plant available water compared to wood ash, limestone and biochar (table 5, figure 1).Increase in VMC could be due to increase in porosity because of lower bulk density (Amini and Movahedi Naeini 2013).Water retention properties of paper sludge amended soil enhanced at higher rate of paper sludge incorporation by increasing the total porosity, potentially high micro pore spaces, due to paper sludge fibers positioned between soil particles (Farhain et al 2022).The organic matter content absorbs and holds high amount of VMC at soil water tension less than 1500 kPa (Amini and Movahedi Naeini 2013).Our study findings were similar to the results reported by Zhang et al (1993), where authors applied paper sludge to sandy textured soil at 246 Mg ha −1 as a soil additive or amendment for alfalfa and bluegrass crop growth and observed increase in VMC by 20 and 74% at −33 kPa and −1500 kPa, respectively.Trépanier et al (1996) reported that paper sludge has water holding capacity of 0.36 cm 3 cm −3 and 0.26 cm 3 cm −3 at −33 kPa and −1500 kPa pressure, respectively, which was found to be greater than that of most mineral soils.Increase in total plant available water has more significant effect than field capacity to influence the crop growth.The magnitude of decrease in bulk density and increase in water holding capacity and soil aggregation in fine sandy loam soil depend upon the rate and frequency or application interval of paper sludge (Zibilske et al 2000).The paper sludge used in all experiments possesses a fibrous structure with a very low bulk density 0.12 g cm −3 (table 3).This characteristic attribute enhanced the micro porosity while decreasing the drainable water in the amended soil (Farhain et al 2022).As a result, the VMC at field capacity increased in the soil amended with paper sludge.The results of the conducted study are in line with findings of Foley and Cooperband (2002) where, they conducted some vegetable rotation experiments on loamy sand with the application of solid PMW and observed the VMC increased at −33 kPa and there was no significant effect at −1500 kPa.However, after second application, authors observed paper mill residue treatments retained from 16% to 45% greater VMC than control and the percentage of VMC at −1500 kPa was from 2% to 50% greater in amended plots compared to control.
Heavy metals (As, Cd, Cr, Co, Pb, Mo and Ni) are non-biodegradable and have some adverse impacts on soil system and plant metabolism and lower the agricultural output.These elements might spread to adjacent environments through leaching to groundwater or surface water (Puga et al 2016).They persist for a long period in contaminated soils and it is costly to remove these heavy metals from soil system.Furthermore, if their quantities exceed the permitted CCME limits, then their bioaccumulation into fruits, vegetables, and forages may have a negative impact on humans and animals' health (Khan et al 2019).The presence of heavy metals in wood ash and paper sludge can restrict their application as agricultural soil amendments because of these negative consequences.In order to diminish the harmful effects of heavy metals and prevent them from being absorbed by plants, it is essential to control the soluble and exchangeable fractions of these heavy metals in soils.Organic amendment like biochar addition along with these PMW may be reliable way to limit the bioavailability of these heavy metals (Karami et al 2011).
In experiments we conducted, it was observed that biochar addition significantly reduced the concentration of different heavy metals (Cd, Co, Cu, Pb and Ni) in the leachate (table 7).There was no significant difference observed between biochar mended and unamended treatments for As, Cr and Mo, although the concentration of these heavy metals were lower in biochar amended treatments leachates compared to unamended treatments (table 7).The accumulation of these heavy metals might be due to rise in pH by enhancement of the metal's retention on the soil surface (Maheswaran 2019).The application of biochar with wood ash and paper sludge absorbed the heavy metals and reduced its concentration in leachates.Biochar decreased the As concentration in collected leachate, although the difference between biochar unamended treatment was not significant (table 7).The biochar has carbonized fractions (CO 3 2-and PO 4 3- ) which can interact with soil contaminants react as adsorbents to reduce their bioavailability.Especially, the level of O-containing carboxyl, hydroxyl, and phenolic functional groups in biochar have strong effect to bind the soil contaminants (Uchimiya et al 2011).
The biochar addition significantly affected the heavy metals concentration in leachates by reducing the total leachate volumes because of increase in water retention in soil columns (table 8).Higher water holding capacity of biochar relative to other organic materials correlates with higher surface area and higher micropore volume (Uchimiya et al 2010a, Ashiq and Vithanage 2020).These characteristics of biochar showed its potential as an environment sorbent for contaminants in soil and water (Maheswaran 2019).The pH changes have a strong effect on immobilization of different heavy metals, as the PMW and biochar have strong liming effect which can promote the mobilization of oxyanions and immobilization of different heavy metals (Almaroai et al 2013, Farhain 2021).Li et al (2016) observed that Cd is a divalent cation, and its sorption behavior is same with Pb, which depends on the types of feedstocks and pyrolysis process.Increasing pH can affect the precipitation of Cd and Pb (Almaroai et al 2013).The minimum pH ranges are 8.8 − 9.8 and 6.1 − 9.1 for Cd and Pb hydroxides precipitation respectively from soil system (Maheswaran 2019).However, the pH range for the soil used in leaching experiments was 6.0 − 6.5 (table 3).Li et al (2017) reported that non-electrostatic mechanisms are dominant factors for Pb sorption in soil system.The Cr leaching is highly dependent on dissolution of Cr carrying oxide and hydroxide.Its solubility is highly controlled by Cr 2 O 3 and Cr (OH) 3 (Komonweeraket et al 2015).The biochar application to mineral soil can reduce the leaching potential of Cr through redox reaction with metals.Chicken manure biochar application to Cr contaminated soil reduced the mobility of Cr (VI) by converting into Cr (III), thus decreased the leaching of Cr from amended soil (Choppala et al 2015).The decrease in Cr concentration in leachate was attributed to adsorption on cation exchange sites and precipitation in form of Cr (OH) 3 in resultant of reduction process of Cr (VI) by releasing the OH -ion (Choppala et al 2015).During different leaching events, heavy metals have different leaching behavior.For most heavy metals, paper sludge + biochar and wood ash + paper sludge + biochar treatments had lower concentrations of heavy metals among all the amended treatments.The PMW as a factor did not show significant difference in Co and Cu concentration in leachate, although biochar addition significantly reduced their concentration during different leaching events (table 7).Oxidation-reduction reactions and acid-base properties of the heavy metals always affect their mobility in soil profile (Hayyat et al 2016).Sometime Cd, Cu and Cr move through the soil pore water (Hayyat et al 2016).Biochar produced from crop straws have more developed pores compared to wood char, since wood char has more lignin contents (Hayyat et al 2016).Uchimiya et al (2010b) conducted some experiments on immobilization of Cu, Cd, Pb and Ni in water and soil and reported that litter derived biochar significantly adsorbs Cu, Cd, Pb and Ni and deceptive that tendency of the exclusion order was Ni < Cd < Cu < Pb.
During different leaching events in the present study, the concentration of heavy metals was higher in nonamended biochar treatments compared to biochar amended treatments (table 7).These findings are consistent with the results of Maheswaran (2019), who conducted laboratory experiments to measure the effect of biochar on leaching and bioavailability of heavy metals on wood ash amended podzolic soil.The author observed that biochar had significant effect in reducing the heavy metals concentration in the leachate and it reduced the bioavailability and plant uptake of Cu, Co, Ni, Cr, Ni, Mo, and Pb. Ozolincius et al (2005) assessed the fertilization effect of wood ash and collected leachates and observed Cd, Cr, Cu and Pb concentrations for all treatments were below the detection limit of the equipment.In our experiment, we observed that leached heavy metals concentrations were quite low in comparison to the quality limits developed by CCME for irrigation water, livestock purpose and drinking water (table 8).In greenhouse experiments conducted by Javed (2021), it was reported that paper sludge alone and with wood ash application increased the Ni, Cd and Pb concentrations in annual ryegrass and kale grown pot soils; though, these values were very below the CCME permitted limits for biosolid application.The author reported that biochar addition decreased Pb concentration by 16% in annual ryegrass shoot whereas, Ni and As concentration by 31% and 65%, respectively in kale shoot.These results suggest that wood ash and paper sludge application with biochar has demonstrated agronomic benefits to both crops and therefore they could be used as a substitute source for liming and nutrients in the agriculture industry.However, before any recommendation to use these PMW as liming agent, we have to conduct different control and field conditions experiment in different soil types.
In this study, wood ash, paper sludge and biochar significantly affected simulated VMC at saturation level (table 9).Application of PMW significantly influenced the time to leachate but biochar did not show significant effect on time to leachate; although, its addition increased the time to leachate and improved the hydrological properties (figure 3).Our study results showed strong agreements between Hydrus-1D simulated and measured leaching time resulting low RMSE and RE % values.Saha (2020) conducted some lab experiments to measure the effect of biochar on nitrogen transport and hydraulic properties of podzolic soil and reported that Hydrus-1D simulated values for time to leachate were close to experimental values.However, variability in hydrological processes was observed by Altdorff et al (2019), where the bottom flux rate significantly decreased with increasing biochar rate.Different researchers have reported that Hydrus-1D simulated values were reliable and reasonable to measure the solute transport (Mo'allim et al 2018).The Hydrus-1D model was found to be an effective tool to estimate the water flow for rainfed conditions for different amount of precipitation and evaporation conditions (Iqbal et al 2020).A high coefficient of determination (R 2 ) and very low RMSE and RE % was observed in our experiment between measured and simulated time to leachate consistent with the findings of Negm et al (2017) and Pal et al (2014).This suggests that simulation of hydrological processes to be accurate and that this model can be used to predict the water flow in various soils amended with different organic amendments (Negm et al 2017, Saha 2020).However, the assessment of leaching potential of heavy metals under field condition is important accompanied by variable environmental factors associated with utilization of wood ash and paper sludge.The optimal amounts should be determined before every application of wood ash and paper sludge to minimize the risk of heavy metals, or these soils should be evaluated annually before and after application of wood ash and paper sludge to verify that heavy metal concentrations are within safe limits.To get a complete picture of the role of biochar in reducing the mobilization of heavy metals; there is a need to conduct long-term field experiments with various crops and agronomic practices on different soils with different biochar types and rates in boreal ecosystem.
The way forward: • Need to develop application guidelines for wood ash and paper sludge as liming material or soil amendment to replace the conventional limestone for agricultural and forest lands under provincial and territorial jurisdiction depending on the climate or ecosystem.Once the application guidelines have been developed in the province, this practice will be beneficial to amend low pH and poorly fertile soils with PMW as well as it significantly reduces the land disposal cost for the paper mill industry.
• Need to conduct comprehensive, multilocational field experiments on the effects of wood ash, paper sludge and biochar on the physicochemical properties of different soils, crop growth and development.This would help to analyze the sustainability of agricultural practices and the environment.
• Further simulation of hydrological properties using the Hydrus-1D model would be helpful to develop suitable approach that could protect the environment from heavy metals contamination and other nutrients leaching by finalizing the application rates of different soil amendments.

Conclusion
Wood ash, paper sludge and biochar application to podzolic soil significantly affected the pH, electrical conductivity, soil organic matter, cation exchange capacity, bulk density, total porosity, field capacity, plant available water and heavy metals leaching.All the treatment combinations tested have optimum pH (6.0 − 6.5) for plant growth, where wood ash and paper sludge had higher effects on increasing the pH and EC compared to limestone (table 5).The paper sludge had a higher effect on improving hydrological properties compared to wood ash and limestone due to its fibrous structure and very low bulk density (figures 1 and 2).Biochar addition significantly improves the physicochemical properties of amended soil (table 5).The treatment paper sludge + biochar was found to be the most effective treatment to improve the pH, electical conductivity, soil organic matter, total porosity, field capacity and plant available water with minimum bulk density among all treatments evaluated in this study.Biochar application has significant effect on reducing the concentration of Cd, Co, Cu, Pb and Ni in leachate from podzols (table 7).It also reduced the concentration of As, Cr and Mo in the leachate, but its effect was not significant (table 7).In general, the study findings suggest that wood ash, paper sludge and biochar have the potential to improve the physicochemical properties of podzolic soil (table 5) and the biochar application at levels used in this study was effective in reducing the heavy metals (table 7) concentration in leachate from amended podzols.Hydrus-1D model prediction showed PMW with biochar has a significant potential to increase the volumetric moisture contents of amended podzolic soil (table 9) and, it also showed simulated leaching time and saturation level similar to measured values (figure 7).For further investigation, microscopic studies need to be done with wood ash, paper sludge and biochar to assess the effect on their structural properties and functional groups which may have prominent role on heavy metals, nutrients adsorption and their mobilization in soil system.
appreciatively acknowledge Dr. Tao Yuan, Lab Coordinator for his valued contribution to conduct the leaching experiment and measuring heavy metals using the ICP-MS.

Table 2 .
Treatments with application rates of lime, wood ash, paper sludge and biochar.Where, W s −saturated sample weight, W 0 −the dried sample weight and V t −the total sample volume.

Table 3 .
Physicochemical properties of soil, wood ash, paper sludge and biochar used in the study.

Table 4 .
Comparison of heavy metals in wood ash, paper sludge and soil with different application guidelines.

Table 5 .
Effect of paper mill wastes and biochar on selected physicochemical properties of podzolic soil.

Table 7 .
Effect of paper mill wastes and biochar on heavy metal leaching from soil material collected from podzolic soil.

Table 8 .
Total leached out heavy metal concentration in leachates comparison with different water quality guidelines.

Table 9 .
Simulated moisture contents and time to saturation under different treatments using Hydrus-1D.

Table 10 .
Hydrus-1D validation of time to start of leachate.