The study was conducted in Moro Local Government Area of Kwara State in North-Central Nigeria in Figure 1. It is situated around latitude N8.94225˚, longitude E4.77804˚, and an altitude of about 300 m above sea level. The area lies within the guinea savanna ecoregion with marked seasonality. It is characterized by a mean annual rainfall of 1200 mm with a wet season that spans April to October and a dry season from November to March. It has a warm average annual temperature of 26.2˚C rising to a peak of 30˚C in March [16] [17] [18] . A reconnaissance conducted in August 2019 as part of this study revealed evidence of various activities in the charcoal value-chain across the study area including kilns, charcoal storage depots, retail points, and haulage. Hence, charcoal business is an important economic activity in the study area.

A total of 19 kilns were purposefully selected from five administrative districts within the study area (Table 1) between July and September 2020. The inclusion criteria include kilns not older two months [11] as well as security accessibility to a sampling point due to the recent ban on charcoal production and activities of the task force set up by the State Government [19] [20] . Locals involved in charcoal production activities were recruited in this regard. Soil core samples were collected using a soil auger from a depth of 0 - 20 cm at each sampling point. The soil cores were taken in duplicates from each kiln (coded CKS for charcoal kiln soil) and from an adjacent control site (coded ACS for adjacent

control soil) about 5 - 15 cm from the edge of the reference kiln. Therefore, 38 CKS along with complimentary 38 ACS were collected in polythene bags and transported to the laboratory.

The soil samples were air-dried at the laboratory and sieved through a 2 mm mesh before analysis to determine the physicochemical properties of the soil samples including soil particle size, bulk density, pH, electrical conductivity, extractable acidity, total nitrogen, available phosphorus, total organic carbon, organic matter, calcium ion, magnesium ion, sodium ion, potassium ion, cation exchange capacity, and heavy metals (copper, manganese, iron, lead, chromium, cadmium).

An improved hydrometer method was used to determine the percent distribution of sand, silt, and clay particles in the soil samples [21] [22] . Soil pH was determined by an electrometric method using a 1:2.5 soil to water ratio, wherein 20 g of soil was added to 50 ml of distilled water and the value measured with the electrode of a pH meter [22] [23] . Electrical conductivity was determined by adding 100 ml distilled water to 20 g of the soil samples and measured using a conductivity meter [24] . Extractable acidity was determined by centrifuging a mixture of the soil sample with BaCl₂-TEA buffer solution then an aliquot of the supernatant solution was titrated with acid [25] . Total nitrogen was determined using the micro-Kjeldahl procedure. Available phosphorous was determined using the Olsen method. Total organic carbon was determined with the Walkey-Black wet oxidation method [25] .

To determine exchangeable Ca²⁺, Mg²⁺, K⁺, Na⁺ and effective cation exchange capacity (CEC) in soil, 30 ml of 1 N NH₃OAC was added to 5 g of the soil sample and was shaken using a mechanical shaker for 2 hrs. The solution was then centrifuged carefully at 2.00 rpm for 5 - 10 mins and the supernatant was carefully decanted into a 100 mL volumetric flask. Another 30 mL of NH₄OAC solution was added and the flask was shaken for 30 minutes. It was then centrifuged, and the supernatant was transferred into the same volumetric flask. The step was repeated thrice, and the supernatants were transferred into the same volumetric flask which was used to mark up with the NH₄OAC solution. The concentrations of these cations were determined using flame photometer and atomic absorption spectrophotometer [25] [26] [27] .

A wet digestion method was used to prepare soil samples for determining the concentration of heavy metals (Cu, Mn, Pb, Fe, Cr, and Cd) using atomic absorption spectrophotometry. A mixture of 1 g soil sample, 5 ml concentrated HCl and 15 ml HNO₃ was heated on a hot plate in a fume-hood at a temperature between 50˚C - 60˚C until the brownish fume color was expelled. The mixture was allowed to cool at room temperature then 5 ml of distilled water added. The resulting mixture was filtered into a clean plastic container using Whatman filter paper and made up to 50 ml in a standard flask with distilled water. Each digested sample was transferred into plastic containers for heavy metal analysis using a BUCK Scientific ACCUSYS 211 Atomic Absorption Spectrophotometer.

The difference in the soil parameters between CKS and ACS was assessed using the Student’s t test with the critical significance level (α) set at 0.05. The relative change in each soil parameter between both site categories was determined using the formula below.

Relative Change ( % ) = P c − P a P a × 100 % (1)

where P_c and P_a are the CKS and ACS soil parameter, respectively.

The variation of the physical properties of the soil at ACS and CKS are presented in Figure 2, while Table 2 outlines the mean ± standard error of the mean, relative change, and statistical difference of these properties. Although the silt and clay fractions of the soil were not significantly different between both sites, they exhibited slight decrease of 3.25% and 1.03% at the charcoal kiln sites, respectively. However, the sand fraction increased by 1.59% at the charcoal sites. These findings are consistent with those of previous researchers [11] [28] . The marked increased in soil temperatures due to charcoal production might have led to the fusion of silt and clay particles into sand-sized ones [29] [30] . This coarsening process may adversely affect the water holding capacity of the severely heated soil surface [31] .

There was also no significant difference in the soil bulk density, however, it slightly reduced by 0.68% at the charcoal kiln sites. Nigussie and Kissi suggest that this could be due to the complex pore structure of charcoal residues left on the kiln site as well as the increased sand fraction at these sites as discussed above [28] .

Soil pH exhibited a very high significant difference (p < 0.001) between the charcoal kiln sites and adjoining control sites (Table 3). The soil in the study area

ns = not significant.

is typically neutral, however, charcoal production appears to have increased the pH by 9.12% in agreement with the findings of Nigussie and Kissi, and Chima et al. [32] [33] . The ash residue at the charcoal kilns has been suggested to be responsible for this increase in pH. Furthermore, the porous nature of charcoal increases exchange of bases (cation exchange capacity) of soils. Thereby improving the possibility of Al and Fe to bind with the exchange site [34] [35] .

The electrical conductivity of the soil exhibited a high significant difference (p < 0.01), increasing by about 17.8% at the charcoal kiln sites relative to the adjacent control sites. This is also indicative of the presence of ash, which is known to have abundance of exchangeable cations. Similar patterns of difference in soil electrical conductivity have been reported by previous researchers [11] [32] .

Figure 3, extractable acidity showed a very high significant difference (p < 0.001) between the charcoal kiln sites and the adjacent control sites. In variance to pH and electrical conductivity, the extractable acidity of the soil reduced by about 24% at the charcoal kiln sites. Nigusiie and Kissi [32] reported a significant negative correlation between extractable acidity and pH/electrical conductivity.

Charcoal production did not significantly affect the soil organic carbon, organic matter, total nitrogen, carbon to nitrogen ratio, and available phosphorus relative to the adjacent control sites. Ogundele et al. reported similar findings for organic carbon and nitrogen in a nearby ecosystem [36] . However, in Figure 4, the observed increase in carbon, nitrogen and organic matter in soils directly beneath the kilns may be due to charcoal residue. Similarly, the increase in available phosphorus by 19.9% in the charcoal kiln soil may be due to wood ash that is characterized by high phosphorus content [28] (Table 4).

Charcoal production significantly affected the cation exchange capacity (p < 0.001), increasing its value by 40.11% relative to the adjacent control sites. This

TOC = Total Organic Carbon, OM = Organic Matter, TN = Total Nitrogen, AvP = Available Phosphorus, C/N = Carbon to Nitrogen ratio, ns = not significant.

conforms with the findings and suggestion of Nigussie and Kissi that this increase could be due to charcoal residue and other charred matter at the kiln sites [32] . Addition of biochar to soil has also been shown to increase cation exchange capacity [37] .

In Figure 5, two of the exchangeable bases (Ca and Mg) significantly increased (p < 0.001) at the charcoal kiln sites by 57.15% and 89.16%, respectively. However, Na slightly increased by 3.30% while K reduced by 0.29%. The release of minerals has been correlated with the deposition of ash due to its richness in basic cations [38] . Awodun et al., Kishor et al., and Nigussie and Kissi all reported a significant increase in exchangeable bases at their respective burn sites [32] [39] [40] . Furthermore, our non-significant findings on exchangeable Na and K are similar to those of Ogundele et al. [36] . This could be due to the proximity of our study locations (Table 5).

Most of the heavy metals investigated did not significantly change between both sites. Only Fe exhibited a significant (p < 0.001) reduction of 28.54% at the charcoal kiln sites. Mn and Cr also slightly reduced by 16.28% and 6.35%, respectively. Lehmann [41] explained that the bioavailability of heavy metals is reduced when COO, OH and other functional groups on the surface of biochar form complexes with heavy metals in Figure 6. Pb and Cd were largely below the detection limits of the methods used in this study. This could be because the study areas are rural with minimal industrial activities (Table 6).

CEC = Cation Exchange Capacity, ns = not significant.

ND = Not Detected, NA = Not Applicable, ns = not significant.