Testing Biochar’s Ability to Moderate Extremely Acidic Soils in Tea-Growing Areas

Abstract

Biochar as a by-product of the carbonization of biomass has an inherent potential to modify acidic soils due to its alkaline nature. To explore the mechanism and effectiveness of biochar, a case study was conducted on severely acidic soils from six fields under tea cultivation in a subtropical zone, comparing rice husk biochars, in three rates (B5, B10, B15 t ha⁻¹), and CaCO₃ as conventional liming practice. The results showed increases in pH of 71.5%, 52.7%, 30.6%, and 29.7% in B15, B10, B5, and CaCO₃-treated soils compared to the control. On average, B15 and B10 treatments resulted in the highest organic matter with 12.3% and 9.7%, respectively. B15, B10, B5, and CaCO₃ caused increases of 196.6%, 173.4%, 129.7%, and 100.9% in base saturation compared to the control, respectively. Also, after the application of B15, B10, and B5 treatments, the effective cation exchange capacity increased by 191.4%, 112.1%, and 39.5%; however, the application of CaCO₃ resulted in a 20.1% decrease. Overall, applying biochar on acidic soils provides adequate negative charges due to its well-extended specific surface area and pore volume, which cause the absorption of additional Al⁺, resulting in ameliorating soil pH. The application of proper biochar could notably be more effective in improving acidic soils than conventional practices such as the overuse of CaCO₃. In this regard, evaluating various biochars in terms of feedstock, pyrolysis conditions, and modification scenarios merits in-depth research in future studies.

1. Introduction

One of the primary issues with the excessive use of chemical fertilizers in farms, especially those situated in wet regions, is the gradual acidity of the soil [1]. Low yields of crops planted in such acid soils are a result of the unavailability, high expense, and poor quality of typical liming substances [2]. One challenging method of managing soil to preserve soil quality and productivity is keeping the soil acidity at a suitable level. Globally, about 3950 million hectares (ha) of arable lands are suffering from acidic pH: 58% of agricultural lands in Asia, 31% of Latin America, and 56% of Africa and North America have a pH of lower than 5.5. At that level of acidity, just 5% of common agricultural products are able to grow [3]. The mismanagement of soil during agricultural practices such as the overuse of chemical fertilization, wastewater irrigation, and intensive tillage are the main reasons for soil pH instabilities [4].

Today, tea is being cultivated as one of the popular beverages with more than 5 million ha in about 66 countries of the world [5]. Typically, regions with altitudes up to 2100 m above sea level, warm, humid climates with 1000 mm of annual precipitation, and acidic soil with a coarse texture are suitable areas for tea growing [6,7]. The majority of the soils in tea-growing regions are Alfisols, Ultisols, and Oxisols, which are typically deficient in primary minerals and bases such as Ca and Mg [8,9,10]. Due to severe weathering and a lot of precipitation, the base saturation in tea land areas is low [11,12]. Heavy precipitation has caused bases to be washed away, resulting in Al³⁺ and its species as well as H⁺ dominating the exchange sites of the soils [13]. Low P and Mo concentrations in the soil solution make acidic soils naturally less nutritious [14]. The most important effects of soil acidification are the dissolution of minerals containing iron, magnesium, and aluminum as well as the substitution of exchangeable base cations such as Ca²⁺, Mg²⁺, and K⁺ by H⁺ and Al³⁺ [15]. Moreover, the most noticeable sign of Al toxicity and one of its primary consequences is the restriction of plant root growth [16]. Overexposure to Al inhibits the development of expanded root tips, root hair formation and cell division [17]. In addition, the detrimental presence of Al prevents plants from absorbing nutrients and water [18].

Tea is among the plants that perform better in acidic soils with a pH between 4.5 and 5.5 [19]. Soil pH is a key factor that affects the absorption of nutrients and their ability to be used by plants. Some of the negative impacts of high pH include Al and manganese toxicity as well as calcium, molybdenum, and boron insufficiency [15]. The presence of one milligram of Al per kilogram of soil limits or stops the growth of plants. The roots of the plants become short and thick in such soils due to the high concentration of dissolved Al and manganese underground, and their growth slows down as a result of insufficient soil penetration [20]. Even the existence of such a circumstance supports the development of the nematode that causes tea root wound disease and reduces the quantitative and qualitative efficiency of the plant [19,21].

The maintenance of soil fertility in the past three decades in tea lands has been controlled using inorganic fertilizers, mainly sulfate of ammonia. However, the oxidation of a mole of NH₄⁺ applied to soil leads to the production of two moles of H⁺. As a result, the use of ammonium sulfate may have played a role in most of the soils’ acidic pH < 4.0. Regarding these challenges, managing the tea plantations’ acidic soils in rain-fed regions is a critical issue to avoid changing the consistency of the soil [1,14,22]. The use of lime is recommended as one of the solutions to overcome acidic soil, while the use of agricultural lime by farmers is challenging due to high haulage costs [23]. To boost the productivity of the soils and crops, farmers who own such acidic farms must be given a viable alternative. Any substitute material for liming acid soils that farmers could find appealing should be reasonably affordable, easily accessible, and ideally located close to the farms.

Given that biochar has inherent alkali properties, using it in agricultural acidic soils could be a possibility [24]. A carbon-rich substance called biochar is produced by pyrolyzing biomass with limited oxygen [25]. As a result of its carbon skeleton structure, biochar is chemically and physiologically more stable in soil than directly adding uncharred material, which has the same carbon equivalent [26]. Based on the type of feedstock and pyrolysis condition, biochar may contain a variety of chemically reactive functional groups and components, which give the substance its high capacity for adsorbing harmful elements like aluminum and manganese from acidic soils [27,28,29,30]. Al(OH)₃ and Al(OH)₄⁻ are examples of less hazardous Al species that can be produced as a result of biochar’s reaction between oxygen functional groups on the surface of biochar and free Al³⁺ in soil [31]. The type of feedstock, pyrolysis temperature, surface area of the biochar, soil characteristics, and regional environmental conditions all play a role in determining the variety and degree of reactions, which takes place in between biochar and the soil’s physiochemical components [25,32].

The plain regions of northern Iran, between the Caspian Sea and the Alborz Mountain range, as well as along the Hyrcanian woodlands, have historically been used for tea planting due to their ideal climate. Unfortunately, the overuse of chemical fertilizers in these areas has resulted in serious acidity and degradation problems in recent years. The continuous use of acid-forming chemical fertilizers such as ammonium phosphate and ammonium sulfate, along with additional influences including excessive precipitation, leaching, and acidic parent materials such as granite, has caused the pH of the soil in most of these lands to significantly decrease and reach about 3.5, which inexorably impacted the health of plants. On the other hand, rice husk is one of the agricultural by-products that is prevalent in literally all northern Iran. The efficiency of rice husk biochar for the purpose of soil pH modification has been considered in some studies [33,34]. The observed increase in soil pH is usually between 1 and 2 units for the rice husk biochar application rates of 3 to 12 t ha⁻¹ [35,36]. In general, while the silica functional groups (Si–OH, Si–O–Si, and Si–H) are dominant in rice husk biochar produced at low pyrolysis temperatures, aliphatic and aromatic C–H functional groups can significantly increase with increasing pyrolysis temperatures [35]. However, rice husk biochar usually maintains the primary skeleton of rice husk with a very porous structure, high surface area and therefore high potential for the absorption of cations [37,38]. RHB usually has an alkaline pH ranging from 7.1 to 10.8, which depends on the pyrolysis temperature [35]. That is a very important characteristic for soil amendment to be used in acidic soils.

Due to the alkaline nature of biochar, especially the potential of rice husk originated biochar in the modification of acidic environments, it is hypothesized that biochar utilization could be considered as an alternative solution for conventional liming practices. However, the application of rice husk biochar as a liming material on acid tea lands in humid, subtropical settings is still mostly unknown. Additionally, to the best of our knowledge, biochar’s remediation capability in northern Iran’s acidic soils has not yet been studied in comparison with the use of CaCO₃ as a traditional liming practice. To effectively ameliorate extremely acidic soils, it is thus important to fill in these gaps. Therefore, the purpose of this study was to determine how effective biochar can be as a liming material on various types of acidic tea-growing regions compared with conventional CaCO₃ application.

2. Materials and Methods

2.1. Study Area Description

Soil samples were collected from 6 separate tea fields from different cities along the Alborz Mountains. The distance from the first sampling point to the last point was about 230 km (Figure 1). The geographical directions were as follows: Fooman (37°05′24.2″ N 49°12′49.6″ E), Rasht (37°05′24.2″ N 49°12’49.6″ E), Lahijan (37°10′48.3″ N 50°02′06.4″ E), Amlash (37°01′05.3″ N 50°02’59.1″ E), Chaboksar (36°57′04.3″ N 50°32′39.3″ E), and Shahsawar (36°45′08.0″ N 50°48’37.4″ E).

Table 1. Environmental description of the study areas.

Property	Fooman	Rasht	Lahijan	Amlash	Chaboksar	Shahsavar
General climate	Humid subtropical	Humid subtropical	Humid subtropical	Humid subtropical	Mediterranean	Mediterranean
Average annual temperature (°C)	19.82 ± 1.34 *	19.41 ± 1.47	18.46 ± 2.45	18.91 ± 0.97	19.33 ± 0.23	20.41 ± 1.23
Average annual precipitation (mm)	110.2 ± 5.64	107.6 ± 2.75	102.4 ± 6.89	89.95 ± 4.53	51.71 ± 9.54	54.58 ± 3.45
Rainy days per year	130.1 ± 1.42	127.1 ± 1.28	120.9 ± 1.37	116.8 ± 1.93	79.89 ± 1.81	84.31 ± 1.21
Relative humidity (%)	74.83 ± 2.43	73.33 ± 3.73	69.82 ± 1.21	68.43 ± 1.75	62.41 ± 3.85	65.85 ± 6.15
Monthly sunshine (h−1)	11.02 ± 1.2	10.75 ± 0.68	10.23 ± 0.94	10.61 ± 0.83	10.43 ± 0.82	11.01 ± 1.34
Parent material	Granite	Granite	Granite	Granite	Phyllite	Phyllite
Clay minerals	Mica, Montmorillonite	Quartz, Mica	Mica, Feldspar	Feldspar, Montmorillonite	Quartz, Chlorite	Chlorite, Sericite mica
Soil taxonomy	Typic Hapludalfs	Typic Hapludalfs	Ultic Hapludalfs	Inceptic Hapludalfs	Typic Dystrudepts	Typic Udorthents
Sand:Silt:Clay (%)	78.3:14.2:7.5	57.1:38.2:4.7	61.4:10.3:28.3	50.1:39.6:10.3	55.7:10.1:34.2	44.6:24.2:31.2
Soil texture	Loamy sand	Sandy clay	Sandy loam	Sandy clay	Sandy loam	Loam

* Standard deviation.

provides some environmental descriptions of the study areas. The average area of the selected tea fields was 30 ha. Random disturbed samples of the plow layer (0–20 cm) of the soils were taken, bulked, homogenized, and sub-sampled for routine chemical characterization and pot experiments. Subsequently, 6 soil samples representing 6 different tea fields were considered for physical and chemical tests. The disturbed samples were air-dried, and a portion was ground and passed through a 2 mm sieve for a three-month incubation pot experiment from 1 May to 30 July 2020. The pots were 35 cm in height by 30 cm wide with a capacity of 6 kg soil.

2.2. Biochar Preparation and Characterization

Air-dried rice husks obtained from farmers’ fields were carbonized in a furnace at a pyrolysis temperature of 500 °C with a heating rate of 10 °C min⁻¹ and residence time of 2 h. The results of some physicochemical analysis of produced biochar are presented in

Table 2. Physicochemical properties of used rice husk biochar.

Property	Value	Property	Value
pH	8.91	O/C	0.15
EC (dS m−1)	0.28	H/C	0.61
CEC (cmol(c+) kg−1)	68.2	BY (%)	40.9
Ca2+Ex (cmol(c+) kg−1)	5.24	MC (%)	0.22
Mg2+Ex (cmol(c+) kg−1)	7.15	VM (%)	30.7
K+Ex (cmol(c+) kg−1)	4.18	AC (%)	17.8
Na+Ex (cmol(c+) kg−1)	1.15	FC (%)	51.3
PAv (mg g−1)	3.76	SSA (m2 g−1)	151.8
C (%)	63.4	BD (g cm−3)	0.23
H (%)	3.24	Vt (cm3 g−1)	0.118
N (%)	3.15	Vmicro (cm3 g−1)	0.067
O (%)	12.4	Vmeso & macro (cm3 g−1)	0.051

EC: electric conductivity, CEC: cation exchange capacity, Ca²⁺_Ex: exchangeable calcium, Mg²⁺_Ex: exchangeable magnesium, K⁺_Ex: exchangeable potassium, Na⁺_Ex: exchangeable sodium, P_Av: available phosphorous, C: carbon, H: hydrogen, N: nitrogen, O: oxygen, BY: biochar yield, MC: moisture content, VM: volatile matter, AC: ash content, FC: fixed carbon, SSA: specific surface area, BD: bulk density, V_t: total pore volume, V_micro: micro-pores, V_meso: meso-pores, and V_macro: macro-pores.

and Figure 2. The pH and EC of biochar were determined using a 1:10 (w/v) ratio suspension of biochar and deionized water [39]. The Brunner, Emmett, and Teller (BET) method was used to calculate the specific surface area of biochar. Biochar bulk density and porosity were measured via mercury porosimetry (AutoPore IV 9500 M, Micromeritics, Norcross, Georgia). To analyze the status of functional groups on the surface of biochar samples, Fourier transform infrared spectroscopy (FTIR) was conducted in the wavenumber region of 400–4000 cm⁻¹ by using an FTIR Spectrum (PerkinElmer, MA, USA) (Figure 2). The ammonium acetate technique was used to determine the cation exchange capacity (CEC) [40]. Ammonium extractable bases, i.e., Ca²⁺, Mg²⁺, K⁺ and Na⁺ in the biochar were extracted sequentially with deionized water and 1 M ammonium acetate, respectively, and the concentrations were read on a Perkin Elmer Analyst 800 atomic absorption spectrometer. Carbon (C), hydrogen (H), and nitrogen (N) were measured by the elemental analyzer (Perkin Elmer 2400 II). The oxygen (O) content of the produced biochar was calculated by subtracting CHN and ash content from 100% biochar.

The biochar yield (BY) was calculated using the initial weight of feedstock and the immediate weight of produced biochar, as shown in Equation (1). The moisture content (MC) of biochar was obtained by weighing the biochar samples before and after drying in an oven at 105 °C for 24 h using Equation (2). The ash content (AC) and volatile matter (VM) of biochars were measured using an electric muffle furnace. To obtain the ash content, 1.5 g of oven-dried biochar samples were weighed into a pre-weighed crucible and heated in air at 600 °C for 6 h. After combustion, the residues were weighed, and the ash content was calculated according to Equation (3). For volatile matter determination, 8 mg of oven-dried biochar samples were burned at 950 °C for 30 min under a nitrogen atmosphere, and then the weight of the remaining mass after combustion was used for volatile matter calculation based on Equation (4). Fixed carbon (FC) was calculated by subtracting the percentages of moisture, ash, and volatile matter from the initial biochar weight, as shown in Equation (5).

$$\mathrm{B}\mathrm{Y}(\mathrm{\%})=[\frac{\mathrm{F}\mathrm{e}\mathrm{e}\mathrm{d}\mathrm{s}\mathrm{t}\mathrm{o}\mathrm{c}\mathrm{k} \mathrm{w}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}\left(\mathrm{g}\right)-\mathrm{B}\mathrm{i}\mathrm{o}\mathrm{c}\mathrm{h}\mathrm{a}\mathrm{r} \mathrm{w}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t} \left(\mathrm{g}\right)}{\mathrm{B}\mathrm{i}\mathrm{o}\mathrm{c}\mathrm{h}\mathrm{a}\mathrm{r} \mathrm{w}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t} \left(\mathrm{g}\right)}]\times 100$$

(1)

$$\mathrm{M}\mathrm{C}(\mathrm{\%})=[\frac{\mathrm{I}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l} \mathrm{b}\mathrm{i}\mathrm{o}\mathrm{c}\mathrm{h}\mathrm{a}\mathrm{r}\left(\mathrm{g}\right)-\mathrm{O}\mathrm{v}\mathrm{e}\mathrm{n} \mathrm{d}\mathrm{r}\mathrm{i}\mathrm{e}\mathrm{d} \mathrm{b}\mathrm{i}\mathrm{o}\mathrm{c}\mathrm{h}\mathrm{a}\mathrm{r} \mathrm{a}\mathrm{t} 105 °\mathrm{C} \left(\mathrm{g}\right)}{\mathrm{O}\mathrm{v}\mathrm{e}\mathrm{n} \mathrm{d}\mathrm{r}\mathrm{i}\mathrm{e}\mathrm{d} \mathrm{b}\mathrm{i}\mathrm{o}\mathrm{c}\mathrm{h}\mathrm{a}\mathrm{r} \mathrm{a}\mathrm{t} 105 °\mathrm{C} \left(\mathrm{g}\right)}]\times 100$$

(2)

$$AC\left(\%\right)=[\frac{Ovendriedbiocharat105°C\left(g\right)-Combustedbiocharat600°C\left(g\right)}{Combustedbiocharat600°C\left(g\right)}]\times 100$$

(3)

$$VM\left(\%\right)=[\frac{Ovendriedbiocharat105°C-Combustedbiocharat950°C\left(g\right)}{Combustedbiocharat950°C\left(g\right)}]\times 100$$

(4)

$$FC\left(\%\right)=100-[Moisturecontent\left(\%\right)+Volatilematter\left(\%\right)+Ashcontent(\%\left)\right]$$

(5)

2.3. Experimental Design

Each soil sample was combined with biochar at three rates of 5 t ha⁻¹ (equal to 12.1 g pot⁻¹), 10 t ha⁻¹ (equal to 24.2 g pot⁻¹), and 15 t ha⁻¹ (equal to 36.3 g pot⁻¹). Also, a lime treatment with an amount of 5 t ha⁻¹ (equal to 12.1 g pot⁻¹) carbonate calcium (CaCO₃) with a purity of 99% was considered as a conventional liming approach in which the amount and type of lime were based on the common practice in local tea gardens. A treatment with no amendment was also considered as the control (CK). Therefore, the experiment was conducted using six soils, five treatments (CK, B5, B10, B15, and CaCO₃), and three replications, in a completely randomized design. The moisture contents of the soils in the pots were kept at 80% field capacity in the greenhouse. The average temperature during the period of incubation was between 27 and 32 °C.

2.4. Soil Characteristics

Soil samples were analyzed at the end of the experiment by the following methods: soil pH and electrical conductivity (EC) in a 1:1 (w:v) by soil-to-water ratio; soil texture by hydrometer; organic carbon (OC) by wet oxidation [41]; available N (NO₃⁻ and NH₄⁺) were determined using a continuous flow analyzer (SEAL Analytical, Norderstedt, Germany) with 1 M KCL extracts [42]; available P was determined using the sodium bicarbonate method [43]. Soil K was determined using the flame photometer method [44]; exchangeable basic cations were analyzed using a 5:50 ratio of soil:ammonium acetate (NH₄Oac)-buffered solution at pH 7, in which the basic cations adsorbed in soil were replaced by NH₄⁺ ions [45] and measured by a spectroscope (ICP-OES, PerkinElmer). The pH values of soil solutions were determined through KCl extraction and were measured potentiometrically; titratable and exchangeable acidity (the total of H⁺ and Al³⁺ ions), by titration with 0.05 M NaOH solution to pH 8.2; and total alkalinity, by titration with 0.01 M H₂SO₄ solution to pH 4.4 [46]. Also, the effective cation exchange capacity (ECEC) was determined by summation of the respective exchangeable bases and exchangeable acidities.

2.5. Data Analysis

A one-way analysis of variance (ANOVA) was performed to examine the significance of differences in soil parameters among treatments. Lowercase letters in the figures indicate statistically significant differences after the least significant difference (LSD) test at p < 0.05. SPSS 22.0 was used to analyze the data. Origin 2022 and Excel 2020 were used to create all the figures.

3. Results

3.1. Changes in Soil pH and Cations

The results of changes in cations are presented in

Table 3. Changes in soil cations’ status under different treatments at the end of the experiment.

Sites	Treatments	pH	EC (dS m−1)	OC (%)	Ex-Base Cations (cmol(c+) kg−1)				Ex-Acid Cations (cmol(c+) kg−1)		Av-N Content (mg kg−1)
					Ca2+	Mg2+	K+	Na+	H+	Al3+	NH4+	NO3−
Fooman	CK	3.52 d	0.35 b	2.49 d	1.11 d	0.57 d	0.23 d	0.75 c	3.020 a	1.82 a	5.32 a	8.54 d
	B5	4.39 c	0.37 b	4.76 b	2.29 c	4.98 c	2.65 c	0.77 c	0.407 b	1.65 a	4.27 b	12.85 c
	B10	5.39 b	0.41 a	6.57 ab	8.87 b	7.58 b	4.96 b	0.92 b	0.041 c	1.26 b	3.24 c	13.45 b
	B15	6.28 a	0.42 a	7.33 a	10.2 a	12.3 a	5.24 a	1.12 a	0.005 d	0.57 c	2.19 d	16.42 a
	CaCO3	4.49 c	0.39 a	2.99 cd	2.58 c	0.42 d	0.25 d	0.18 d	0.324 b	1.09 b	5.38 a	8.34 d
Rasht	CK	4.01 c	0.22 b	1.85 e	0.67 e	0.69 d	0.36 d	0.83 b	0.977 a	1.46 a	6.36 a	7.15 c
	B5	4.72 b	0.28 ab	3.65 c	5.72 c	3.83 c	1.41 c	0.91 b	0.191 b	1.42 a	5.54 b	10.72 b
	B10	5.64 a	0.29 ab	5.35 b	10.3 b	6.23 b	2.87 b	0.87 b	0.023 c	0.98 c	5.11 b	11.14 b
	B15	6.22 a	0.32 a	7.21 a	13.6 a	9.72 a	4.92 a	1.03 a	0.006 d	0.84 c	3.47 c	15.53 a
	CaCO3	4.71 b	0.31 a	2.62 d	2.46 d	0.59 d	0.39 d	0.84 b	0.195 b	1.02 b	6.21 a	6.72 c
Lahijan	CK	3.16 c	0.49 b	1.44 e	1.01 d	1.12 d	0.68 d	0.35 c	6.918 a	1.78 a	7.95 a	9.09 d
	B5	5.01 b	0.51 b	3.01 c	2.42 c	1.56 c	1.13 c	0.42 b	0.098 c	1.74 a	6.98 ab	11.45 c
	B10	5.41 a	0.53 ab	4.85 b	4.13 b	4.83 b	3.22 b	0.46 b	0.039 d	1.56 b	5.47 b	13.73 b
	B15	5.97 a	0.56 a	6.92 a	7.29 a	9.21 a	5.58 a	0.51 a	0.011 e	1.02 c	3.32 c	15.47 a
	CaCO3	4.47 bc	0.43 c	2.81 d	2.63 c	1.07 d	0.76 d	0.39 bc	0.339 b	1.39 b	7.54 a	8.17 d
Amlash	CK	3.29 d	0.36 b	2.35 e	1.12 e	0.42 d	0.28 c	0.62 b	5.129 a	1.48 a	6.25 a	7.44 d
	B5	4.35 c	0.39 ab	4.55 c	4.35 c	1.84 c	0.31 c	0.95 a	0.447 b	1.01 b	5.37 b	9.75 c
	B10	5.42 b	0.41 a	6.28 b	6.84 b	3.53 b	1.94 b	0.91 a	0.038 d	0.86 c	3.62 c	12.42 b
	B15	6.38 a	0.43 a	7.79 a	10.7 a	6.35 a	3.34 a	0.96 a	0.004 e	0.51 d	2.06 d	14.71 a
	CaCO3	4.81 c	0.42 a	3.55 d	2.31 d	0.62 d	0.29 c	0.76 b	0.155 c	0.91 b	5.89 a	6.02 d
Chaboksar	CK	4.21 c	0.69 b	2.19 d	0.69 e	1.29 d	0.56 c	0.21 c	0.617 a	1.69 a	6.69 a	7.46 d
	B5	5.59 b	0.71 ab	3.01 c	1.16 d	4.15 c	0.64 b	0.32 b	0.026 c	1.58 b	6.28 a	10.43 c
	B10	6.24 a	0.73 a	5.44 b	3.23 b	6.29 b	0.94 a	0.36 ab	0.006 d	1.55 b	4.37 b	13.76 b
	B15	6.66 a	0.76 a	6.28 a	7.16 a	8.82 a	0.99 a	0.41 a	0.002 d	1.23 bc	2.12 c	16.28 a
	CaCO3	5.32 b	0.71 ab	2.35 cd	2.24 c	1.32 d	0.54 b	0.23 c	0.048 b	1.12 c	6.05 a	7.11 d
Shahsavar	CK	3.97 c	0.37 b	2.53 d	0.86 e	1.02 c	0.87 c	0.43 c	1.072 a	1.41 a	6.78 a	7.08 a
	B5	4.68 b	0.39 b	3.89 c	5.35 c	1.52 b	0.89 c	0.44 c	0.209 b	1.02 b	5.44 b	8.74 b
	B10	5.49 a	0.42 a	5.31 b	7.84 b	1.97 ab	0.95 b	0.59 b	0.032 d	0.71 c	3.63 c	12.63 c
	B15	6.13 a	0.43 a	7.21 a	11.5 a	2.19 a	1.26 a	0.73 a	0.007 e	0.39 d	2.12 d	14.32 d
	CaCO3	4.73 b	0.38 b	3.35 cd	2.24 d	1.09 c	0.92 b	0.42 c	0.186 c	0.65 c	6.33 a	7.13 a

EC: electric conductivity, OC: organic carbon, Ca²⁺_Ex: exchangeable calcium, Mg²⁺_Ex: exchangeable magnesium, K⁺_Ex: exchangeable potassium, Na⁺_Ex: exchangeable sodium, H⁺: hydrogen, Al³⁺: aluminum, NH₄⁺: ammonium, NO₃⁻: nitrate. Significant differences between treatments are shown in lower case letters (LSD test, p < 0.05).

. All treatments significantly increased pH values in all soil sites. The lowest pH was related to Lahijan soil with a value of 3.16, in which B15 treatment caused an 88.9% increase (5.97), which was significantly higher than the other treatments. Meanwhile, the application of CaCO₃ in the same soil increased the pH to 4.81, with a 41.4% increase rather than control. Also, the increase in biochar application rates significantly increased the pH in all sites. Based on the average of changes from all sites, increases of 30.6%, 52.7%, 71.5%, and 29.7% were obtained from B5, B10, B15, and CaCO₃, respectively. In the case of EC, the application of B5 did not show a significant effect; however, an increase in biochar rates slightly caused an increase in EC values. On average, B15, B10, and CaCO₃ significantly increased EC values by 20.9%, 15.1%, and 10.4% compared to the control. The application of biochar treatments in all six soils significantly increased the OC content, and the increase in biochar yield had a significant effect on its effectiveness. Although CaCO₃ showed an increase in OC content as well, its effect in Fooman, Chaboksar, and Shahsavar was not significant. Overall, B15, B10, B5, and CaCO₃ had 244.8%, 169.4%, 80.4%, and 41.4% increases in OC on average, respectively. All treatments effectively increased soils’ base cations, however, the effect of biochar treatments was much more significant than CaCO₃. In the case of Ca²⁺, although there are no significant differences between B5 and CaCO₃ in Fooman and Lahijan, B5 showed significantly higher values of Ca²⁺ in other sites in comparison to CaCO₃. As for Mg²⁺, the application of CaCO₃ did not show significant differences from the control in all six soils. On average, CaCO₃ was shown as the lowest effectiveness treatment with 175.2%, 1.9%, 5.8%, and 5.6% increases in Ca²⁺, Mg²⁺, K⁺, and Na⁺, respectively. However, the highest base values were related to B15 with 1067%, 1035%, 896.7%, and 56.5% increases in Ca²⁺, Mg²⁺, K⁺, and Na⁺ compared to the control, respectively. As for acid cations, the application of biochar significantly decreased the values of H⁺, Al³⁺, and NH₄⁺, and an increase in the biochar application rate caused a significant decrease in those values. The application of B15 and B10 caused significant decreases in H⁺ by 99.7% and 98.5% in comparison to the control. However, CaCO₃ and B5 resulted in 89.4% and 88.9% decreases in H⁺. On average, B15 and CaCO₃ caused a greater decrease in Al³⁺ with 42.3% and 29.1% decreases on average across all six sites, respectively. Also, the value of NH₄⁺ in all soil was significantly affected by the application of biochar, which resulted in 14.1% (B5), 35.5% (B10), and 61.1% (B15) decreases on average. On the other hand, NO₃⁻ showed a reverse trend compared to NH₄⁺. An increase in the application rate of biochar resulted in significant increases in NO₃⁻ content: 53.4% (B5), 80.1% (B10), and 116.3% (B15). However, the application of CaCO₃ did not show a significant difference from the control in both cases of NH₄⁺ and NO₃⁻ contents.

3.2. Soil Al and Ca Saturation

The results of Ca and Al saturations are presented in Figure 3. In Fooman, the highest Ca saturation was obtained after the application of CaCO₃ (44.3%), which was significantly higher than the other treatments. Also, B10 and B15 with Ca saturation levels of 35.6% and 33.3% did not show significant differences from each other. However, in the same soil, Al saturation in the CaCO₃ treatment was not significantly different from the control. On the other hand, the application of biochar significantly decreased the Al saturation compared to the control; the highest biochar rate (B15) resulted in the lowest Al saturation (1.8%). Almost the same pattern was observed for Lahijan soil. However, of the four other sites, B15 resulted in the highest Ca saturation, which was significantly higher than the CaCO₃ treatment. Based on the average across all sites, B15, B10, B5, and CaCO₃ caused increases of 309.5%, 272.6%, 202.1%, and 250.3% in Ca saturation compared to the control, respectively. Also, the average decrease in Al saturation compared to the control obtained from all six sites was B15 (82.4%), B10 (61.1%), B5 (27.3%), and CaCO₃ (15.1%).

3.3. Soil Effective Cation Exchange Capacity (ECEC)

The results of acid saturation (AS) and base saturation (BS), as well as changes in effective cation exchange capacity (ECEC), are shown in Figure 4. According to the results, all treatments significantly decreased AS in all six sites. Also, the increase in biochar application from B5 to B15 had a significant effect regarding the greater decrease in AS. The most effective treatment in decreasing AS was B15 with an 86.9% decrease on average for all six sites. Meanwhile, CaCO₃ showed lower effectiveness (44.1% decrease in AC compared to control) in comparison to the other biochar treatments. In some sites, such as Lahijan and Chaboksar, the CaCO₃ and B5 treatments showed similar effectiveness in AS without any significant difference from each other. In the case of base saturation, the highest BS for all six sites was obtained from B15. In the cases of Lahijan and Chaboksar, the application of CaCO₃ and B5 resulted in the same BS percentage (roughly about 63%). Based on the average changes in BS obtained from all sites, B15, B10, B5, and CaCO₃ resulted in increases of 196.6%, 173.4%, 129.7%, and 100.9% compared to the control, respectively. The application of biochar significantly increased the ECEC of soils from all sites. Also, an increase in the biochar application rate had a significant effect on the increase in ECEC. The average of ECEC obtained from all six sites showed that B15, B10, and B5 caused an increase in ECEC by percentages of 191.4%, 112.1%, and 39.5%, respectively. However, the application of CaCO₃ resulted in a decrease in ECEC of 20.1% on average.

3.4. Changes in Available NPK

Figure 5 presents the results of changes in NPK affected by different rates of biochars and CaCO₃. The application of biochar significantly increased the available N in all sites. The highest available N was obtained from the B15 treatment with a 26.1% increase in comparison to the control based on the average of all six sites. However, the application of CaCO₃ had a significant negative effect on available N in soil with an average decrease of 6.1% compared to the control. Also, available P and K in the presence of CaCO₃ showed 8.2% and 6.8% increases compared to the control. However, based on the average available P from all six sites, B15, B10, and B5 cause increases of 228.5%, 118.2%, and 50.3% compared to the control. Also, the application of B15, B10, and B5 on average resulted in an increase in K availability by 192.9%, 95.8%, and 47.4% in comparison to the control.

3.5. Pearson Correlation between Soil Characteristics

The Pearson correlations between characteristics of soil affected by different treatments are given in Figure 6. Based on the results, pH showed a positive correlation with OM, Ca²⁺, Mg²⁺, and K⁺ with determine coefficients (R²) of 0.37, 0.84, 0.75, and 0.75, respectively. Also, there was a significant negative correlation between ECEC and AS with R² = −0.69. The available N, P, and K showed a negative correlation with Al saturation (−0.73, −0.81, and −0.76, respectively) as well as a positive correlation with OM (0.83, 0.88, and 0.76, respectively).

4. Discussion

In the current study, although low application rates of biochar and calcium carbonate showed the same effectiveness in modifying the soil pH issue, the increase in biochar application rate was more effective than calcium carbonate. Three mechanisms are put forward for how these modifications work: (1) supplying an adequate absorbent for acidic ascorbates through well-expanded surface area and porosity, (2) loading exchangeable base cations from their inherent structure, and (3) nutrient balancing through an increase in soil organic carbon. Firstly, the alkaline properties of biochar are greatly influenced by the high specific surface area, porosity, and adequate negative charges [39,47]. Although the role of base cations in plant nutrition and balancing soil acidity is proven, however, the high solubility of basic cations, particularly Ca²⁺, and the limited solubility of acidic cations like Al³⁺ constantly maintain a high level of soil acidity [15,48,49,50]. High annual precipitation contributes to the leaching of alkaline cations and excess Al³⁺ in the soil matrix, making this issue more significant in those regions [51,52]. Therefore, it may be inferred that the addition of liming practices does not ensure an improvement in soil pH if free acidic cations are not stabilized. However, in this study, biochar performed this function perfectly due to its abundance of negative charges, which led to a high capacity for the absorption of acidic cations such as Al³⁺ and H⁺ [27,31,53]. This claim is supported by a considerable drop in acidic cations in biochar treatments when compared to the control and CaCO₃ treatments (Table 3). Some other studies likewise observed that applying biochar decreased the exchangeable acidity of acidic soils and increased the sorption capacity for nutrients [54,55].

Oxygen-containing functional groups on the surface of biochars, such as carboxyl and hydroxyl, play a key role in maintaining free Al³⁺ as the most effective ion in acidic soils [56,57]. These functional groups can create surface compounds with acidic cations due to their negative charges, which reduces the amount of Al³⁺ in the soil matrix, resulting in decreasing soil acidity [31,58]. The FTIR analysis of our used biochar shows a sharp peak at 3420 cm⁻¹ which confirms the presence of carboxyl and hydroxyl functional groups in the structure (Figure 2). That is why the Al saturation of all six soils showed a descending trend with the increase in biochar application rate (Figure 3). Although the application of CaCO₃ decreased the Al saturation significantly, the concentration of Al is still higher than when using biochar treatments. Also, the formation of micro-pores of biochar during high pyrolysis temperatures causes a notable specific surface area that would offer more contacts for acidic ions absorption, and the growth of meso-pores (2–10 nm) in the biochar would be advantageous for substance diffusion into the inner structure of biochar [59]. Both of those types of pores result in a wide range of channels, which is consistent with the high specific surface area of biochar, oxygen functional groups and thus the sorption of free acidic ions in soil. The results of the basic analysis of biochar in Table 1 confirm the potential of used biochar in absorbing Al³⁺ in acidic soils due to its high specific surface area and various types of pores.

However, types of feedstock and pyrolysis conditions can be influential on the surface functional groups of biochar [60]. In general, biochars produced at pyrolysis temperature higher than 400 °C are susceptible to having a more alkaline nature [61]. The increase in pH with increasing pyrolysis temperature may be caused by the conversion of basic cations on the surface of the biochar to alkaline compounds such CO₃²⁻ [62]. As we produced our biochar at 550 °C, it might have benefited from CO₃²⁻ in its structure. Furthermore, the ash content of biochar can make it less able to absorb substances by blockage pores and reducing its surface area available for absorption [63]. Crop-based biochar is considered to have a lower ash concentration than manure-based biochar, which results in a more porous structure and formed surface area [64]. It might be another explanation for our utilized biochar, which was made from rice husk and has a high specific surface area. It has been observed that the addition of biochar increased the effective cation exchange capacity (ECEC) of acidic soils in rainy climates due to the presence of a large negative charge on its surface [54,65]. This is the main reason for the significant increase in ECEC and base saturation of biochar treatments in comparison to the control and CaCO₃ (Figure 4). Also, the positive correlation between ECEC and BS confirms this fact (Figure 6).

As the second mechanism of pH adjustment in biochar-amended soil, biochar can provide various basic cations along with Ca²⁺. Biochar contains Ca²⁺, Mg²⁺, and K⁺, and the incorporation of biochar can raise the amount of exchangeable base cations of acidic soils [66,67]. In contrast to applying CaCO₃, which simply increases the soil’s Ca²⁺ level, the application of biochar can maintain the balance of Ca²⁺, Mg²⁺, and K⁺ in acidic soils (Table 3). The chemical analysis of biochar clearly shows its potential for loading base cations into the soil due to their availability on the biochar structure (Table 1). That is why biochar treatments significantly increased all basic cations in soils rather than CaCO₃, which only had an influence on Ca²⁺. The same effect of CaCO₃ and biochar treatments can be observed in the result of Ca saturation in soils (Figure 3). In general, adding biochars to acidic soil can cause their base cations to be released, which are then able to engage in exchange reactions with Al³⁺ and H⁺ and reduce the acidity of the soil [31,55].

Furthermore, the modification of acidic soils is significantly affected by the impact of biochar on soil organic carbon [24,68]. Biochar’s enormous specific surface area and porous structure, which enable it to retain moisture and support a variety of microorganisms, can efficiently stimulate microbial activity in soil [69]. Therefore, it is expected that the breakdown of soil organic matter and the mineralization of nutrients will both show an upward trend with the consideration of the increased rate of biochar application. This process notably increases the soil organic carbon accessibility of available nutrients for plant roots [70] and eventually the elevation of plant growth. Additionally, plant yield could be enhanced by enhancing the soil nutrient-holding capacity and root expansion, which can also improve the plant’s ability to take in more nutrients [35]. It has been reported that the use of biochar alone significantly increased yield gains when compared to the control [71]. It worth noting that cellulosic biochars such as husk and straw have a much higher specific surface area and porous structure than readily degradable biochars like sludge biochar [29]. This finally results in the cations required by the plant being absorbed and retained rather than being leached away, which has a substantial impact on the plant’s growth [72]. Furthermore, the pyrolysis process may have left some readily available nutrients in biochar, known as volatile matter, which can be directly incorporated into the soil when in contact with soil particles [73]. Thus, the increase in soil organic carbon (Table 3) and NPK (Figure 5) could be explained by those reasons. With a closer look at Table 3, it is obvious that the use of biochar resulted in a drop in NH₄⁺ and a rise in NO₃⁻ levels, which is mostly seen in the boosted conversion of NH₄⁺ to NO₃⁻ [42]. Biochar application could stimulate nitrogen mineralization due to its high surface area and potential for the activity of microorganisms [74]. According to several reports, biochar indirectly aided in the catalytic oxidation of NH₄⁺ to NO₃⁻ by boosting the diversity of components in soil ammonia-oxidizing bacteria [75]. In addition, applying biochar to the soil increased the activity of nitrifying bacteria and accelerated the nitrification process [76]. As a result, the NO₃⁻ content increased and the NH₄⁺ content dropped as the biochar application rate was raised [77]. The negative correlation between OC and NH₄⁺, as well as the positive correlation between OC and NO₃⁻ (Figure 6), is obviously consistent with the role of nitrifying bacteria as a function of the increase in soil OC following the application of biochar.

5. Conclusions

The effectiveness of different levels of biochar and the conventional liming approach was investigated to ameliorate extremely acidic soils utilized for tea cultivation in a subtropical region. Although soil calcium can be increased by calcium carbonate, the created alkaline condition is unable to completely neutralize the quantity of free acidic cations because of the high solubility and instability of basic ions. However, biochar has a significant capacity to absorb free acid cations from the soil due to the many functional groups and substantial negative charges in its porous surfaces. Furthermore, biochar’s large specific surface area, expanded porosity, and high micro-pores volume provide ideal conditions for microbial activity, the breakdown of organic matter, and the availability of the macronutrients and basic cations that plants require. Therefore, in extremely acidic areas in subtropical climates, such as tea growing, the application of biochar can surely be a more effective and affordable alternative to traditional low-efficiency liming methods. Although long-term studies could provide us with a better understanding of the usefulness of biochar in neutralizing acidic cations, however, its resilience against decomposition and excessive precipitation due to the carbon skeleton structure is still reliable.