Effects of Combined Applications of Biogas Slurry and Biochar on Phosphorus Leaching and Fractionations in Lateritic Soil

Abstract

Diverse soil phosphorus (P)-leaching phenomena induced by environmental disturbance have gained increasing attention. Two kinds of typical organic materials, biochar and biogas slurry, (BS) are widely utilized to amend agricultural soil, but there is little research that gives insight into their co-effects on soil P-leaching and corresponding mechanisms. Herein, a total of six treatments (viz., control, 2% (w/w) biochar, low ratio BS with or without 2% (w/w) biochar, high ratio BS with or without 2% (w/w) biochar) were conducted to investigate the P-leaching and fraction transformation mechanisms. The column experiment results showed that compared to control, sole BS application or biochar both can slightly enhance the soil-P loss by 134.8% and 39.8%. High ratios of BS induced higher P loss than the low ratios of BS by 125.1%. In comparison with the sole BS treatment, combined BS and biochar application increase P loss but result in less soil leaching of basic cations. The incubation experiment results showed that the enhanced P-leaching in combined BS and biochar treatment is probably attributable to the enhanced soil pH, decreased DPS, soil P adsorption capacity, and transformation of moderately labile Fe–P into labile P. This research helps in understanding the abiotic process of biochar and BS in promoting soil P-leaching and soil-P management using biochar and biogas slurry.

1. Introduction

The aggravated phosphorus (P) leaching from farmland soil has become an important source of agricultural non-point source pollution. It is reported that 300 × 10⁴ t to 400 × 10⁴ t of P₂O₅ are annually lost from farmland soil to waterbodies globally [1]. Meanwhile, various organic wastes (e.g., straw [2], manure [3], straw dust [4], etc.) and their derived materials (e.g., biochar [5], biogas slurry [6], etc.) are returned to the intensive farmland soils to amend soil and realize organic waste reutilization. However, soil P-leaching and non-point source pollution can be enhanced [7,8] and made more severe after these organic wastes are applied in soils. Therefore, it is necessary and significant to pay attention to relevant mechanisms of soil P-leaching after these organic wastes are returned to soil [9,10].

Biochar is a kind of carbonaceous materials which is invariably used to sequester photo-synthetical carbon and hence is favored for moderating the climate [11]. Biochar surface is normally constituted of nitrogen, sulfur, and oxygen-based functional groups, which are responsible for biochar’s negatively charged surface [12]. Hence, biochar addition usually increases the negative charge of soil particle surface and decreases the affinity of P towards biochar-amended soils due to their enhanced electrostatic repulsion [13]. For instance, biochar enhanced P availability in sandy soils and P leaching in loamy soils [14]. However, few reports revealed the P-leaching mechanisms induced by biochar application in acidic soils. In addition, biogas slurry (BS), being a liquid fraction from anaerobic digestion of organic waste, is often applied to agriculture fields. In China, the annual product of BS is 0.4 billion tonnes [9], and 33% of farmers are using this product as fertilizer [6]. It has been reported that the application of concentrated BS could bring significant changes to tomato production, including increases in organic matter and available N and P [15,16]. In addition, Du et al. [17] showed that medium substitution of chemical fertilizer with BS is an adoptable strategy to guarantee high crop yield, improve nitrogen use efficiency, and reduce nitrogen loss. Tang et al. [18] also found that BS addition can accelerate the turnover rate of the soil organic carbon and therefore favors substitution of chemical fertilizer in systems of straw return with BS addition. Ultimately, highly concentrated BS may lead to nutrient leaching when the nutrient provided is beyond the crop requirements [16]. However, the influence of sole BS application on soil P-leaching was not conclusive or systematically investigated [6,8].

As two kinds of widely sourced, safe, and multifunctional organic-waste-derived materials, biochar and biogas slurry can play different roles in soil amendments, and therefore, they were always co-applied in farmland soils to achieve efficient amendment. For example, Abdo [19] co-applied the biochar and BS in sandy soil, and this field research indicated that this combination of 10 and 15 Mg ha⁻¹ biochar and BS can reduce soil bulk and increase the field water capacity. Guo et al. [20] investigated the effect of cow dung biochar addition on nutrient leaching of BS-amended acidic yellow soil, and found that the increasing biochar and BS application amount can both enhance the availability and leaching amount of the soil phosphorus and potassium. Lateritic red soil is a sort of zonal acidic soil distributed in India, Mexico, China, etc. [21]. Ma et al. [22] reported that, even when laterite soil is enriched with Fe minerals, P still can transport as if affected by varied pH and total organic carbon content. However, little research has given deep insight into the effects of co-application of biochar and BS on lateritic red soil P-leaching and corresponding mechanisms. Thus, the effects of BS and biochar on P losses from lateritic red soil need more investigation. Previous studies have revealed that the critical abiotic mechanisms of soil P-leaching mainly include the soil particle-adsorption capacity [23], soil P-saturation degree [2], and transformation of P fractions [24]. In the current study, it is hypothesized that additional BS in biochar-amended soil will further increase soil P-leaching in comparison to solely biochar-amended soil by reducing soil P-adsorption performance and transformation of labile P. Herein, the laboratory column and incubation experiments were conducted with lateritic soil by investigating the above-mentioned processes before and after biochar and BS addition to achieve the following research objectives: (i) investigation of synergistic effects of BS and biochar on soil P-leaching and fraction transformation; (ii) explanation of the mechanism differences between sole BS and biochar in promoting soil P-leaching; (iii) exploration into if higher ratios of BS could promote more P-leaching compared to low BS ratios, and corresponding mechanisms in soil.

2. Materials and Methods

2.1. Soil, Biochar, and Biogas Slurry Samples

The laterite soil was collected from Baini Town, Sanshui District, Foshan City, China. The cropping system at this experiment site was a cabbage–wax gourd–rice rotation. Chemical fertilizer was applied as the basal fertilizer with application rates of 60 kg N ha⁻¹, 60 kg P ha⁻¹, and 60 kg K ha⁻¹ in each plant season. The urea fertilizer was applied at the rates of 100, 50, and 50 kg N ha⁻¹ as the topdressing for cabbage, wax gourd, and rice planting, respectively. In total, the N application was 160 kg ha⁻¹, 110 kg ha⁻¹, and 110 kg ha⁻¹ for cabbage, wax gourd, and rice. The cabbage, wax gourd, and rice seasons were from November to March, April to June, and June to October. The experimental soil (0–20 cm) was collected before the cabbage season at the end of October in 2021 from fifteen sites at the experiment field. Afterwards, the soil samples were transported to laboratory, air-dried, and ground to pass through a 2 mm sieve for following experiments. The laterite soil clay, silt, and clay contents were 35.6%, 41.5%, and 22.0%, with a pH of 5.42, cation exchange capacity (CEC) of 11.2, total P of 1.55 g kg⁻¹, total N of 0.549 g kg⁻¹, total K of 17.8 g kg⁻¹, Olsen-P of 74.4 mg kg⁻¹, and CaCl₂-extracted P of 6.59 mg kg⁻¹. Biochar was produced from corn stalk. Specifically, the air-dried corn stalk was crushed to pass through a 0.25 mm sieve and then pyrolyzed at 500 °C for 2 h under nitrogen atmosphere. The biochar had a pH of 10.9, with total organic carbon of 613 g kg⁻¹, total phosphorus of 1.24 g kg⁻¹, CaCl₂-extracted P of 9.50 mg kg⁻¹, and Olsen-P of 95.2 mg kg⁻¹. The BS originated from anaerobically digested chicken manure waste which was collected from the layer industry in Bijie City, Guizhou Province, China. The BS had a pH of 7.56 and TN of 756 mg L⁻¹, NH₄⁺-N of 598 mg L⁻¹, 15.2 mg of L⁻¹ NO₃⁻-N, total phosphorus (TP) of 49.5 mg L⁻¹, and total potassium (TK) of 298 mg L⁻¹.

2.2. Experiment Design

Column experiments and incubation experiments were included in the current research, which aimed to explored the coupling effects of biochar and BS on soil P-leaching and fraction transformation, respectively. In both experiments, a total of six treatments (

Table 1. Total amounts of nutrients input in each treatment.

Treatments	BS Volume (mL)	Biochar Application Rate (%)	P2O5 (mg per Column)
BS0-0	0	0	0
BS0-2	0	2	4.96
BS100-0	100	0	4.95
BS100-2	100	2	9.91
BS200-0	200	0	9.90
BS200-2	200	2	14.9

Note: BS0-0: the control; BS0-2: application of 2% (w/w) biochar; BS100-0: 100 mL BS; BS100-2: 100 mL BS with 2% (w/w) biochar; BS200-0: 200 mL BS; BS200-2: 200 mL BS with 2% (w/w) biochar. The same below.

) were performed (viz. control (BS0-0), 2% (w/w) biochar (BS0-2), low ratio BS without (BS100-2) or with 2% (w/w) biochar (BS100-2), high ratio BS without (BS200-0) or with 2% (w/w) biochar (BS200-2)). For the column experiment, the biochar was mixed with 200 g of air-dried soil to make the biochar-to-soil content (w/w) 0 and 20 g kg⁻¹. The polyvinyl chloride columns were 5.0 cm in diameter and 15 cm in height and inserted with nylon meshes with 60 μm pore size to prevent soil loss from the column bottoms. After the column assemblies were completed, they were pre-incubated by adding deionized water (DI) to 70% field capacity to equilibrate the microbe community for 10 days. Afterwards, the BS was applied at 0, 100, and 200 mL per column; the total nutrient amount of each treatment is given in Table 1. The BS was diluted with DI to 1000 mL, and 200 mL was applied every three days. A total of five leaching events were conducted within 15 days and the leachate was collected separately and stored at −4 °C for further analysis.

In addition, the incubation experiments were conducted to explore the changes of the P fractions and adsorption performance, which included a total of six treatments (Table 1). In detail, 300 g of air-dried soil ground was passed through a 2 mm sieve and thoroughly mixed with biochar in s 0.25 L cylindrical polyethylene tank in triplicate. Then, a certain volume of BS was added to the cylindrical polyethylene tank to achieve the 70% field water capacity. Afterwards, each tank was covered with perforated parafilm and incubated at 25 °C for 30 days.

2.3. Analysis of Incubated Soil and Leachate

The soil pH was determined at the soil-to-water ratio (1:10, w/w), and the soil total P was determined by peroxydisulfate digestion and colorimetric method [1]. The CaCl₂-extracted P and Olsen-P were detected through colorimetric method, after the soil samples were respectively extracted by 0.01 M CaCl₂ solution and 0.5 M NaHCO₃ solutions [25]. Diverse soil P fractions were determined through the Kuo and Sparks [26] methods. In brief, the NH₄Cl-P, Al-P, Fe-P, Occluded-P (O-P), and Ca-P were respectively extracted with NH₄Cl (1 M, pH 5.5), NH₄F (0.5 M, pH 8.2), NaOH (0.1 M, pH 12), Na₃C₆H₅O₇ (0.3 M 20 mL)-Na₂S₂O₄ (1 g)-NaOH (0.5 M 5 mL) (pH 13), and H₂SO₄ (0.25 M, pH 1). The inductively coupled plasma optical emission spectroscopy (ICP-OES, ICAP6300, Thermo Fisher Scientific, Waltham, MA, USA) was used to determine the Fe, Al, P, Mg, and Ca content in soil samples extracted by Mehlich 3 solution and in leachate samples digested by inverse aqua regia [1,25].

2.4. P Adsorption in Soils or Soil–Biochar

To evaluate the P-adsorption characteristics of soils or biochar-amended soils in the incubation experiment, the 2 mg soil samples were added to 20 mL P solutions of 20, 40, 60, 80, 100, 120, 160, and 240 mg P·L⁻¹ in 50 mL centrifuge tubes. These were shaken for 24 h (200 rpm, 25 °C). After, the equilibrium P concentration of filtered solution was detected using colorimetrical method. The batch experiment data was fitted by adsorption isotherm models (Langmuir (Equation (1)) and Freundlich (Equation (2))):

$$q_e=\frac{q_m{K}_L{C}_e}{{1+K}_L{C}_e}$$

(1)

$$q_e{=\mathrm{K}}_{\mathrm{F}}{C}_e^{1/n}$$

(2)

C_e (mg·L⁻¹), q_m (mg·kg⁻¹), and n refer to the P equilibrium concentration, the maximum adsorption capacity, and the nonlinearity constant, respectively. The K_L and K_F are adsorption constants related to the binding strength. The P buffer capacity (PBC) and the degree of soil P saturation (DPS) were calculated based on Equations (3) and (4); PBC and DPS refer to the difficulty level of soil-P concentration change and the risk of soil-P loss.

$${\mathrm{PBC}=q}_m{\times K}_L$$

(3)

$${\mathrm{DPS}=\mathrm{M}3-\mathrm{P}/[\mathrm{M}3-\mathrm{P}+q}_m]\times 100$$

(4)

2.5. Statistical Analysis

Excel 2007 and SPSS 18.0 were used for a statistical analysis of the data. One-way ANOVA and Duncan’s method were used for ANOVA and multiple comparison (α = 0.05). Excel 2007 software was used for plotting. Data in the chart are mean ± standard error.

3. Results and Discussion

3.1. Soil Labile P, P Fractionation, and Soil pH Changes

In terms of pH analysis, biochar treatments significantly (P < 0.05) improved the soil pH by 0.5–0.6 units, which was similar to previous studies [5,27]. Interestingly, it wads found that addition of BS can reduce the pH to some extent compared to those treatments with the same amount of biochar addition (

Table 2. The pH and contents of different fractions of inorganic phosphorus (Pi) of the diverse treatments after 30 days incubation.

Treatments	pH	NH4Cl-P	Al-P	Fe-P	O-P	Ca-P	Total Pi
		mg kg−1
BS0-0	5.53 d	35.6 f	178 d	328 a	469 b	165 d	1176
BS0-2	6.12 a	43.8 e	194 c	308 b	476 a	206 b	1228
BS100-0	5.62 cd	50.6 d	187 cd	318 ab	471 a	173 cd	1199
BS100-2	6.03 b	65.8 b	219 b	275 c	458 c	215 ab	1233
BS200-0	5.75 c	62.8 c	215 b	268 c	475 a	180 c	1201
BS200-2	6.13 a	78.6 a	226 a	252 d	462 bc	228 a	1247

Note: Different letters of the same parameter among the treatments indicate there is significant difference among them (P < 0.05).

), which may be ascribed to buffer maters (e.g., phosphate compounds and organic acidic) inhibiting the pH-increasing effect. In addition, lower pH will also prevent the P precipitation with cation ions like Ca²⁺ and Mg²⁺ [28]. Therefore, inhibited pH increase can potentially contribute to the increase of the soil-P loss.

It has been reported that biochar and BS can show positive or negative effects on enhancing soil labile P content or P availability. In addition, the CaCl₂–P was the most labile P form in soil, and the Figure 1a showed that sole biochar (BS0-2) and BS (BS100-0) treatments significantly increase the soil CaCl₂–P content. Specifically, the soil BS treatments (BS100-0 and BS200-0) showed higher CaCl₂-P content, which is probably attributable to the competition between higher labile organic matter and soluble phosphate on soil adsorption sites [8]. In addition, compared with the sole biochar or BS treatment, co-application treatments (BS100-2 and BS200-2) showed higher CaCl₂–P content, which implies that co-application of biochar and BS can more significantly (P < 0.05) enhance soil labile P content. This phenomenon also indicates that additional biochar application in BS-amended laterite acidic soil may carry a potential soil P-leaching risk. In terms of soilP bioavailability analysis, a similar trend was found in changes of the soil Olsen-P. Specifically, BS200-2 can most significantly (P < 0.05) increase the soil Olsen-P content, which was similar to previous studies [9,29] and can probably be explained by the following two reasons: (i) a higher amount of labile P was used, but less adsorption sites were occupied by the low-molecular-weight organic matter; (ii) biochar alkalinity promoted transformation of the moderately labile P (e.g., Fe/Al bounded P) into labile P. In short, BS can more significantly (P < 0.05) promote the P loss and availability in comparison to the biochar input when the total P inputs are equal (Table 1), and higher BS input showed increasing P-leaching loss and availability.

To further investigate if the biochar and BS input can transform the inorganic P fractions and afterwards affect the P-leaching degree, diverse inorganic P fractions were investigated by chemical sequential method [2,23,30]. Table 2 shows that NH₄Cl−P was slightly higher than the Olsen-P in each treatment and in a similar trend as Olsen-P. Obviously, NH₄Cl−P as one indicator of the soil P lability was most significantly (P < 0.05) increased in BS200-2, while the Al−P increase and the Fe−P was reduced in BS200-2. It has been reported that pH increase promotes the P transform from Fe−P to Al−P and labile P through a microbe-induced Fe reduction process that takes the enriched soil soluble organic matter as an electron donor [24]. It is noteworthy that co-application of BS and biochar showed synergistic effects on reduction of soil Fe–P (Table 2). This result implies that the increase of the labile P in BS200-2 can also be attributed to the P transformation induced by soil pH increase and the Fe reduction process, excepting the direct input of labile P from BS and biochar. For the conditions (BS100-0 and BS200-0) without biochar addition, changes in NH₄Cl−P, Al−P and Fe−P were similar, except for the Ca−P. It is widely acknowledged that sole biochar application (BS0-2) can increase the Ca−P content [1,25]. Interestingly, it was found that Ca–P showed high content in co-biochar and BS treatments, which may be a result of Ca precipitation induced by higher pH caused by biochar addition. Furthermore, soluble Mg²⁺ also can inhibit Ca precipitation, as the Mg²⁺ can inhibit the formation of Ca nucleus [25]. Therefore, higher pH in biochar treatments may reduce the more soluble magnesium cation content and finally result in enhanced Ca precipitation. From the perspective of soil P transformation, co-addition of biochar and BS can better transform the Fe−P into labile and moderately labile P, even though it simultaneously enhances Ca−P in comparison with the sole BS treatments, which implies that long-term co-application of biochar and BS posed soil P-leaching risk with simultaneously increased soil total P content.

3.2. P Adsorption on Biochar and Biogas-Slurry-Amended Soils

The soluble phosphate adsorption towards soil particles was critical to maintain the soil P to prevent P-leaching. Thus, evaluation of the P adsorption towards soil particles was significant. DPS was also a key soil parameter; it indicates the saturation degree of the soil-P adsorption site. Figure 2 clearly shows that the control treatment yielded the highest P-adsorption performance. As shown in

Table 3. Phosphorus sorption data represented by Langmuir and Freundlich; M3-extracted elements and DPS at 30 days incubation.

Treatments	Langmuir				Freundlich			M3-Al (mg kg−1)	M3-Fe (mg kg−1)	M3-Mg (mg kg−1)	M3-Ca (mg kg−1)	M3-P (mg kg−1)	DPS (%)
	KL (L mg−1)	qm (mg kg−1)	PBC (L kg−1)	R2	1/n	KF (mg kg−1)(L mg−1)−1/n	R2
BS0-0	0.063	243	15.3	0.992	0.549	25.9	0.975	584	682	150	342	86.5	26.3
BS0-2	0.082	164	13.4	0.995	0.520	21.2	0.969	595	675	175	352	91.5	35.8
BS100-0	0.064	143	9.15	0.990	0.548	14..0	0.957	591	688	216	412	98.6	40.8
BS100-2	0.056	116	6.50	0.996	0.532	11.5	0.963	584	679	186	422	105	47.5
BS200-0	0.053	111	5.88	0.996	0.543	10.89	0.951	591	688	286	476	101	47.6
BS200-2	0.022	80.4	1.77	0.981	0.710	2.8	0.963	586	681	290	486	118	59.5

, the Langmuir model well fitted the P-adsorption data (R² = 0.981–0.992). The biochar and BS treatments showed lower q_m and PBC than the control treatment. After the sole biochar addition (BS0-2), the maximum adsorption capacity decreased, while the DPS was also reduced. This was similar with previous research [14], which was attributed to the fact that biochar addition can induce higher amounts of dissolved organic matter to compete with P and reduce the adsorption site through Fe oxide reduction. More importantly, compared with the sole biochar treatment (BS0-2), co-BS and biochar treatment (BS100-2) and sole BS treatment (BS100-0) reduced the soil P adsorption capacity by a further 12.8–29.3%, despite the high Ca and Mg ion content in BS [31]. This further confirmed that dissolved organic matter in BS plays an important role in reducing the soil P−adsorption performance and promoting P-leaching. It is noteworthy that the co-application of biochar and BS showed synergistic effects on decreasing the soil-P adsorption capacity and enhancing the DPS, which may be attributed to those reasons induced by sole biochar addition. Soil basic cations extracted with Mehlich 3 solution can reveal the available adsorption sites to some extent [1,25]. Obviously, the highest total amounts of M3−Fe, M3−Al, M3−Ca, and M3−Mg were found in BS200-2, but the lowest adsorption was found in BS200-2, which implied that the BS200-2-introduced cations became more complexed with organic matter and failed to supply adsorption sites for soil−P retention.

3.3. Effects of Biochar and Biogas Slurry on Soil P-Leaching

The above evidence has proved that co-application of biochar and BS not only enriched the soil total P and P bioavailability in lateritic soil, but also enhanced the soil-P loss by reducing the soil-P adsorption sites and increased the DPS. The above incubation experiment results imply that co-application of biochar BS shows potential P-loss risk, which was further examined through column experiment in the current study (Figure 3). The column experiment results showed that the sole biochar or BS treatment slightly increased the molybdate reactive P and total P content compared to control, which was similar to a previous study [1]. It is noteworthy that sole BS treatment (BS100-0) showed higher P-leaching amounts than those of sole biochar treatment (BS0-2), while they induced the same amounts of P₂O₅ into the columns (Table 1). This trend among diverse treatments was in line with the CaCl₂–P results from the sole BS treatment, wherein CaCl₂–P showed higher content than the sole biochar treatment. In addition, higher application rates of BS (BS200-0) increased the soil P-leaching than the lower application rate of BS (BS100-0), which accorded with P-transformation results; BS200-0 can reduce moderately labile P and enhance the labile P significantly (P < 0.05) (Table 2). More importantly, biochar addition significantly (P < 0.05) increased the soil P-leaching based on BS addition. For instance, the BS200-2 antically showed the highest P-leaching on day 3, and then gradually the P-leaching content decreased, implying that it is necessary to pay attention to P-leaching at the initial remediation stage when the combined application of biochar and BS is conducted. In addition, this P-leaching phenomenon also encourages the modification of the combined application strategy of biochar and BS, e.g., some cost-effective minerals like dolomite [25] or modified biochars [32], showing that P-adsorption performance can be co-applied with biochar and BS to avoid the initial P-leaching under this condition.

Beyond the the P-leaching phenomenon, the leaching of diverse soil basic cation ions was also determined. As shown in

Table 4. The accumulative leaching amount of the Mn, Ca, and Mg in the leachate.

Element	Treatments
	BS0-0	BS0-2	BS100-0	BS100-2	BS200-0	BS200-2
	mg
Mn	226 ± 19.3 a	143 ± 15.9 c	197 ± 11.0 b	136 ± 24.8 c	228 ± 14.5 a	148 ± 17.6 c
Ca	8.74 ± 2.31 d	11.3 ± 2.16 c	18.1 ± 2.3 b	13.4 ± 1.70 c	25.2 ± 2.44 a	14.9 ± 2.57 b
Mg	3.09 ± 1.42 d	4.87 ± 1.49 d	11.0 ± 4.01 b	7.21 ± 2.10 c	19.5 ± 3.25 a	8.81 ± 3.12 bc

Note: Within a panel, data attached with the same letter are not significantly (P < 0.05) different from each other.

, the accumulative leaching amount of BS200-0 was the highest (Total P: 0.653 mg; molybdate reactive P: 0.512 mg), but the BS200-2 was lower, which was probably due to BS200-2 showing higher pH induced by biochar addition and hence can precipitate these soil basic cation ions. It might also be suggested that sole application of BS may induce the significant cation leaching. Previous research also indicated that BS and concentrated BS were abundant in Ca and Mg ions [31], hence, addition BS may cause higher Ca and Mg leaching compared to control.

In summary, the enhanced P-leaching in the lateritic soil induced by co-application of biochar and BS are mainly attributable to the following causes: (i) the soil DPS and P-adsorption capacity were significantly reduced; (ii) the soil labile P content was largely increased while the soil-P availability was enlarged; (iii) the dissolved organic matter introduced by the BS and biochar deactivated the P-adsorption sites; (iv) enhanced microbes mediated Fe oxide reduction and Fe–P release.

4. Conclusions

The current research investigated the effects of co-application of biochar and BS on soil P-leaching and their potential mechanisms. Compared to control, sole BS application or biochar both can slightly enhance the soil P loss by 134.8% and 39.8%. In comparison with sole BS ratio treatment, high ratios of BS can induce higher P loss, by 125.1%, in lateritic soil. Compared to the sole BS treatment, combined BS and biochar showed more severe P loss but less soil basic cation leaching which may be a result of the enhanced soil pH, decreased DPS, soil-P adsorption capacity, and transformation of moderately labile Fe–P into labile P. This study offers deep insights into the mechanisms of lateritic soil P-leaching. Future exploration should focus on how these combinations promote P-leaching or transformation through biotic processes, exploring their applications in practical field experiments, and reducing the P loss through modified strategies of co-application of biochar and BS.