Effect of Biochar and Inorganic Fertilizer on Soil Biochemical Properties in Njoro Sub-County, Nakuru County, Kenya

Declining soil fertility is a major constraint to potato farming, the second most important food crop in Kenya. The objective of the study was to determine the effect of different rates of biochar and inorganic fertilizer on some soil properties; soil pH, soil phosphomonoesterases, inorganic nitrogen and extractable phosphorus. The study was conducted for two seasons (short and long rains) at two locations (Egerton University agricultural field and farmer’s field in Mau Narok) using a split-plot design in a randomized complete block (RCBD) arrangement with variety as the main plot and soil amendments as the subplot. Biochar and Diammonium Phosphate (DAP) at 0, 5, and 10 t ⋅ha ^−1 and 0, 250, and 500 kg ⋅ha ^−1 respectively, were applied, resulting in nine treatment combinations. Two potato varieties ( Shangi and Destiny) were used in the study. A combination of 5 t ⋅ha ^−1 biochar and 500 kg ⋅ha ^−1 DAP and sole application of biochar at 5 t ⋅ha ^−1 resulted in an increase of 1.25, 2.54 units in soil pH in two seasons, respectively. Similarly, a combination of 5 t ⋅ha ^−1 biochar and 250 kg ⋅ha ^−1 DAP increased soil available phosphorus by 105 units from 30.7 mg ⋅kg ^−1 to 136 mg ⋅kg ^−1. The application rate of 5 t ⋅ha ^−1 biochar with 250 or 500 kg ⋅ha ^−1 DAP significantly increased soil nitrate by 102.11 and 116.14 units, respectively. Soils amended with biochar at 5 t ⋅ha ^−1 combined with 500 kg ⋅ha ^−1 DAP, 10 t ⋅ha ^−1 of biochar combined with either 250 kg or 500 kg of DAP gave the highest alkaline enzymes (mM pNP × kg ^−1 × h ^−1). However, the highest acid soil phosphomonoesterases were obtained under the sole application of DAP at 500 ha ^−1. Thus, using biochar with chemical fertilizer seems a plausible option to ameliorate the declining nutrient base of farmland in Kenya, which could sustainably support potato growth.

• Biochar
• Inorganic Nitrogen
• Phosphorus
• Soil pH
• Phosphomonoesterases

1. Introduction

Productive soil is an indispensable resource to smallholder farming communities of Sub-Saharan Africa (SSA) in addressing food, nutritional and income insecurity [1] . However, the declining soil fertility due to continuous nutrient mining, imbalanced organic matter (OM) levels and reduced nutrient replenishment efforts by farmers such as inappropriate use of organic and chemical fertilizer application, pose a significant threat to sustainable crop productivity and farmers’ livelihoods [2] [3] [4] [5] . To mitigate the low soil fertility issue, farmers continuously focus on the application of chemical fertilizers as sole soil amendment to replenish the depleted nutrients for improved crop yield [6] . However, due to the intensified soil degradation in SSA, the soils have limited OM and nutrient reserves resulting in reduced soil responsiveness to chemical fertility. Furthermore, given the high cost of fertilizers, farmers have been forced to apply lower rates than recommended, which insufficiently affects optimal crop yield. Thus, the application of chemical fertilizers solely provides limited success in unravelling the low soil fertility [7] . Therefore, this calls for sustainable integrated soil fertility management (ISFM) approaches that will enhance soil fertility through restoration of OM and nutrient reserves [8] .

The integration of biochar with chemical fertilizers has recently become a common ISFM approach [9] . Biochar is a carbon (C)-rich organic material made by thermal degradation of biological matter under anaerobic conditions [10] [11] [12] . Biochar possesses several inherent qualities namely, high organic carbon content, high specific surface area, high porosity, as well as alkaline pH that have been reported to significantly improve various soil physicochemical and biological properties that impact soil productivity and crop yield [13] [14] [15] . Several studies have shown that the incorporation of biochar to soil improves soil structure, soil aggregate stability and aeration, water-holding capacity, nutrient cycling and microbial diversity [16] [17] [18] [19] [20] . Elsewhere, it has been proved that biochar also influences the cation exchange capacity, increases pH, nutrient retention [16] [17] . For instance, studies by Bai et al. [21] and Xu et al. [22] , indicate that addition of biochar not only enhances soil organic nitrogen (nitrate and ammonium) but also minimizes its loss to leaching, possibly because organic carbon in biochar gets sequestered upon application to the soil. Secondly, biochar makes phosphorus readily available by decreasing soil acidity and increasing cation exchange capacity [12] [23] . Due to biochar’s ash content and liming capacity, it favors the growth of crops that do not thrive well in acidic soils [23] [24] [25] . In addition, biochar alters soil’s microbial activity by increasing acid and alkaline phosphomonoesterase enzymatic activity depending on the soil pH and thus ameliorating organic P mineralization [24] [26] . Thus, combining biochar with chemical fertilizers will enhance soil fertility through restoration of OM and nutrient reserves, hence improving crop yields [27] . Biochar in form of charcoal dust is readily available in SSA and would act as a cheap and sustainable soil amendment to resource-constrained farmers [23] , which has enabled biochar to gain global attention over the past years [28] [29] [30] .

Despite the various benefits of biochar, only limited research on the effect of biochar on soil properties has been done in Kenya that has resulted in low utilization of biochar in farming. Unfortunately, little is known about the right combination of biochar and other fertilizers and its effect on soil properties under potato (Solanum tuberosum L) production, which is the second important crop after maize in Kenya. Hence, this study was conducted to determine the effect of different rates of biochar and inorganic fertilizer on soil phosphomonoesterases, inorganic nitrogen, extractable phosphorus, and soil pH over two cropping seasons. The findings from this study would be a scientific basis to advocate for integrated biochar and chemical fertilizer application in Kenya.

2. Materials and Methods

2.1. Study Site Description

Field-based experiments were carried out for two cropping seasons (short rains of September to December 2020 and long rains of April to August 2021) at two sites in Njoro Sub-county, Nakuru County. Both study sites are situated in the Agro-ecological zone III [31] . The first study site was in field 17 at Egerton University demonstration farm (0˚23'S; 35˚35'E) at an altitude of 2238 m above sea level and receives bi-modal rainfall of approximately 1000 mm annually with average temperatures of 18˚C - 21˚C [31] . It has Mollic Andosols soils that are moderately fertile with good drainage, friable and smeary feel, dark reddish-brown color, and sufficient depth. The second site was at a farmer’s field (0˚39'S; 35˚57'E), located in Mau Narok ward, Mathanga Uta village, Nakuru county, at 2900 m above sea level. The area also receives bimodal rainfall with an annual average of 1300 mm, minimum temperatures of 6˚C to 14˚C, and maximum temperatures of 22˚C to 26˚C [31] . This site commonly has Mollic Andosols soils characterized by crumby structure, good drainage, friable consistence, dark brown loams with clay loam texture, and medium organic matter [31] . These site characteristics provide suitable conditions for potato growth [31] .

Prior to experimental set up, the soil was sampled at soil depths (0 - 15 cm and 15 - 30 cm) and analyzed for total nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), sodium (Na), cation exchange capacity (CEC), soil pH, total organic carbon (OC) and soil texture class as described by Okalebo et al. [32] (Table 1).

2.2. Source of Varieties, Biochar, and Fertilizers

Two potato (Solanum tuberosum L) varieties (Shangi and Destiny) were used for the study and were both sourced from Agricultural Development Corporation (ADC), Molo. These varieties are the most preferred by farmers in Nakuru because they are high-yielding and quick maturing. They also have moderate resistance to most diseases, especially late blight (Phytophthora infestans), potato virus X (Potato virus X), and potato leaf roll virus (Potato leaf roll virus) [33] [34] . Biochar (charcoal dust) was sourced from charcoal producers in Nakuru County and thoroughly mixed to make a composite. Before application, the composite biochar was crushed and sieved through a 3 mm mesh to produce a relatively fine powder [35] , a biochar sample was analysed for total N, P, K, and Mg. The inorganic fertilizer, Diammonium Phosphate, DAP (18:46:0), was purchased from Meya Agri Traders Limited in Nakuru town, Kenya.

2.3. Treatment Combination and Experimental Design

The field experiments were set up in Randomized Complete Block Design (RCBD) at each study site with three blocks and two main treatments (varieties and soil amendment combinations). The treatments were laid in a split-plot pattern in each block with potato variety as main-plot and soil amendments as sub-plots. The main two treatments included potato variety with two levels (Shangi and Destiny). The second treatment included an integrated soil amendment, which was combined three DAP levels (0, 250, 500 kg∙ha⁻¹), and three biochar levels (0, 5, and 10 t∙ha⁻¹) to make nine soil-amendment combinations i.e. B0D0 (control), B0D250 (250 kg∙ha⁻¹ DAP), B0D500 (500 kg∙ha⁻¹ DAP), B5D0 (5 t∙ha⁻¹ biochar), B5D250 (5 t∙ha⁻¹ biochar with 250 kg∙ha⁻¹ DAP), B5D500 (5 t∙ha⁻¹ biochar with 500 kg∙ha⁻¹ DAP), B10D0 (10 t∙ha⁻¹ biochar), B10D250 (10 t∙ha⁻¹ biochar with 250 kg∙ha⁻¹ DAP), B10D500 (10 t∙ha⁻¹ biochar with 500 kg∙ha⁻¹ DAP).

2.4. Soil and Biochar Analysis

Soil samples were taken twice i.e. before planting at two depths (0 - 15 cm and 15 - 30 cm) and after harvesting at one depth (0 - 15 cm) using soil auger in zigzag sampling pattern. All soils were air dried and sieved using a 2mm sieve except those for soil phosphomonoesterases and inorganic nitrogen that were sampled and kept in a cool ice box with ice cubes to avoid drying. On the other hand, biochar was crushed and sieved through a 3 mm mesh [35] .

Total carbon was determined by the Walkley-Black method using sulphuric acid and aqueous potassium dichromate. Cation exchange capacity (CEC) and exchangeable cations were extracted in 1M ammonium acetate as described by Okalebo et al. [32] . Calcium and magnesium were determined by atomic absorption spectrophotometry, and sodium and potassium were measured using flame photometry [32] . Soil texture was determined by the hydrometer (Bouyoucos) method [32] . Soil and biochar pH was measured in a 1:2.5 (w/v) soil-water solution using a digital pH meter [32] .

Soil samples for phosphomonoesterases enzymes analysis were randomly obtained from the rhizosphere by carefully excavating the soil next to the roots using a soil auger to minimize bulk soil i.e., soil outside the rhizosphere or soil not penetrated by roots was avoided. The soil samples were transferred into a clean bucket and thoroughly mixed to get a composite sample. Of the composite sample, 50 g was placed in bag and taken to laboratory for further analysis. To ensure the soil samples were not exposed to high temperature during transit to laboratory, samples were transported in a cool box filled with ice cubes. At arrival in the laboratory, samples were stored at 4˚C in the refrigerator until analysis [36] . The activity of phosphomonoesterase enzymes was determined by quantifying the amount of p-nitrophenol produced after a 37˚C incubation period of 1 hour for hydrolysis [37] . As described by Tabatabai et al. [38] , 1 g of soil was assayed at pH 6.5 and pH 11 for acid and alkaline phosphomonoesterases respectively, and incubated with 1 ml of p-nitrophenyl phosphate solution at 37˚C for 1 hour. Then, the yellow colour intensity of p-nitrophenol was quantified using a spectrophotometer at wavelength of 400 nm. Phosphomonoesterase enzymes’ activity was measured as mM pNP × kg⁻¹ × h⁻¹ i.e. number of moles of p-nitrophenol produced by 1 kg of a soil at 37˚C per hour [38] . Inorganic soil nitrogen ( NO 3 − -N and NH 4 + -N ) were extracted in 0.5 M K₂SO₄ and measured by the colorimetric method [32] . For biochar, total N and P were determined by the digest method, where the biochar was treated with hydrogen peroxide, sulphuric acid, selenium, and salicylic acid [32] . While for soils, total N was determined by the Kjeldahl method and extractable P by Olsen method [32] (Table 1).

Table 1

Biochar and soil chemical and physical properties at the onset of field trials

Study site	Soil Sample				Biochar
	Egerton		Mau-Narok
Soil depth (cm)	0 - 15	15 - 30	0 - 15	15 - 30
Total nitrogen (gkg−1)	0.24	0.24	0.26	0.24	0.35
Phosphorus (mg∙kg−1)	18.50	22.00	30.70	27.60	0.08
Potassium (Cmol∙kg−1)	1.80	1.40	1.36	0.56	0.73
Calcium (Cmol∙kg−1)	9.00	10.20	4.60	3.80	1.19
Magnesium (Cmol∙kg−1)	5.02	4.85	3.36	3.26	0.22
Sodium (Cmol∙kg−1)	0.20	0.30	0.90	0.50	--
Cation exchange capacity (Cmol∙kg−1)	29.00	32.00	24.30	25.70	--
Soil pH-H2O (1:1)	6.01	6.15	5.00	5.20	10.2
Total Org. Carbon (g∙kg−1)	2.59	2.60	2.87	2.59	--
Acid phosphomonoesterases (mM pNP × kg−1 × h−1)	100	--	98	--	--
Alkaline phosphomonoesterases (mM pNP × kg−1 × h−1)	87	--	102	--	--
Sand %	58.00	60.00	56.00	56.00	--
Silt %	14.00	12.00	14.00	14.00	--
Clay %	28.00	28.00	30.00	30.00	--
Texture class	SCL	SCL	SCL	SCL	--

SCL—Sandy clay loam.

2.5. Data Analysis

Normality test was done on the data to ensure its meets the assumptions of Analysis of variance (ANOVA). Analysis of variance (ANOVA) was thereafter done using the General Linear Model procedures of SAS 9.3 version. Treatment means that were significantly different were separated using Tukey’s honestly significant difference at P < 0.05.

3. Results

3.1. Soil pH

Soil pH was significantly affected by the soil amendments applied (P ≤ 0.001) (Table 2). Treatments without biochar i.e., plots that received only DAP and the control significantly lowered soil pH compared to treatments when biochar was added in both sites. This was consistent across the two sites and seasons. At Egerton, treatment B5D500 increased soil pH from the initial 6.01 to 6.55 and 7.25 in fields planted with destiny and shangi respectively in both seasons. This was not significantly different from other treatments that had biochar. At Mau Narok in the short rain season, soil amendment B5D0 gave the highest soil pH of 7.54 from the initial soil pH of 5.00 which was not significantly different from B10D250 and B10D500 on soils planted with destiny. However, in the same season under soils grown with shangi, B5D500 had the highest soil pH of 7.39 from the initial soil pH of 5.00. Still, this was not significantly different from the soil amendments of B5D0, B5D250 B10D500. In the long rain season, B5D0 had the highest soil pH for both varieties that was not significantly different from all the other amendments that had biochar (Table 2).

Table 2

Biochar and DAP effects on soil pH at two agro-ecological zones in Kenya

Treatment	Egerton				Mau Narok
	Short rain season		Long rain season		Short rain season		Long rain season
	Destiny	Shangi	Destiny	Shangi	Destiny	Shangi	Destiny	Shangi
B0D0	5.66abc	5.99bc	5.05c	4.99c	5.43d	5.31d	4.97c	5.29bc
B0D250	5.55bc	5.75c	4.90c	5.07c	5.46d	5.55d	5.24bc	5.05c
B0D500	5.42c	6.14abc	5.27bc	5.19bc	5.36d	5.60cd	5.19bc	5.29bc
B5D0	6.01abc	6.23abc	5.93a	5.71ab	7.54a	6.68ab	5.69abc	6.24a
B5D250	6.19abc	6.77abc	5.65ab	5.93a	6.43c	6.61ab	5.49abc	5.72abc
B5D500	6.55a	7.25a	5.65ab	5.79ab	6.58bc	7.39a	6.07ab	6.29a
B10D0	6.07abc	6.86abc	6.01a	5.24bc	6.49bc	6.58b	6.28a	6.51a
B10D250	5.92abc	6.78abc	5.70ab	5.75ab	7.26a	6.33bc	5.82abc	5.76abc
B10D500	6.38ab	7.04ab	5.82ab	6.02a	7.11ab	6.85ab	6.08ab	6.00ab
MSD	0.95	1.12	0.57	0.64	0.69	0.78	1.03	0.87

Means with the same letter(s) within a column are not significantly different at (p < 0.05), MSD: Minimum Significance Difference, B: Biochar, D: DAP.

3.2. Soil Phosphomonoesterases

Both acid and alkaline phosphomonoesterase enzymes were significantly affected by soil amendment (P ≤ 0.001) (Table 3), (Table 4). There were significant differences in the response of both enzymes to the applied treatment factors. Sole application of biochar was not significantly different from unamended soils for the acid enzymes. Combination of biochar and DAP induced significant increases in soil alkaline enzymes. Generally, the controls (soils with no amendment) recorded the lowest levels of alkaline enzymes that were not significantly different from alkaline enzymes under sole application of DAP. In Egerton during the short rains, soil amendment B5D500 under both varieties, gave the highest alkaline enzymes. However, in the long rains, soils under B10D500 planted with Shangi gave the highest alkaline enzymes while for Destiny the highest was under B10D250 but still these two were not significantly different from B5D500 (Table 3). In Mau Narok, B10D500 amended soils gave the highest alkaline enzymes for both varieties during both rainfall seasons. For the acid soil phosphomonoesterases in Mau Narok, the highest levels were under the recommended rate of DAP followed by integration of biochar and DAP which were not significantly different (Table 4).

3.3. Soil Inorganic Nitrogen

Soil ammonium and nitrate were significantly influenced by incorporated soil

Table 3

Effect of biochar and DAP on soil acid and alkaline phosphomonoesterases at Egerton, Kenya

Egerton
Variety Treatment	Short rain season (Oct - Dec 2020)				Long rain season (April - July 2021)
	Destiny		Shangi		Destiny		Shangi
	Acid enzyme	Alkaline enzyme	Acid enzyme	Alkaline enzyme	Acid enzyme	Alkaline enzyme	Acid enzyme	Alkaline enzyme
B0D0	165.49cd	61.69d	127.49b	76.67d	56.81cd	23.85c	90.46a	29.50b
B0D250	241.18a	36.29e	114.84b	87.33d	76.81bc	28.92bc	98.94a	33.68ab
B0D500	261.77a	102.09c	185.97a	109.79c	101.02a	36.44abc	115.48a	32.89ab
B5D0	135.25d	122.91abc	118.96b	119.00bc	53.14d	32.44abc	84.81a	29.33b
B5D250	230.41ab	137.48a	163.03a	150.26a	91.47ab	37.88ab	101.88a	35.98ab
B5D500	188.46bc	139.13a	163.91a	144.55a	95.05ab	37.94ab	112.78a	38.24ab
B10D0	155.53cd	106.24bc	104.48b	110.04c	59.22cd	35.93abc	83.39a	27.44b
B10D250	182.07c	124.39abc	161.49a	131.86ab	84.97ab	43.83a	100.49a	36.99ab
B10D500	166.80cd	133.87ab	162.68a	144.41a	90.96ab	43.43a	100.91a	43.45a
MSD	44.76	27.79	33.81	21.67	23.19	13.23	NS	11.16

Means with the same letter(s) within a column are not significantly different at (p < 0.05), MSD: Minimum Significance Difference, NS: Not Significantly Different, B: Biochar, D: DAP. Units: units (mM pNP × kg⁻¹ × h⁻¹).

Table 4

Effect of biochar and DAP on acid and alkaline phosphomonoesterases at Mau Narok, Kenya

Mau Narok
Variety Treatment	Short rain season (Oct – Dec 2020)				Long rain season (April - July 2021)
	Destiny		Shangi		Destiny		Shangi
	Acid enzyme	Alkaline enzyme	Acid enzyme	Alkaline enzyme	Acid enzyme	Alkaline enzyme	Acid enzyme	Alkaline enzyme
B0D0	109.46cd	23.41d	87.15def	25.92d	92.67abc	18.73b	85.31ab	28.07c
B0D250	135.06bc	88.71c	136.52bc	36.73d	101.66abc	18.42b	97.30ab	29.47c
B0D500	174.54a	94.36c	179.49a	106.94b	108.22ab	30.83ab	111.19a	30.39c
B5D0	101.91de	123.40ab	76.43ef	116.16b	74.10c	32.52a	94.03ab	31.37c
B5D250	111.98cd	140.01a	107.72cde	123.43b	107.85ab	37.94a	106.30ab	38.93abc
B5D500	154.44ab	135.40a	134.12bc	120.79b	112.52a	37.76a	113.43a	42.61abc
B10D0	69.61e	90.72c	72.22f	75.43c	80.67bc	33.51a	75.81b	36.38bc
B10D250	154.69ab	106.61bc	114.45cd	103.27b	90.77abc	40.76a	86.77ab	49.84ab
B10D500	146.18ab	143.62a	156.27ab	150.36a	88.76abc	44.13a	93.20ab	51.99a
MSD	33.09	26.29	34.28	25.96	27.98	13.55	30.81	15.53

Means with the same letter (s) within a column are not significantly different at (p < 0.05), MSD: Minimum Significance Difference, NS: Not Significantly Different, B: Biochar, D: DAP. Units: units (mM pNP × kg⁻¹ × h⁻¹).

amendments (P ≤ 0.001) (Table 5), (Table 6). Applied soil amendment of B5D500 gave the highest soil nitrate in Egerton for both short and long rain seasons for both varieties The lowest soil nitrate concentration was observed where no soil amendments were applied (Table 5). Conversely, in Mau Narok, B5D0 and B5D250 gave the highest soil nitrate in short and long rain seasons under Shangi Like Egerton, the control treatment gave the lowest soil nitrate in Mau Narok (Table 6). Still in Egerton, soil amendment B5D500 gave the highest soil ammonium in Egerton in the short rain season for both varieties. However, in the long rain season at Egerton, treatments B5D500 and B5D250 were not significantly different for soil ammonium (Table 5). The same trend was observed in Mau Narok for soil ammonium (Table 6).

3.4. Extractable Phosphorus

Soil phosphorus significantly varied with the applied soil amendments (P ≤ 0.001). Significant increases in soil P were observed from applying the recommended rate of DAP and the combination of biochar and inorganic fertilizer, DAP. Low soil P levels were found in soils under sole biochar and unammended soils. Soil P levels differed during the growing seasons (P ≤ 0.001) (Table 7). The highest soil phosphorus of 136 mg∙kg⁻¹ was observed under B5D250 grown with Shangi variety in the short rain season at Mau Narok. The lowest 2.0 mg∙kg⁻¹ was found in soils amended with B5D0 during the long rains planted with Destiny in

Table 5

Effect of biochar and DAP on soil nitrate and ammonium at Egerton, Kenya

Egerton
Variety Treatment	Short rain season				Long rain season
	Destiny		Shangi		Destiny		Shangi
	Nitrate-N	NH 4 +	Nitrate-N	NH 4 +	Nitrate-N	NH 4 +	Nitrate-N	NH 4 +
B0D0	57.34d	11.97e	84.74d	23.11c	35.78ef	42.49d	26.92f	13.81d
B0D250	112.79bc	29.49cd	174.58b	30.46bc	81.43d	56.96c	58.90de	28.12bc
B0D500	136.46ab	41.75bc	187.88ab	38.53ab	129.12c	78.11b	103.55bc	38.52ab
B5D0	72.15cd	15.54e	93.98d	22.83c	25.55f	25.37ef	20.22f	20.74cd
B5D250	136.17ab	47.63ab	202.11ab	36.46ab	159.57b	84.98ab	120.84b	38.53ab
B5D500	160.39a	57.19a	216.14a	48.11a	194.67a	92.14a	154.57a	50.18a
B10D0	58.84d	13.01e	109.37d	20.73c	18.49f	18.07f	36.33ef	14.33d
B10D250	122.10ab	20.32de	142.70c	24.58c	71.43d	35.28de	76.53cd	21.49cd
B10D500	117.07ab	30.77cd	174.43b	30.45bc	55.76de	57.37c	86.94cd	28.27bc
MSD	43.49	13.19	30.05	11.78	28.43	13.95	30.82	12.05

Means with the same letter(s) within a column are not significantly different at (p < 0.05), Nitrate-N: Nitrate Nitrogen (MgNO₃/L), NH 4 + : Ammonium nitrogen ( MgNH 4 + /L), MSD: Minimum Significance Difference, B: Biochar, D: DAP.

Table 6

Effect of biochar and DAP on soil nitrate and ammonium at Mau Narok, Kenya

Mau Narok
Variety Treatment	Short rain season				Long rain season
	Destiny		Shangi		Destiny		Shangi
	Nitrate-N	NH 4 +	Nitrate-N	NH 4 +	Nitrate-N	NH 4 +	Nitrate-N	NH 4 +
B0D0	84.32d	18.38ef	131.76c	13.19e	39.63b	76.64d	34.76f	63.67d
B0D250	117.86cd	36.25bcd	190.89b	34.58cd	137.00a	113.04c	61.43ef	146.74c
B0D500	140.05bc	45.27b	242.36a	48.76bc	146.92a	153.07b	118.88cd	191.65b
B5D0	103.38cd	15.03f	116.45c	15.67e	36.24b	40.73e	82.41de	68.09d
B5D250	179.37ab	43.70bc	134.12c	61.33b	151.63a	177.18a	162.61ab	276.43a
B5D500	206.98a	74.50a	185.52b	88.09a	165.74a	185.21a	173.12a	265.21a
B10D0	117.62cd	33.40d	68.17d	25.33de	40.67b	37.28e	65.84ef	59.62d
B10D250	178.59ab	27.17de	134.67c	33.31cd	123.59a	38.00e	106.69cd	82.14d
B10D500	173.32ab	33.97cd	128.85c	38.01cd	73.59b	65.16d	123.65bc	88.95d
MSD	40.79	9.97	37.89	16.65	47.78	21.44	39.42	33.04

Means with the same letter(s) within a column are not significantly different at (p < 0.05), Nitrate-N: Nitrate Nitrogen (MgNO₃/L), NH 4 + : Ammonium nitrogen ( MgNH 4 + /L), MSD: Minimum Significance Difference, B: Biochar, D: DAP.

Table 7

Effect of biochar and DAP on soil phosphorus (P) at two agro-ecological zones in Kenya

Variety	Short rain season				Long rain season
	Egerton		Mau Narok		Egerton		Mau Narok
	Destiny	Shangi	Destiny	Shangi	Destiny	Shangi	Destiny	Shangi
B0D0	16.63e	33.90ab	29.73d	11.02d	4.40a	3.53a	4.92a	4.23a
B0D250	28.70bc	24.37bc	45.47c	19.48d	3.87a	6.20a	3.05a	4.12a
B0D500	38.32ab	30.17ab	98.17a	37.27c	4.16a	7.60a	5.53a	3.37a
B5D0	17.60de	39.83a	23.70de	16.10d	4.20a	4.50a	2.00a	3.30a
B5D250	26.89cd	40.07a	33.73cd	136.00a	4.87a	4.52a	3.62a	3.77a
B5D500	40.20a	36.60a	15.48e	87.83b	6.78a	5.30a	2.10a	3.20a
B10D0	16.23e	32.07ab	90.37a	17.10d	3.88a	5.73a	6.23a	4.37a
B10D250	28.30bc	24.07bc	100.47a	37.47c	3.48a	5.60a	2.80a	4.53a
B10D500	29.90bc	18.80c	74.47b	42.90c	4.53a	8.12a	3.12a	5.28a
MSD	10.12	11.15	10.12	11.15	NS	NS	NS	NS

Means with the same letter(s) within a column are not significantly different at (p < 0.05), P: Phosphorus (mg∙kg⁻¹), MSD: Minimum Significance Difference, NS: Not Significantly Different, B: Biochar, D: DAP.

Mau Narok. However, the low soil phosphorus under B5D0 was not significantly different from soil phosphorus levels of other soil amendments. In Egerton, during the short rains, the highest soil phosphorus was under soils amended with B5D500 planted with Shangi and was not significantly different from soils under a recommended rate of DAP all planted with Destiny. Soil amendment B10D0 had the least soil phosphorus, which was not significantly different from B5D0, and the control which were all planted with Destiny. However, in the long rains, the highest soil phosphorus was found in B10D500 planted with Shangi and did not differ significantly from the soil phosphorus in all the other soil amendments. In Mau Narok, during the short rains, B5D250 treated soils planted with Shangi had the most soil phosphorus. Unamended soils planted with Destiny had the least soil phosphorus. Soils amended with B10D0 had the highest soil phosphorus in the long rain season but were not significantly different from all the other soils under the different soil amendments (Table 7).

4. Discussion

4.1. Soil pH

Soil pH is a very vital soil chemical property that affects nutrient availability, and consequently impacts crop performance. It influences the availability of plant nutrients, mainly nitrogen and phosphorus [39] [40] . If the soils are very acidic (<5.5), there is a tendency for some nutrients to be fixed, leading to their imbalances in the soil [41] . Initially, before biochar was added, soil pH of the two sites was acidic, i.e., 6.01 and 5.00 at Egerton and Mau Narok respectively, which is typical of most potato-growing areas in Kenya. With biochar application, this rose to acceptable range of 5.5 - 6.5 necessary for potato growth in Kenya. Soil acidity in these areas is attributed to constant use of chemical fertilizers, mostly DAP, which supply H⁺ ions whose accumulation in soil will acidify the soils in the long run [41] . The cheap and environmentally friendly biochar thus stands as a right amendment to raise the soil pH to suitable range for potato growth.

In this study, there was an increase in soil pH after applying biochar amendments, mainly at the end of the first season, either singly or in combination with inorganic fertilizer. Increments of biochar from 5 t∙ha⁻¹ to 10 t∙ha⁻¹ either in sole or in combination with fertilizer resulted into either an increase or a decrease in soil pH. The increase in soil pH could be due to alkaline nature of biochar; thus, there was a liming effect of biochar on the soils. These results agree to findings by Mensah et al. [23] . Many studies have found similar results of increased soil pH after applying biochar to acidic soils [14] [42] [43] [44] . Biochar is also known to produce carbonates that are liming in nature that aid in raising the soil pH by reacting with soil hydrogen ions [45] . However, in the long rain season, there was a decline in soil pH after applying biochar. This decrease in soil pH can be attributed to the soil's buffering capacity. Meng et al. [46] and Wang et al. [47] established that different soil types have different buffering capacities which determine their response to changes in their soil pH. Elsewhere, studies have observed a decline in soil pH after biochar use [48] [49] [50] [51] . This could be due to oxidation of COO- a form of acidic carboxyl groups [52] . Yang et al. [53] also found out that biochar caused a sharp decline in soil pH after biochar application in which the biochar effect could not be manifested, which could have been due to the short period of two months. This calls for long-term biochar studies to adequately quantify the amendment's sustainability. On the other hand, sole application of DAP lowered soil pH due to its acidifying character. This is because this fertilizer promotes nitrification leading to a lower soil pH [48] [54] . However, integration of biochar and DAP showed an increase in soil pH. This is consistent with the results obtained by Chan et al. [55] who found an increase in soil pH when biochar was combined with nitrogenous fertilizer of ammonium nitrate.

4.2. Soil Phosphomonoesterases

Soil phosphorus enzymes, especially the soil phosphomonoesterases, play a pivotal role in mineralizing organic P to inorganic P that can be utilized by plants [56] . The soil phosphomonoesterases are pH-dependent with acid phosphomonoesterases thriving in acidic soils ranging from 4 to 6.5 while the alkaline phosphomonoesterases are dominant in alkaline soils of pH 9 to 11 [57] . The sites in this study were generally acidic, explaining the dominance of acid phosphomonoesterases over alkaline phosphomonoesterases.

Soils treated with DAP showed high soil enzyme levels; the inorganic fertiliser was source of substrate to the enzymes [58] . Similar results were also reported by Rejsek et al. [58] . Also Margalef et al. [26] reported increased phosphomonoesterases activities after applying inorganic nitrogenous fertilisers. Acid phosphomonoesterases were higher than the alkaline phosphomonoesterases after amendment with biochar. Similar results were obtained by Antonious et al. [59] after amending soils with biochar where the acid phosphomonoesterases were raised by 115% whereas the alkaline phosphomonoesterases did not show significant differences. In this study, biochar increased soil enzyme levels. This is attributed to biochar’s potential to refine soil microbial activity by promoting a conducive microhabitat and modifying the rhizosphere [60] . However, some studies have reported low enzyme levels under biochar amended soils due to the biochar inhibiting the enzymes or substrate [48] . There was reduced enzyme activity in the long rain season. This is attributed to the dry season and the reduced soil nutrients. Phosphomonoesterase activity is driven by soil moisture and soil nutrients in which the enzymes’ activities decrease more during dry soil periods [26] . The low reduced enzyme activity could also have been due to spraying the potatoes with chemicals to control diseases. These chemicals inhibit soil enzymatic activity where the enzymes are denatured and inhibited by the chemical compound in the pesticides [61] . In a study done by Maria et al. [61] , Ridomil fungicide that was also used in this study decreased acid phosphomonesterases by 73%. Literature also shows a decrease in soil P enzymes following fumigation [62] .

4.3. Inorganic Soil Nitrogen

Soil inorganic nitrogen (SIN) is important to plants since plants take up inorganic nitrogen in the forms of ammonium and nitrate [63] [64] [65] . The highest increases in soil nitrate and ammonium for the two sites in the two seasons were observed in plots where biochar interacted with diammonium phosphate. This is attributed to reduced nutrient leaching and mineralization which improves nutrient availability [66] [67] . Elsewhere, studies have shown that biochar significantly enhances soil inorganic nitrogen [67] [68] . Also, the combination of biochar and DAP could increase both ammonium and nitrate because of the synergistic effect of the two fertilizers. [39] reported an increase in soil inorganic nitrogen due to synergism between biochar and NPK fertilizer in soils grown with maize for one season. It is suggested that biochar could have enabled this by decreasing the leaching of nitrogen ions, enhancing nitrification by promoting the nitrifiers’ microbial activity and reducing the rate of volatilization. However, the combination of DAP at recommended rate of 500 kg∙ha⁻¹ with biochar at 10 t∙ha⁻¹ resulted in lower inorganic nitrogen than with 5 t∙ha⁻¹, and this could be due to the increased soil C/N ratio that led to immobilization of nitrogen. This was also reported by Huang et al. [69] , who observed a decline in mineral nitrogen when biochar application increased from 10 to 30 t∙ha⁻¹. These results show that biochar can adsorb ammonium nitrogen [70] . The performance of biochar in soils depends on various factors such as environmental factors, fertilization, soil and biochar properties, biochar application rates and climatic conditions [70] .

Addition of biochar to soils has shown a positive, negative or neutral effect on soil inorganic nitrogen [66] . This is evidenced in this study in which there were positive results in the first season which declined in the second season, and this was due to the dry season that was experienced in the second season. Soil inorganic nitrogen concentration also varied across the sites since the sites have different soil properties and variation in soil inorganic nitrogen was attributed to climatic conditions. Generally, the use of biochar in combination with DAP gave the highest inorganic nitrogen content in both seasons across sites

A single application of biochar at B5D0 and B10D0 gave low soil inorganic nitrogen results and this was due to the low nitrogen content of 0.35 g∙kg⁻¹ in the biochar used. This is in line with Mensah et al. [23] who also attributed the low soil inorganic nitrogen to the low nitrogen in the biochar used. Furthermore, the biochar could have retained the ammonium and nitrates, reducing their concentration in the soil solution [71] . This could also be attributed to the immobilization of N [66] . Addition of inorganic fertiliser gave high soil inorganic nitrogen. This is because inorganic fertilizers readily release nutrients to the soil and enhance nitrification by availing the substrate to the microbes [66] . Additionally, there could have been reduced soil inorganic nitrogen losses like immobilisation and denitrification [48] . This study showed varying trends across the two seasons, similar results were reported by McElligott et al. [72] who suggested long-term studies to track the effectiveness of biochar as an amendment.

4.4. Soil Phosphorus

Phosphorus availability in the soil depends on several factors, such as soil pH and organic matter. Phosphorus tends to be fixed in strongly acidic soils, mainly below 5.5 [73] . This leads to nutrient deficiency in most crops [74] . However, potatoes can tolerate acidic soils. Phosphorus availability for potatoes can be increased by raising the pH to 6 - 7 [41] . Across the two seasons and sites, there was an increase in soil phosphorus from the sole application of DAP and the combination of DAP with biochar. An explanation for this is the ready supply of phosphorus through the chemical fertilizer and additive effect associated with biochar given the optimal pH which made phosphorus readily available [43] .

Additionally, the biochar promoted phosphorus mineralization from organic P to inorganic P. These results are in line with those of Farooque et al. [75] , in which biochar application significantly increased available soil phosphorus. Some studies attribute the positive effect of biochar on soil phosphorus to the phosphorus contained in the biochar itself [74] . However, for this study, the biochar phosphorus was very low at 0.08 mg∙kg⁻¹ and could not lead to a more significant increment in soil phosphorus. The P differences in sites are because the performance of biochar in soils relies mainly on environmental conditions, soil properties, and climatic conditions [76] [77] . At 10 t∙ha⁻¹ of biochar, available soil phosphorus was reduced, and this may be due to immobilization in which the phosphates are strongly sorbed on the biochar surfaces.

One of the main mechanisms in which biochar increases soil available P is the enhancement of arbuscular mycorrhiza, which promotes P availability and uptake [60] . Literature showed that the enzymatic activity of phosphatases declined during insufficient soil moisture, thus reducing phosphorus mineralization, resulting in low available phosphorus [26] . The low phosphorus levels in the long rain season could be due to the drop in soil pH since increased acidity tends to fix soil P, making it unavailable in the soil. Furthermore, positive effects of biochar are usually observed in strongly acidic and nutritionally poor soils [77] . However, the study sites were not deficient in P, which could have led to lower soil phosphorus content after biochar incorporation. Also, the low P levels could be attributed to immobilisation and sorption of P onto the biochar surfaces [77] .

5. Conclusion

Declining soil fertility lowers potato productivity in Kenya. Sustainable strategies need to be put in place to solve this problem. Here we propose the use of biochar at 5 t∙ha⁻¹ combined with DAP at 500 kg∙ha⁻¹ (B5D500) that synergistically led to better chemical soil properties and enzyme build up. The obtained results from this study show that the combination of biochar and inorganic fertilizer had a significant effect on selected soil properties. The decline in soil properties in the second season after the amendment application for some soil properties calls for long-term studies to evaluate the long-term effects of the use of biochar on soil properties.

Acknowledgements

Special thanks to Mastercard Foundation @ RUFORUM through Transforming African Agricultural Universities to meaningfully contribute to Africa’s growth and development (TAGDev) program at Egerton University for funding this research.

Conflicts of Interest

The authors declare no conflicts of interest regarding the publication of this paper.

References

1. Stewart, Z.P., Pierzynski, G.M., Middendorf, B.J. and Prasad, P.V.V. (2020) Approaches to Improve Soil Fertility in Sub-Saharan Africa. Journal of Experimental Botany, 71, 632-641. https://doi.org/10.1093/jxb/erz446

2. Wawire, A.W., Csorba, á., Tóth, J.A., Michéli, E., Szalai, M., Mutuma, E., et al. (2021) Soil Fertility Management among Smallholder Farmers in Mount Kenya East Region. Heliyon, 7, e06488. https://doi.org/10.1016/j.heliyon.2021.e06488

3. Tully, K., Sullivan, C., Weil, R. and Sanchez, P. (2015) The State of Soil Degradation in Sub-Saharan Africa: Baselines, Trajectories and Solutions. Sustainability, 7, 6523-6252. https://doi.org/10.3390/su7066523

4. Bado, V.B. and Bationo, A. (2018) Chapter One-Integrated Management of Soil Fertility and Land Resources in Sub-Saharan Africa: Involving Local Communities. In: Sparks, D.L., Eds., Advances in Agronomy, Academic Press, Cambridge, 1-33. https://doi.org/10.1016/bs.agron.2018.02.001 https://www.sciencedirect.com/science/article/pii/S006521131830018X

5. Nair, K.P. (2019) Soil Fertility and Nutrient Management. In: Nair, K.P., Ed., Intelligent Soil Management for Sustainable Agriculture: The Nutrient Buffer Power Concept, Springer International Publishing, Cham, 165-189. https://doi.org/10.1007/978-3-030-15530-8_17

6. Ma, H., Egamberdieva, D., Wirth, S., Li, Q., Omari, R.A., Hou, M., et al. (2019) Effect of Biochar and Irrigation on the Interrelationships among Soybean Growth, Root Nodulation, Plant P Uptake, and Soil Nutrients in a Sandy Field. Sustainability, 11, Article 6542. https://doi.org/10.3390/su11236542

7. Beesigamukama, D., Mochoge, B., Korir, N., Musyoka, M.W., Fiaboe, K.K.M., Nakimbugwe, D., et al. (2020) Nitrogen Fertilizer Equivalence of Black Soldier Fly Frass Fertilizer and Synchrony of Nitrogen Mineralization for Maize Production. Agronomy, 10, Article 1395. https://doi.org/10.3390/agronomy10091395

8. MacCarthy, D.S., Darko, E., Nartey, E.K., Adiku, S.G.K. and Tettey, A. (2020) Integrating Biochar and Inorganic Fertilizer Improves Productivity and Profitability of Irrigated Rice in Ghana, West Africa. Agronomy, 10, Article 904. https://doi.org/10.3390/agronomy10060904

9. Frene, J.P., Frazier, M., Gardner, T.G. and Senwo, Z.N. (2022) Validation of Soil Enzyme Activity Assay for a Biogeochemical Cycling Index in Biochar Amended Soils. Advances in Enzyme Research, 10, 61-73. https://doi.org/10.4236/aer.2022.103004

10. Liu, W.Y., Yan, X., Li, J., Jiao, N. and Hu, S. (2017) Biochar Amendments Increase the Yield Advantage of Legume-Based Intercropping Systems over Monoculture. Agriculture, Ecosystems & Environment, 237, 16-23. https://doi.org/10.1016/j.agee.2016.12.026

11. Shikorire, T.J., Asudi, G.O., Ng’ang’a, M.M., Kirubi, G. and Hassanali, A. (2019) Analysis of Emission Profiles from Charcoal Produced from Selected Tree Species by Different Pyrolysis Methods. International Journal of Environmental Science and Technology, 16, 5995-6004. https://doi.org/10.1007/s13762-019-02220-x

12. Zhang, S.Y, Wu, Z., Yan, X., Gunina, A., Kuzyakov, Y., et al. (2020) Effects of Six-Year Biochar Amendment on Soil Aggregation, Crop Growth and Nitrogen and Phosphorus Use Efficiencies in a Rice-Wheat Rotation. Journal of Cleaner Production, 242, Article ID: 118435. https://doi.org/10.1016/j.jclepro.2019.118435

13. Tian, X., Li, Z., Wang, Y., Li, B. and Wang, L. (2021) Evaluation on Soil Fertility Quality under Biochar Combined with Nitrogen Reduction. Scientific Reports, 11, Article No. 13792. https://doi.org/10.1038/s41598-021-93200-0

14. Aneseyee, A.B. and Wolde, T. (2021) Effect of Biochar and Inorganic Fertilizer on the Soil Properties and Growth and Yield of Onion (Allium cepa) in Tropical Ethiopia. The Scientific World Journal, 2021, e5582697.

15. Yu, L., Lu, X., He, Y., Brookes, P.C., Liao, H. and Xu, J. (2017) Combined Biochar and Nitrogen Fertilizer Reduces Soil Acidity and Promotes Nutrient Use Efficiency by Soybean Crop. Journal of Soils and Sediments, 17, 599-610. https://doi.org/10.1007/s11368-016-1447-9

16. Jia, Y., Hu, Z., Ba, Y. and Qi, W. (2021) Application of Biochar-Coated Urea Controlled Loss of Fertilizer Nitrogen and Increased Nitrogen Use Efficiency. Chemical and Biological Technologies in Agriculture, 8, Article No. 3. https://doi.org/10.1186/s40538-020-00205-4

17. Obia, A., Mulder, J., Hale, S.E., Nurida, N.L. and Cornelissen, G. (2018) The Potential of Biochar in Improving Drainage, Aeration and Maize Yields in Heavy Clay Soils. PLOS ONE, 13, e0196794. https://doi.org/10.1371/journal.pone.0196794 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5947891/

18. Qian, L., Chen, L., Joseph, S., Pan, G., Li, L., Zheng, J., et al. (2014) Biochar Compound Fertilizer as an Option to Reach High Productivity but Low Carbon Intensity in Rice Agriculture of China. Carbon Management, 5, 145-154. https://doi.org/10.1080/17583004.2014.912866

19. Sika, M.P. and Hardie, A.G. (2014) Effect of Pine Wood Biochar on Ammonium Nitrate Leaching and Availability in a South African Sandy Soil. European Journal of Soil Science, 65, 113-119. https://doi.org/10.1111/ejss.12082

20. Yang, F., Sui, L., Tang, C., Li, J., Cheng, K. and Xue, Q. (2021) Sustainable Advances on Phosphorus Utilization in Soil via Addition of Biochar and Humic Substances. Science of the Total Environment, 768, Article ID: 145106. https://doi.org/10.1016/j.scitotenv.2021.145106

21. Bai, S.H., Reverchon, F., Xu, C.Y., Xu, Z., Blumfield, T.J., Zhao, H., et al. (2015) Wood Biochar Increases Nitrogen Retention in Field Settings Mainly through Abiotic Processes. Soil Biology and Biochemistry, 90, 232-240. https://doi.org/10.1016/j.soilbio.2015.08.007

22. Xu, C.Y., Hosseini-Bai, S., Hao, Y., Rachaputi, R.C.N., Wang, H., Xu, Z., et al. (2015) Effect of Biochar Amendment on Yield and Photosynthesis of Peanut on Two Types of Soils. Environmental Science and Pollution Research, 22, 6112-6125. https://doi.org/10.1007/s11356-014-3820-9

23. Mensah, A.K. and Frimpong, K.A. (2018) Biochar and/or Compost Applications Improve Soil Properties, Growth and Yield of Maize Grown in Acidic Rainforest and Coastal Savannah Soils in Ghana. International Journal of Agronomy, 2018, Article ID: 6837404. https://doi.org/10.1155/2018/6837404

24. Jin, Y., Liang, X., He, M., Liu, Y., Tian, G. and Shi, J. (2016) Manure Biochar Influence upon Soil Properties, Phosphorus Distribution and Phosphatase Activities: A Microcosm Incubation Study. Chemosphere, 142, 128-135. https://doi.org/10.1016/j.chemosphere.2015.07.015

25. Faloye, O.T., Alatise, M.O., Ajayi, A.E. and Ewulo, B.S. (2017) Synergistic Effects of Biochar and Inorganic Fertiliser on Maize (Zea mays) Yield in an Alfisol under Drip Irrigation. Soil and Tillage Research, 174, 214-220. https://doi.org/10.1016/j.still.2017.07.013

26. Margalef, O., Sardans, J., Fernández-Martínez, M., Molowny-Horas, R., Janssens, I.A., Ciais, P., et al. (2017) Global Patterns of Phosphatase Activity in Natural Soils. Scientific Reports, 7, Article No. 1337. https://doi.org/10.1038/s41598-017-01418-8

27. Yadav, V., Karak, T., Singh, S., Singh, A.K. and Khare, P. (2019) Benefits of Biochar over other Organic Amendments: Responses for Plant Productivity (Pelargonium graveolens L.) and Nitrogen and Phosphorus Losses. Industrial Crops and Products, 131, 96-105. https://doi.org/10.1016/j.indcrop.2019.01.045

28. Rees, F., Simonnot, M.O. and Morel, J.L. (2014) Short-Term Effects of Biochar on Soil Heavy Metal Mobility Are Controlled by Intra-Particle Diffusion and Soil pH Increase. European Journal of Soil Science, 65, 149-161. https://doi.org/10.1111/ejss.12107

29. Madiba, O.F., Solaiman, Z.M., Carson, J.K. and Murphy, D.V. (2016) Biochar Increases Availability and Uptake of Phosphorus to Wheat under Leaching Conditions. Biology and Fertility of Soils, 52, 439-446. https://doi.org/10.1007/s00374-016-1099-3

30. Meng, C., Tian, D., Zeng, H., Li, Z., Yi, C. and Niu, S. (2019) Global Soil Acidification Impacts on Belowground Processes. Environmental Research Letters, 14, Article ID: 074003. https://doi.org/10.1088/1748-9326/ab239c

31. Wang, X., Tang, C., Mahony, S., Baldock, J.A. and Butterly, C.R. (2015) Factors Affecting the Measurement of Soil pH Buffer Capacity: Approaches to Optimize the Methods. European Journal of Soil Science, 66, 53-64. https://doi.org/10.1111/ejss.12195

32. Ezike, O.B. (2016) Impact of Biochar, Cattle Manure and Mineral Fertilizer on Soil Properties and Grain Yield of Maize (Zea mays L.) in the Guinea Savannah Zone of Ghana. Master’s Thesis, Kwame Nkrumah University of Science and Technology, Kumasi Ghana.

33. Prommer, J., Wanek, W., Hofhansl, F., Trojan, D., Offre, P., Urich, T., et al. (2014) Biochar Decelerates Soil Organic Nitrogen Cycling but Stimulates Soil Nitrification in a Temperate Arable Field Trial. PLOS ONE, 9, e86388. https://doi.org/10.1371/journal.pone.0086388

34. Sarfraz, R., Shakoor, A., Abdullah, M., Arooj, A., Hussain, A. and Xing, S. (2017) Impact of Integrated Application of Biochar and Nitrogen Fertilizers on Maize Growth and Nitrogen Recovery in Alkaline Calcareous Soil. Soil Science and Plant Nutrition, 63, 488-498. https://doi.org/10.1080/00380768.2017.1376225

35. Zhang, M., Riaz, M., Zhang, L., El-desouki, Z. and Jiang, C. (2019) Biochar Induces Changes to Basic Soil Properties and Bacterial Communities of Different Soils to Varying Degrees at 25 mm Rainfall: More Effective on Acidic Soils. Frontiers in Microbiology, 10, Article 1321. https://doi.org/10.3389/fmicb.2019.01321 https://www.frontiersin.org/article/10.3389/fmicb.2019.01321

36. Cheng, C.H., Lehmann, J., Thies, J.E., Burton, S.D. and Engelhard, M.H. (2006) Oxidation of Black Carbon by Biotic and Abiotic Processes. Organic Geochemistry, 37, 1482-1488. https://doi.org/10.1016/j.orggeochem.2006.06.022

37. Yang, Y., Ma, S., Zhao, Y., Jing, M., Xu, Y. and Chen, J. (2015) A Field Experiment on Enhancement of Crop Yield by Rice Straw and Corn Stalk-Derived Biochar in Northern China. Sustainability, 7, 13713-13725. https://doi.org/10.3390/su71013713

38. Kaboneka, S., Ong’or, B., Chantal, K., Nsavyimana, G., Donatien, B. and Népomuscène, N. (2019) Effects of Urea and Di-Ammonium Phosphate Application on Acidification of Three Burundi Representative Soils. International Journal of Advances in Scientific Research and Engineering, 5, 183-191. https://doi.org/10.31695/IJASRE.2019.33474

39. Chan, K.Y., Van Zwieten, L., Meszaros, I., Downie, A. and Joseph, S. (2007) Agronomic Values of Greenwaste Biochar as a Soil Amendment. Soil Research, 45, 629-634.

40. Tang, X., Bernard, L., Brauman, A., Daufresne, T., Deleporte, P., Desclaux, D., et al. (2014) Increase in Microbial Biomass and Phosphorus Availability in the Rhizosphere of Intercropped Cereal and Legumes under Field Conditions. Soil Biology and Biochemistry, 75, 86-93. https://doi.org/10.1016/j.soilbio.2014.04.001

41. Adetunji, A.T., Lewu, F.B., Mulidzi, R. and Ncube, B. (2017) The Biological Activities of β-Glucosidase, Phosphatase and Urease as Soil Quality Indicators: A Review. Journal of Soil Science and Plant Nutrition, 17, 794-807. https://doi.org/10.4067/S0718-95162017000300018

42. Rejsek, K., Vranova, V., Pavelka, M. and Formanek, P. (2012) Acid Phosphomonoesterase (E.C. 3.1.3.2) Location in Soil. Journal of Plant Nutrition and Soil Science, 175, 196-211. https://doi.org/10.1002/jpln.201000139

43. Antonious, G.F., Turley, E.T. and Dawood, M.H. (2020) Monitoring Soil Enzymes Activity before and after Animal Manure Application. Agriculture, 10, Article 166. https://doi.org/10.3390/agriculture10050166

44. Solaiman, Z.M., Abbott, L.K. and Murphy, D.V. (2019) Biochar Phosphorus Concentration Dictates Mycorrhizal Colonisation, Plant Growth and Soil Phosphorus Cycling. Scientific Reports, 9, Article No. 5062. https://doi.org/10.1038/s41598-019-41671-7

45. Maria Mihaela, M., Badulescu, L. and Israel-Roming, F. (2018) Effect of Pesticides on Enzymatic Activity in Soil. Bulletin of University of Agricultural Sciences and Veterinary Medicine Cluj-Napoca Animal Science and Biotechnologies, 75, 80-84. https://doi.org/10.15835/buasvmcn-asb:2017.0040

46. Riah, W., Laval, K., Laroche-Ajzenberg, E., Mougin, C., Latour, X. and Trinsoutrot-Gattin, I. (2014) Effects of Pesticides on Soil Enzymes: A Review. Environmental Chemistry Letters, 12, 257-273. https://doi.org/10.1007/s10311-014-0458-2

47. Bronson, K., Schepers, J.S. and Raun, W. (2008) Forms of Inorganic Nitrogen in Soil. In: Schepers, J.S. and Raun, W.R., Eds., Nitrogen in Agricultural Systems, John Wiley & Sons, Hoboken, pp. 33-38.

48. Carlisle, E., Myers, S.S., Raboy, V. and Bloom, A.J. (2012) The Effects of Inorganic Nitrogen form and CO2 Concentration on Wheat Yield and Nutrient Accumulation and Distribution. Frontiers in Plant Science, 3, Article 195. https://doi.org/10.3389/fpls.2012.00195 https://www.frontiersin.org/articles/10.3389/fpls.2012.00195/full

49. Zhang, P., Dumroese, R.K. and Pinto, J.R. (2019) Organic or Inorganic Nitrogen and Rhizobia Inoculation Provide Synergistic Growth Response of a Leguminous Forb and Tree. Frontiers in Plant Science, 10, Artticle 1308. https://doi.org/10.3389/fpls.2019.01308 https://www.frontiersin.org/article/10.3389/fpls.2019.01308

50. Nguyen, T.T.N., Xu, C.Y., Tahmasbian, I., Che, R., Xu, Z., Zhou, X., et al. (2017) Effects of Biochar on Soil Available Inorganic Nitrogen: A Review and Meta-Analysis. Geoderma, 288, 79-96. https://doi.org/10.1016/j.geoderma.2016.11.004

51. Huang, L.Q., Fu, C., Li, T.Z., Yan, B., Wu, Y., Zhang, L., et al. (2020) Advances in Research on Effects of Biochar on Soil Nitrogen and Phosphorus. IOP Conference Series: Earth and Environmental Science, 424, Article ID: 012015. https://doi.org/10.1088/1755-1315/424/1/012015

52. Zhang, L., Jing, Y., Chen, C., Xiang, Y., Rezaei Rashti, M., Li, Y., et al. (2021) Effects of Biochar Application on Soil Nitrogen Transformation, Microbial Functional Genes, Enzyme Activity and Plant Nitrogen Uptake: A Meta-Analysis of Field Studies. GCB Bioenergy, 13, 1859-1873. https://doi.org/10.1111/gcbb.12898

53. Huang, M., Zhang, Z., Zhai, Y., Lu, P. and Zhu, C. (2019) Effect of Straw Biochar on Soil Properties and Wheat Production under Saline Water Irrigation. Agronomy, 9, Article 457. https://doi.org/10.3390/agronomy9080457

54. Adekiya, A.O., Agbede, T.M., Olayanju, A., Ejue, W.S., Adekanye, T.A., Adenusi, T.T., et al. (2020) Effect of Biochar on Soil Properties, Soil Loss, and Cocoyam Yield on a Tropical Sandy Loam Alfisol. The Scientific World Journal, 2020, Article ID: 9391630. https://doi.org/10.1155/2020/9391630

55. Lehmann, J. and Rondon, M. (2006) Bio-Char Soil Management on Highly Weathered Soils in the Humid Tropics. In: Biological Approaches to Sustainable Soil Systems, CRC Press, New York.

56. Kristin, M. (2011) Biochar Amendments to Forest Soils: Effects on Soil Properties and Tree Growth. Master’s Thesis, University of Idaho, Moscow.

57. Johan, P.D., Ahmed, O., Omar, L. and Hasbullah, A. (2021) Phosphorus Transformation in Soils Following Co-Application of Charcoal and Wood Ash. Agronomy, 11, Article 2010. https://doi.org/10.3390/agronomy11102010

58. Bayu, D., Dejene, A., Alemayehu, R. and Gezahegn, B. (2017) Improving Available Phosphorus in Acidic Soil Using Biochar. Journal of Soil Science and Environmental Management, 8, 87-94. https://doi.org/10.5897/JSSEM2015.0540

59. Farooque, A.A., Zaman, Q., Abbas, F., Hammad, H.M., Acharya, B. and Easu, T. (2020) How Can Potatoes Be Smartly Cultivated with Biochar as a Soil Nutrient Amendment Technique in Atlantic Canada? Arabian Journal of Geosciences, 13, Article No. 336. https://doi.org/10.1007/s12517-020-05337-3

60. Nigussie, A., Kissi, E., Misganaw, M. and Ambaw, G. (2012) Effect of Biochar Application on Soil Properties and Nutrient Uptake of Lettuces (Lactuca sativa) Grown in Chromium Polluted Soils. American-Eurasian Journal of Agricultural & Environmental Sciences, 12, 369-376.

61. Bista, P., Ghimire, R., Machado, S. and Pritchett, L. (2019) Biochar Effects on Soil Properties and Wheat Biomass vary with Fertility Management. Agronomy, 9, Article 623. https://doi.org/10.3390/agronomy9100623

62. Muthoni, J. and Nyamongo, D.O. (2009) A Review of Constraints to Ware Irish Potatoes Production in Kenya. Journal of Horticulture and Forestry, 1, 098-102.

63. Neina, D. (2019) The Role of Soil pH in Plant Nutrition and Soil Remediation. Applied and Environmental Soil Science, 2019, e5794869. https://doi.org/10.1155/2019/5794869

64. Kizito, S., Luo, H., Lu, J., Bah, H., Dong, R. and Wu, S. (2019) Role of Nutrient-Enriched Biochar as a Soil Amendment during Maize Growth: Exploring Practical Alternatives to Recycle Agricultural Residuals and to Reduce Chemical Fertilizer Demand. Sustainability, 11, Article 3211. https://doi.org/10.3390/su11113211

65. Tabatabai, M.A. and Bremner, J.M. (1969) Use of p-Nitrophenyl Phosphate for Assay of Soil Phosphatase Activity. Soil Biology and Biochemistry, 1, 301-307. https://doi.org/10.1016/0038-0717(69)90012-1

66. Paz-Ferreiro, J., Trasar-Cepeda, C., Leirós, M.C., Seoane, S. and Gil-Sotres, F. (2009) Biochemical Properties in Managed Grassland Soils in a Temperate Humid Zone: Modifications of Soil Quality as a Consequence of Intensive Grassland Use. Biology and Fertility of Soils, 45, 711-722. https://doi.org/10.1007/s00374-009-0382-y

67. Bottomley, P.J., Angle, J.S. and Weaver, R.W. (2020) Methods of Soil Analysis, Part 2: Microbiological and Biochemical Properties. John Wiley & Sons, Hoboken, 1152.

68. Major, J. (2009) A Guide to Conducting Biochar Trials. International Biochar Initiative, NY, 32.

69. Mumia, B, Muthomi J.W., Narla, R.D., Nyongesa, M.W. and Olubayo, F.M. (2018) Seed Potato Production Practices and Quality of Farm Saved Seed Potato in Kiambu and Nyandarua Counties in Kenya. WJAR, 6, 20-30. https://doi.org/10.12691/wjar-6-1-5

70. Hellmuth, H. (2019) Effects of Potato-Legume Intercropping and Variety on Potato Performance in Different. Master’s Thesis, University of California, Davis.

71. Okalebo, J.R., Gathua, K.W. and Woomer, P.L. (2002) Laboratory Methods of Soil and Plant Analysis: A Working Manual. Vol. 2, Second Edition, Sacred Africa, Nairobi, 131.

72. Jaetzold, R., Schmidt, H., Hornetz, B. and Shisanya, C. (2007) Farm Management Handbook of Kenya: Subpart B1a Southern Rift Valley Province. Ministry of Agriculture, Kenya, in Cooperation with the German Agency for Technical Cooperation (GTZ).

73. Li, Y., Hu, S., Chen, J., Müller, K., Li, Y., Fu, W., et al. (2018) Effects of Biochar Application in Forest Ecosystems on Soil Properties and Greenhouse Gas Emissions: A Review. Journal of Soils and Sediments, 18, 546-563. https://doi.org/10.1007/s11368-017-1906-y

74. Cornelissen, G., Pandit, N.R., Taylor, P., Pandit, B.H., Sparrevik, M. and Schmidt, H.P. (2016) Emissions and Char Quality of Flame-Curtain “Kon Tiki” Kilns for Farmer-Scale Charcoal/Biochar Production. PLOS ONE, 11, e0154617. https://doi.org/10.1371/journal.pone.0154617

75. Coomes, O.T. and Miltner, B.C. (2017) Indigenous Charcoal and Biochar Production: Potential for Soil Improvement under Shifting Cultivation Systems. Land Degradation & Development, 28, 811-821. https://doi.org/10.1002/ldr.2500

76. Puehringer, H. (2016) Effects of Different Biochar Application Rates on Soil Fertility and Soil Water Retention in On-Farm Experiments on Smallholder Farms in Kenya. SLU, Dept. of Soil and Environment, Uppsala. https://stud.epsilon.slu.se/8955/

77. Rawat, J., Saxena, J. and Sanwal, P. (2019) Biochar: A Sustainable Approach for Improving Plant Growth and Soil Properties. Biochar—An Imperative Amendment for Soil and the Environment, 1-17. https://doi.org/10.5772/intechopen.82151 https://www.intechopen.com/chapters/65070