1 Introduction

Biochar has gained attention over the last decade due to its ability to improve soil fertility, plant growth and water retention in poor quality soils, while also providing carbon sequestration (Kätterer et al. 2019). Biochar provides several benefits to the soil by increasing its nutrient contents and water retention capacity. This is due to the cation exchange capacity and porous nature of the biochar, which is essential in promoting plant growth and development (Esmaeelnejad et al. 2017). However, reports on biochar’s ability to improve plant growth and soil properties are mixed, depending primarily on the structure and quality of the original soil (Paz-Ferreiro et al. 2016). Many studies have been undertaken to compare different types of biochars (Hansen et al. 2016), in different soils (Günal et al. 2018) and occasionally in comparison with other organic waste derived soil amendments (Rezaee et al. 2021). However, there are no studies that compare the import of higher quality soils such as potting mixture (PM) which may be more economic or straightforward.

PM is an organic and nutrient rich medium that is much more efficient for agriculture practice in comparison to sandy and clayey soils (Dahanayake et al. 2013). It is frequently used to build up suitable quality soil mass in small-scale areas of concentrated horticulture (Danish et al. 2022). However, the relative benefits of importation of high-quality soil like PM into a poor-quality sandy soil compared with an addition or combination with biochar is unknown. While PM contains high organic content, it may also suffer high evapotranspiration and water/nutrient leaching (Richard 1996). The addition of organic amendments such as compost with biochar has proven to improve soil properties such as enhanced water and nutrient retention, promoting microbial communities, and increasing plants yield and quality (Liu et al. 2021). The application of PM with biochar could potentially result in improving poor textured sandy soil for high water retention to support plant growth and development. However, an investigation is needed to support this theory.

Therefore, the hypothesis put forward in this study is that the application of biochar and PM on sandy soil might enhance plant growth, soil nutrients and water holding capacity individually, and that their combined application may bring additional benefits due to their different characteristics. The current study aims to address this research gap by investigating the individual and combined effect of PM and biochar in a sandy soil of Qatar, focusing on their impact on plant growth. Furthermore, the research also intends to define the effect of PM and biochar amendment on soil nutrient retention and plant uptake of nutrients. The basil (Ocimum basilicum) plant was considered for the current investigation as it requires moderately fertile well drained soil conditions, maximum sunlight and hot climates (Makri and Kintzios 2008), making it suitable for the proposed study soils and climate. The biochar in this study was produced from cabbage waste. Cabbage is widely consumed globally with a production of 60 million tonnes and about 30 to 50% of cabbage becomes waste from harvest to plate (Hossain et al. 2014). Additionally, Pradhan et al., (2020b) has demonstrated through characterization that it has properties that should be amenable for agricultural use, which this study also aims to confirm. Therefore, the primary objective of this paper is to characterize and analyze the effect of combined PM and cabbage waste biochar applications on sandy soil, while observing the growth of basil plants in treated soil mixtures.

2 Materials and Methods

2.1 Materials

Cabbage waste was collected from a university canteen at Hamad bin Khalifa University, Qatar. After segregation, the waste was washed thoroughly, chopped in to small pieces ranging from 3 to 5 cm in size and dried in an oven at 75 °C (Mazac 2016). After drying, the wastes were then ground and sieved to a size of < 75 µm. The cabbage waste was pyrolyzed at 360 °C under a heating rate of 5 °C min−1 with a residence time of 15 min in a muffle furnace (Lindberg/Blue, Thermo Scientific) following a previous characterization and optimization study by Pradhan et al., (2020b). Sandy soil was used for the pot test collected from a golf course (25°18′06.0"N 51°25′31.6"E). The PM was bought from a local supermarket. The soil was oven-dried and passed through a 2 mm sieve to remove stones and other debris.

2.2 Biochar and Soil Characterization

Elemental analysis (C,H,N) of biochar and soil was performed using a combustion type elemental analyzer (EA 3000, EuroVector). Three replicates of each sample were used for determining the elemental composition.

Proximate analysis was performed to determine the percentage of fixed carbon (FC), volatile matter (VM) and ash present in the biochar and soil according to the standard procedure D7582-15 (ASTM 2015) using a thermogravimetric analyzer (SDT-2960, Thermal Instruments). The heating rate followed a linear scale from 40 to 105 °C at a heating rate of 5 °C min−1 and was held at a temperature at 105 °C for 7 min to eliminate moisture content. The temperature was then raised to 850 °C to determine the ash content. The following formula was used to calculate the fixed carbon content (Pradhan et al. 2020b):

$$FC\% = 100\% - (VM +Ash)\%$$
(1)

The composition of sand (2 to 0.05 mm), silt (0.05 to 0.002 mm), and clay (< 0.002 mm) were determined by following the method of Whiting et al. (2015). The percentage of sand, silt, and clay were used in reference with the USDA soil texture triangle to determine the soil texture (USDA 2017). Chemical analysis of the materials was performed using Fourier transform infrared (FT-IR) spectroscopy. Samples were prepared by crushing in a mortar with KBr at a ratio of 1:100 and analyzed in a frequency range of 4000 to 400 cm−1 by measuring 64 scans of each sample at a resolution of 4 cm−1 on a Nicolet iS50 (Thermo Scientific).

The surface chemistry of the biochar, soil and PM was studied by X-ray photoelectron spectroscopy (XPS) using an ESCALAB 250Xi (Thermo-Fisher). The instrument utilizes an electron beam of a monochromatic Al Kα anode with a photon energy of 1486.6 eV for excitation of the samples to obtain photoelectron spectra. The spot size was approximately 0.65 mm. The samples were ground into fine powder before the analysis. A beam incident angle of 45° was utilized for sample placement. The binding energy range for the survey scan ranged from 0 to 1351 eV with a pass energy of 100 eV. The scans for elemental regions of C, N and O involved a pass energy of 20 eV and step size energy of 0.1 eV. Each energy position was calibrated to the C1 line at 284.8 eV for extraneous hydrocarbon. Data was interpreted and deconvoluted by utilizing a Gaussian–Lorentzian value of 30% for optimization of the spectra in Advantage software (Thermo Scientific).

The surface characteristics of the biochar and PM were determined by scanning electron microscope (SEM; Quanta 650 FEG) of gold sputtered samples at a 5 kV acceleration voltage.

The electrical conductivity (EC) and pH of biochar, PM and soil were determined by using a multimeter with EC and pH probes (Orion Star A121 and A329, Thermo Scientific). The samples were prepared at a ratio of 1:10 with Milli-Q water and mixed at a speed of 150 rpm for 1 h (Lee et al. 2013).

2.3 Determination of Nutrients Content

The micronutrient contents of oven-dried (70 °C) plant biomass, PM and soil samples were measured by inductively coupled plasma optical emission spectroscopy (ICP-OES 5110, Agilent) after first performing microwave-assisted digestion. The plant biomass 100 mg was weighed with three replicates of each treatment and digested with 8 mL HNO3 and 2 mL of H2O2 in microwave-assisted digester (Ethos UP, Milestone). For the biochar, PM, and soil (pre- and post- harvest) 100 mg was digested with 8 mL of HNO3, 5 mL of HCl, and 2 mL of H2O2. The temperature was set at 200 °C with a ramp rate of 13 °C min−1, a pressure of 90 bar and a residence time of 30 min. Then 2 mL of HCl was added and kept for 24 h to digest the leftover biomass and soil. The concentration of phosphate (PO43−) in plant and soil samples were measured by a segmented flow analyzer (Sans + , Skalar) based on the manufacturer’s prescribed ammonium heptamolybdate and potassium antimony (III) oxide tartrate method.

2.4 Pot Test

A pot experiment using basil (Ocimum basilicum) was conducted in an outdoor area with a mean temperature of 27 °C and range of 22 to 40 °C. The planting area received north and west facing sun and the pots were rotated daily to avoid shading biases. Three identical pots were used for each test condition, with a diameter of 135 mm and a height of 105 mm. Each pot was planted with 50 mg of basil seeds. The test conditions evaluated are outlined in Table 1 and considered varying fractions of biochar, PM, or a mixture of both applied at a mass ratio of 0%, 2%, or 6%. After seeding, a watering of 150 mL tap water was done to moisten the soil for seed germination. The subsequent watering throughout the study consisted of 150 mL water on a daily basis. The drainage losses were not recorded during the experiment. The number of seeds germinated were counted after 15 days in each treatment. After germination, the plant heights were measured throughout the experiment at regular intervals of 10 days. The plants were harvested after 70 days before the start of the flowering stage. The plants were carefully removed from the pots and the shoots and roots were separated at the base of stem. The biomass and length of shoots and roots was recorded separately. Measures relating to plant growth such as number of leaves, chlorophyll, leaf relative water contents (LRWC), and membrane stability index (MSI) were taken during the experiment after 30 days of germination and described in the following section. After harvesting, the soil from each treatment was cleaned from any debris such as roots hairs, leaves or any other contaminants. The soil was then air dried and packed in sealed bags for further analysis.

Table 1 A detailed description of the seven test conditions used in this study describing the mass percentage of biochar and PM applied to the soil
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2.5 Leaf-Based Assessment of Plant Growth

2.5.1 Chlorophyll Content of Leaves

Fresh leaves were carefully cut from the base of the stem with a sharp blade and immediately transferred to black polythene bags and placed in an ice bath. 10 mL of 80% acetone was added to 100 mg of leaves from each treatment and ground in a pestle and mortar placed in an ice bath. The samples were centrifuged at 1000 rpm for 5 min, the supernatant was collected, and then analyzed by UV spectrophotometer. Chlorophyll a (Chla), chlorophyll b (Chlb), carotenoids, and total chlorophyll were determined according to Eq. 2–5 (Lichtenthaler 1987).

$$Chla = 12.25 {A}_{663.2}- 2.79{ A}_{646.8}$$
(2)
$$Chlb = 21.50 {A}_{646.8}- 5.10 {A}_{663.2}$$
(3)
$$Carotenoids = \frac{1000{A}_{470}- 1.82{Chla}-85.02{Chlb}}{198}$$
(4)
$$Total Chlorophyll = Chl a+ Chl b$$
(5)

where A is the value of the absorbance obtained at different wavelengths of 663.2, 646.8, and 470 nm.

2.5.2 Leaf Relative Water Content

The leaf relative water content (LRWC) of plants signifies the equilibrium between transpiration and water movement to the leaf tissues (Lugojan and Ciulca 2011). The younger and fully expanded leaves were used for assessing LRWC and samples were taken from each replication of every treatment. The leaves were cut from the base with a sharp blade and the fresh weight (FW) was taken immediately before soaking in distilled water for 24 h at room temperature. After drying with tissue paper, the turgid weight (TW) was taken and then the leaves were transferred to an oven at 65 °C for 72 h until a constant dried weight (DW) was obtained. The relative water content of leaves was determined by following the standard procedure reported by Lazcano‐Ferrat and Lovatt, (1999) and given below in Eq. 6.

$$LRWC (\%)= \frac{FW-DW}{TW-DW}\times 100$$
(6)

2.5.3 Membrane Stability Index (MSI)

The membrane stability index (MSI) relates to the plant response to stress imposed by heat and drought conditions. The determination of MSI was conducted following the method described by Sairam et al. (1997), in which 0.01 g of leaf discs were used from every treatment. Rinsed leaf samples were transferred into test tubes and 15 mL of Milli-Q water was added prior to incubation at room temperature for 24 h as described by Petrov et al. (2018). The initial EC (EC1) was measured, then the samples were placed in a boiling water bath for 30 min at a temperature of 100 °C. After cooling the final EC was measured (EC2). MSI was calculated according to Eq. 7:

$$MSI\left(\%\right)=\left(1-\frac{{EC}_{1}}{{EC}_{2}}\right)\times 100$$
(7)

2.6 Soil Water Retention

Water retention was measured by an incubation experiment monitoring weight loss with time. Cylindrical containers having diameter 60 and height 100 mm were used. The soil was added with biochar and PM treatments as described in Table 1 with 3 replications. The initial dry weight of the containers and soil was recorded (W0). Then 60 mL of water (Ww) was added to each container gradually and allowed to drain for 3 h before weighing again (Wt). The containers were then transferred to an incubator set at 45 °C and weighed at regular intervals until dry. The water retention was then calculated according to Eq. 8:

$$WR\left(\%\right)=\frac{({W}_{0}+{W}_{w})-{W}_{t}}{{(W}_{0}+{W}_{w})}\times 100$$
(8)

2.7 Statistical Analysis

Statistical analysis was performed for different fractions of biochar and PM applications with plant and soil parameters using analysis of variance (ANOVA) in the Statistix 10.0 software package. All-pairwise comparison tests were done to find homogenous groups using Tukey’s Least Significant Difference (LSD) test with a significance level of 0.05.

3 Results

3.1 Characterization of Soil, Biochar and PM

3.1.1 General Characteristics

A detailed physicochemical property of the soil, PM and biochar is described in Table 2. PM showed a high EC of 1032 µS cm−1, while both soil and biochar had a lower EC of 883 and 559 µS cm−1, respectively. The soil texture was categorized as sandy loam according to the USDA soil texture triangle (USDA 2017). The soil and biochar were mildly alkaline, while PM was mildly acidic. Biochar had a higher content of carbon, hydrogen and nitrogen than potting mix. The soil analysis showed very low contents of carbon, hydrogen and nitrogen which represents the poor quality of typical sandy soil in Qatar. Potassium (K) and zinc (Zn) were highest in biochar, followed by PM. Biochar and PM shared a similar manganese (Mn) content and for all three elements values were higher than those in the soil. In contrast, the sandy soil had the highest aluminum content, followed closely by PM, but was much lower in biochar.

Table 2 The physical and chemical properties of the soil, biochar, and PM
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3.1.2 FT-IR Analysis

Cabbage waste biochar and PM showed similar FT-IR spectra consistent with organic material, which was distinctly different to that of the soil samples. The bands observed between 2750 and 3450 cm−1 represent the existence of aliphatic amine, carboxylic acid or alcohol compounds having stretching vibrations of N–H, C–H and O–H groups, respectively. The lowest stretching of these groups was found in the soil sample. The peak around 3418 cm−1 wavelength (Fig. 1) showed less C–H (hydroxyl group) stretching in biochar compared to PM due to the loss of these groups during pyrolysis (Chen et al. 2012; Pradhan et al. 2020a). The bands at the wavelength between 1000 to 1400 cm−1 can be ascribed to aromatic ester compounds (Chen et al. 2012). In addition, the presence of peaks in the wavelength range of 1600 to 1750 cm−1 represent the stretching of C = O and N–H in the samples (Chen et al. 2012; Pradhan et al. 2020a). The soil samples shows peaks from 700 to 800 cm−1 which are distinct from the other samples and represent O-Si(Al)-O and Si–Si bending and stretching vibrations associated with the presence of quartz (Theodosoglou et al. 2010) and/or aluminum ions, which were present as confirmed by ICP-OES.

Fig. 1
figure 1

FT-IR analysis of biochar, PM (potting mix) and the soil used for the pot test

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3.1.3 XPS Analysis

XPS spectra for biochar, PM, and soil are shown in Fig. 2. The survey scans of C1s, N1s, and O1s (Fig. 2a) show the existence of carbon, nitrogen, and oxygen elements. Peaks in the C1s spectra in Fig. 2b, and 2c were found around 289, 287, and 285 eV, and are attributed to the presence of carbonyl (C = O, –C–O) and sp2 hybridized carbon (C–C) (Pradhan et al. 2020a). For the sandy soil (Fig. 2d), the peaks were very low for C–C and C-O as well as for elemental carbon (Fig. 2a), indicating its highly mineral nature.

Fig. 2
figure 2

(a) XPS spectrum of biochar, PM (potting mix), and soil. C1s scan of (b) biochar, (c), PM, and (d), soil. Fig b, c, and d shows the presence of different carbonyl and carbon compounds in the samples

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The deconvolution of soil XPS spectra also showed peaks at 530.4, 439.2, and 102.6 eV, that could be attributed to the presence of Mn2p, Ca2s and Si2p (Lacerda et al. 2020), consistent with typical minerals found in sandy soil. A small peak of carbon was also observed in soil. The presence of Lorentzian peaks deconvoluted from N1s spectra were observed at the binding energy of 400, and 399 eV in biochar, PM, and soil. This can be due to the presence of amides and amines. FT-IR also confirms the presence of these functional groups in the samples.

3.1.4 SEM Analysis

The structural morphology of biochar and PM are shown in Fig. 3. Biochar showed a highly porous structure which could be efficient to improve soil water retention capacity and nutrients uptake (Ma et al. 2016). The larger cracks and disturbed porous structure are due to the fibrous nature of the feedstock, but numerous smaller holes were also present and attributed to discharge of volatized compounds during the pyrolysis process. The surface structure of PM shown in Fig. 3(d-f) presented a less structured material that was mainly fractal aggregates with the presence of some fibers. The aggregates and fibers of PM were mainly smooth on the surface, having some major cracks but with lower porosity than biochar.

Fig. 3
figure 3

SEM micrographs of (a-c) for biochar, and (d-f) for PM (potting mix) at different magnifications. Image magnifications as follows: Fig. (a) and (d) at 1000 ×, Horizontal frame width (HFW) = 414 µm Fig (b) and (e) at 5000 ×, HFW = 82.9 µm and Fig (c) and (f) at 10,000 ×, HFW = 41.4 µm. The pores in sample (c) are identified in the image

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3.2 Effect of Biochar and PM Application to Sandy Soil on Plant Growth

Figure 4 Provides the germination and biomass development of basil during the pot testing. The maximum improvement in germination was observed for B6P6, giving rise to a 32% increase over the control (p = 0.02) (Fig. 4b). All other treatments containing biochar also showed statistically significant improvement, with the exception of B6P0 which led to a significant reduction in seed germination of 29% (p = 0.04). PM only treatments did not show significant improvement over the control.

Fig. 4
figure 4

Plant growth measurements showing (a) plant length from 15 to 55 days before harvesting the plants, (b) number of plants germinated in each treatment over the test, (c) number of leaves per plant at harvest, (d) fresh shoot weight at harvest, (e) fresh root weight at harvest, (f) dry shoot weight at harvest, (g) dry root weight at harvest, (h) leaf length at harvest and (i) root length at harvest. The treatment details are as follows: B0P0 = control treatment, B0P2 = 2% PM, B2P0 = 2% B, B0P6 = 6% PM, B6P0 = 6% B, B2P2 = 2% B + PM, B6P6 = 6% B + PM. Error bars show the standard deviation of three replicates. Letters above the bars represent the statistical significance between the treatments. Conditions sharing the same letter are not significantly different (p > 0.05)

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The plant length was observed over sixty days of the pot test and is shown in Fig. 4a. The basil plant in B6P0 showed a restricted growth during the first fifteen days at the seedling stage, which was equivalent to only 60% of that in the control treatment (B0P0). Over the early periods of the test, B2P0 and B2P2 containing moderate (2%) biochar loading showed the best growth amongst the conditions. However, as the test progressed, those samples containing the higher 6% biochar loading, both with (B6P6) and without (B6P0) PM became the tallest basil plants. In comparison, samples containing only PM (B0P2 and B0P6) showed a steady but slower rate of growth through the test, being roughly half the height of B6P0 at the end of the test. B6P0 showed a relatively higher shoot growth than B2P2 (27% more) and B6P6 (11% more) before the crop harvest and was 147% higher compared to the control treatment (B0P0).

The plant leaves in the control condition (B0P0) had a restricted growth with an average of 4 leaves per plant (Fig. 4c). The 2% PM (B0P2) treatment showed a similar average number of leaves at 5 leaves per plant (p = 0.944). However, all other treatments showed statistically significant improvements and followed a similar trend to plant growth. The maximum average number of leaves per plant was 20, recorded in B6P0, and was statistically different (p < 0.05) to the 16 leaves per plant observed in the next best condition of B6P6. Leaf length also showed a similar trend. B6P0 had the maximum leaf length of 27 mm, which was 10 mm more than the plants in the control condition (Fig. 4h). However, most treatments were statistically similar to the control, with B6P0 and B6P6 notably better and statistically similar to each other. A significant (p < 0.05) increase of 50% and 46% in root length was observed in B6P0 and B6P6 over the control condition, giving root lengths of 10.4 ± 0.60 cm and 10.1 ± 0.62 cm respectively. Although all other conditions were not statistically different to the control, even the most limited improvement for the 2% PM treatment (B0P2) was 17% more than control.

The maximum fresh shoot weight of 9.4 g was recorded in B6P6 (Fig. 4g) which was 3 g more than the second highest shoot weight in condition B6P0 (p = 0.05) and 7.6 g greater than the control (B0P0) – a weight increase of more than 5 times. Similar observations were noticed for fresh root weight which was maximum in the B6P6 condition at 5.9 g (Fig. 4h). B2P2 condition gave the next greatest increase, but this was not statistically different to the B6P0 condition. Similar results were observed in case of dry mass of both shoots (Fig. 4i) and roots (Fig. 4j). The greatest increase in biomass of B6P6 is attributed to the high density of plants and number of leaves, enhancing both fresh and dry biomass values.

3.3 Effect of Biochar and PM on Photosynthesis

The effect of different treatments of biochar and PM on chlorophyll, carotenoids, and chlorophyll a/b ratio in plants is shown in Fig. 5. The addition of 6% biochar without (B6P0) and with PM (B6P6) significantly enhanced the leaf chlorophyll content (p < 0.05) by 9 and 8 µg g−1 of leaf biomass, respectively (Fig. 5a). The increase in chlorophyll content by increasing the loading rate of biochar as compared to PM, or biochar and PM, at the same level was notably more pronounced. Similar trends in chlorophyll b concentration were also observed, with the highest concentration occurring in B6P0 condition leaves (2.5 ± 0.17 µg g−1) followed by B6P6 (2.3 ± 0.09 µg g−1) as shown in Fig. 5b.

Fig. 5
figure 5

(a) Chlorophyll a, (b) chlorophyll b, (c) total carotenoid concentrations in plant leaf samples in µg g.−1 of fresh weight (FW), (d) Chlorophyll a to b ratio, (e) MSI (membrane stability index), and (f) LRWC (leaf relative water contents). The treatment details are as follows: B0P0 = control treatment, B0P2 = 2% PM, B2P0 = 2% B, B0P6 = 6% PM, B6P0 = 6% B, B2P2 = 2% B + PM, B6P6 = 6% B + PM. Statistical analysis of all-pairwise comparisons were applied with LSD test and α value of 0.05. Error bars represent the standard deviation of 3 replicates. The letters on the bars represent the statistical significance between the treatments. Conditions sharing the same letter are not significantly different (p > 0.05)

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The carotenoids were promoted most under the highest application of biochar with PM (B6P6 condition) at a content of 3.5 ± 0.13 µg g−1, followed by B6P0 with 3.3 ± 0.17 µg g−1 (Fig. 5c). However, B2P2 also showed high carotenoids contents compared to control with a value of 2.8 ± 0.12 µg g−1. Chlorophyll a/b ratio was also altered by the addition of both biochar and PM (Fig. 5d). There was an almost 100% increase in the ratio of chlorophyll a to b for condition B6P0 while B6P6 showed an increase of 90% (Fig. 5d) compared to the control.

3.4 Effect on Membrane Stability Index and Leaf Relative Water Content

Membrane stability index (MSI) and the leaf relative water content (LRWC) of leaves showed a mostly similar trend to photopigment leaf concentrations (Fig. 5e and f). The control showed the lowest MSI of 80% and B6P0 showed the highest MSI of 92%. There was no significant difference between B0P2 and the control (p = 0.902), but most of the other treatments showed a significant improvement (p < 0.05). Application of biochar to the soil with and without PM showed an increase of MSI which was more than that for the same loading rate of PM only to the soil. Treatments containing PM (alone or combined) showed no statistical improvement between 2 and 6% loading, while discernable difference existed for biochar loading levels.

Only a slight variation of LRWC was found among the treatments because sufficient irrigation was provided throughout the experiment. Most of the treatments were not significantly different from the others but two clear trends were observed in which 1) amendments containing biochar increased RWC more than those containing only PM and 2) increasing amendment loading increased the LRWC. All treatments except B0P2 were statistically different from the control (p = 0.05) which had a LRWC of 93% as shown in Fig. 5f. The highest LRWC was observed for B6P0 with a value of 98%, followed closely by B6P6.

3.5 Effect of Biochar and PM on Nutrient of Plant Biomass

The phosphorus (P) concentration of the basil plant roots in the control condition was 0.09 mg g−1. The maximum P content occurred in B6P0 (0.70 mg g−1), with B6P6 and B2P2 statistically similar (p > 0.05), as shown in Fig. 6a. For potassium (K), the plant roots showed an increasing trend with the application of biochar and/or PM (Fig. 6b). At 2% loadings biochar (B2P0) increased K by 65% compared to sandy soil, while PM (B0P2) increased K by 23%. The largest K contents increases over the control were achieved by B6P0 (147%) and B6P6 (133%) (Fig. 6b). The concentration of zinc (Zn) showed a relatively similar trend to K (Fig. 6c), except B0P6 which was greater than B2P2 (p < 0.05). All treatments except 2% PM (B0P2) showed an increase in root Zn content. The highest Zn concentration of 0.11 mg g−1 was recorded in the roots of B6P0, which was 55% greater than in the roots of the control. For manganese (Mn), the lowest concentration detected in roots was 0.05 mg g−1 for the control. Application of either PM or biochar increased Mn levels. The supplement of 6% biochar (B6P0) showed the highest Mn concentration of 0.12 mg g−1, corresponding to a 147% increase over the control, followed by B6P6, which was statistically similar with 122% increase over the control. The aluminum (Al) concentration in plant roots showed a significant reduction (p < 0.05) with all biochar and PM treatments relative to the control. In B0P0, the Al content was 2.7 mg g−1, as displayed in Fig. 6e. The largest reduction compared to the control was 63% for the B6P0 condition and the second largest reduction of 54% was observed for B6P6.

Fig. 6
figure 6

Nutrient concentration in roots and shoots of basil plant under investigation (a) PO3−-P, (b) potassium (K), (c) zinc (Zn), (d) manganese (Mn), and (e) aluminum (Al), contents in mg g.−1 of plant root and shoot biomass. The treatment details are as follows: B0P0 = control treatment, B0P2 = 2% PM, B2P0 = 2% B, B0P6 = 6% PM, B6P0 = 6% B, B2P2 = 2% B + PM, B6P6 = 6% B + PM. Error bars represent the standard deviation between 3 replicates of each treatment. The letters on the bars represent the statistical significance between the treatments. Treatments sharing a common letter show no significant difference (p > 0.05)

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The impact of biochar application on shoot nutrient content is shown in Fig. 6. The lowest concentration of P was observed in the control condition having a concentration of 0.6 mg g−1 (Fig. 6a) while addition of the smallest fraction (2%) of biochar (B2P0) increased phosphorus content by 35% compared with the control (B0P0). The highest concentration of phosphorus was recorded in B6P0 (1.1 mg g−1) followed by B6P6 (1 mg g−1), with no significant difference between these two highest contents. The control (B0P0) had the lowest K concentration at 1.35 mg g−1, as shown in Fig. 6b, with the 2% PM treatment not showing a significant difference. The highest K content was recorded in B6P0 (6% biochar) which contained double the concentration of the control at 3.7 mg g−1. This was statistically different to all treatments except B6P6 which had a 140% increase over the control. Zn also showed similar behavior in the shoot biomass as to what was observed in the roots, with the exception that all treatments showed a statistically significant difference to the control (0.09 mg g−1), as shown in Fig. 6c. B6P0 measured the highest Zn content, which was 67% higher than the control, but was statistically similar to the other 6% amendment treatments of B0P6 and B6P6. Mn content in the shoots of plants also followed the same trend as in the roots. Both B6P0 and B6P6 showed an increase of ~116% for Mn content in the shoots (Fig. 6d). For Al, the maximum shoot concentration was observed in the control with 0.76 mg g−1. The highest biochar-comprising conditions, B6P0 and B6P6, had the lowest Al content (Fig. 6e), which was 49% and 45% lower than the control, respectively.

3.6 Nutrients Retention in the Soil

The P concentration in the soil was lowest in the control treatment (B0P0) at 0.34 mg g−1 (Table 3). The highest concentrations were found in B6P6 followed by B6P0 at 0.62 and 0.56 mg g−1, respectively. The retention of K was also higher in the combined treatment of biochar and PM, with B6P6 having 0.183 mg g−1, while the 2% application of PM (B0P2) did not show a statistical improvement over the control (p = 0.986). B6P6 had the highest Zn and Mn concentrations. For Zn, the value was 0.0265 mg g−1, which was statistically larger than all other treatments. In addition, all biochar and PM treatments differed significantly from the control condition of 0.0225 mg g−1. In contrast, for Mn four treatments showed no statistical difference to the control, with both biochar only conditions, B2P0 and B6P0, reporting slightly lower Mn concentrations (0.200 and 0.204 mg g−1, compared to 0.205 mg g−1). Only the two conditions containing 6% PM (B0P6 and B6P6) had slightly higher Mn concentrations at 0.217 and 0.222 mg g−1, respectively. The highest Al concentration was in B6P6 with 9 mg g−1 followed by B6P0 with 8.2 mg g−1 (statistically different to each other).

Table 3 The elemental composition of the soil from different treatments after harvesting the plants
Full size table

4 Discussion

The soil used in this study was consistent with typical alumina sand. It had a high silica content based on the results of the XPS, FT-IR and elemental analysis and was alkali. These types of soils are generally less desirable for plant growth due to their low organic content. Surprisingly, it had the lowest electrical conductivity amongst the three materials analyzed. This was consistent with it having the lowest K, Zn and Mn amongst the samples, though it did have the highest Al content which was much higher than the sum of the other elements. According to Raiola et al., (2014), sandy soils of tropical areas are usually high in Al, however, Al is often not in a form readily available to plants (Lin et al. 2010). The fraction of biochar or PM in each tested condition were low, and given most elements had similar orders of magnitude amongst the soil, biochar and PM, the final concentrations in the mixed soils should be similar.

However, the pH was a notable difference between soil, biochar and PM. This means that, ion mobility may differ significantly, with neutral and acidic conditions favoring movement of most ions. The more favorable pH of the biochar and PM indicate that the biochar and PM treatments could potentially improve the soil fertility by exchanging nutrients such as K, Zn and Mn to the soil. This is because it increases the availability of cations in the soil by altering its surface charge (Barrow and Hartemink 2023; Naeem et al. 2016). Most biochar studies report material with an alkali pH due to its higher ash content and the loss of acidic functional groups during the pyrolysis process (Tomczyk et al. 2020). The biochar in this study had a lower H/C and O/C ratio, consistent with reduced loss of important functional groups. This is likely from the low pyrolysis temperature of 360 °C, which may have a large influence on nutrient adsorption (Bonanomi et al. 2017). This is consistent with FT-IR and XPS which showed the presence of functional groups in biochar samples (as well as PM samples).

Sandy soil is known as a poor soil for agricultural applications because of its low water and nutrient retention. It is therefore not surprising that the addition of either biochar or PM increased the plant growth and development. All treatments were consistent in their promotion of growth at the higher of the loadings (6%), whether this was biochar, PM or a mixture of the two. It is expected that continued increase of PM up to or close to 100% would continue to improve plant germination and growth which is why soil importation is frequently utilized. In contrast, biochar may only provide additional benefit for a limited increase in amendment percentage. This limit was not found in this study, but reports of optimal values differ significantly between studies, with a number of studies finding optimal fractions at lower loading values (Kammann et al. 2017; Are 2019). Lebrun et al., (2021) found that 3% application of biochar yielded the best growth of Linum usitatissimum in a heavy metal polluted soil. For basil in a sandy soil, the limit may be close to 6% given the negative impact observed on germination in the B6P0 condition.

While retarded germination is commonly observed with biochar, it is often linked to its alkaline nature and immobilization of nutrients (Solaiman et al. 2012). This explanation does not align with the neutral biochar pH (7.50) observed in this study, which was more neutral than either native soil (pH = 8.45) or the PM (pH = 5.84). Similarly, nutrient uptake was best with biochar in this study. The reduction in germination could also be attributed to the bi-phasic nature of biochar where it has previously been observed that low biochar concentrations enhance plant growth, possibly due to stimulating the defensive mechanism of plants to different stresses, but becomes inhibitory at higher biochar loading rates (Kammann et al. 2015). This is likely linked to the specific chemical composition of the biochar or stimulation of soil microbial communities, as Bu et al., (2020) found that the application of 5% woodchip biochar resulted in a significant reduction in germination rate of Robinia pseudoacacia L, while the same loading of rice husk biochar stimulated germination, despite no significant difference in soil pH. It is unlikely to be related to the porosity and water retention capacity of the soil, as biochar B6P0 had the third highest water retention after B6P6 and B2P2 (Online Resource 1). The improvement in the presence of PM may be due to the further enhancements in water retention, but is more likely linked to binding of biochar to PM, reducing direct contact and concentration of biochar particles around seedlings (Wang et al. 2017).

The response of basil plants to biochar and PM differed significantly. Biochar induced significantly better growth than PM at both application rates when applied individually, despite its lower available organic matter. This cannot be explained by pH, since basil can grow in a wide pH range but has a preference towards acidic soils which is improved with PM application. When comparing the performance of biochar vs biochar-PM mix, results were generally similar with regards to plant growth, nutrient contents, germination and root length. However, the mixed amendment provided greater biomass growth in terms of both root and shoot mass, and for the 2% loading greater shoot length. This could potentially be explained by pH, since both PM and biochar are more acidic than the soil, but is more likely associated with water retention (Online Resource 1) due to the increased fines and decreased porosity resulting from adding amendments. Schulz et al., (2013) performed a somewhat similar study that compared biochar and compost mixtures for the growth of oats, in which the addition of biochar and compost together gave the greatest biomass yield. They attributed the performance to the higher C and N content of the soil. The slight increase in performance of many parameters observed in the B6P0 condition can be explained by the reduced germination in this condition, leading to less competition for space, nutrients and light for emergent seedlings.

From an economic perspective, potting mix costs in the order of $100 USD/m3, compared with a price 3–5 times that value for biochar. Given that these price differences exceed the improvement of biochar growth over potting mixture, careful evaluation is required in the future when determining biochar as an amendment option. It is possible that much higher PM additions could be utilized at a similar cost to the current biochar loadings. Further studies should consider whether higher PM loadings could provide similar benefits and be an economic alternative to the tested biochar loadings. Certainly, when comparing the lower biochar loading of this study (B2P0) against the higher PM loading (B0P6) the latter provides a slight improvement in shoot productivity at a slightly lower cost. However, productivity is still well below that from B6P0, B2P2 or B6P6.

The plant growth depends on several factors including the plant’s ability to utilize nutrients from the soil media and convert these into the biomolecules necessary for cell generation and metabolic regulation. Similar relative uptake of nutrients between the conditions was observed for the roots and shoots. In the case of P, the lowest concentration was recorded in the control (B0P0) while the B6P0 and B6P6 displayed high accumulation. This could be due to the slow release of phosphorus into the soil from the biochar and PM, thereby promoting its uptake by plants. Sulemana et al., (2021) also observed that addition of composted biochar resulted in significant increase in P uptake by maize plants.

K uptake showed a similar trend. Wang et al. (2018) in their study reported the biochar application increased the K uptake in both wheat and maize plants and that the biochar increases the microbial activity enhancing transformations of K to forms available to plants. Poormansour et al. (2019) also reported a maximum increase in the K uptake by fava beans upon the application of biochar, associating this with the higher content of K in the biochar. In this study, the biochar K content was roughly an order of magnitude higher than PM and two orders of magnitude higher than the soil. However, if the behavior was simply due to the availability of K, it would be expected that B6P6 would have the highest K uptake. The potential competition between the plants as described by Craine and Dybzinski, (2013) may explain the discrepancy. B6P6 showed roughly twice the total biomass growth of B6P0, while the inclusion of 6% PM would not double the K available, therefore resulting in a reduced per plant concentration of K.

The highest concentration of Mn was found in both B6P0 and B6P6. Biochar has the capability to either immobilize or mobilize Mn and other related active metals by changing oxidation states of these compounds because of its pH and redox conditions (Kumar et al. 2018). The soil retention and plant supply of Mn is of vital importance due to its contribution in improving antioxidant defense and photosynthetic pigment production (Ye et al. 2020). This was evident from their positive correlation (r = 0.918) between Mn and chlorophyll activity (Online Resource 2).

Biochar treatments showed a statistically significant improvement in reducing Al content of roots relative to PM only treatments, however, differences between biochar and the combined biochar + PM treatments were less clear. Al is a toxic compound for most plants and is a severe issue for agriculture. Toxicity of Al causes restricted plant growth, as well as biomass and root elongation, as observed in B0P0 of our study. The effective role of biochar in reducing this toxicity and uptake effect are likely linked to adsorption and ion exchange with Al, changing the form and availability of Al in the soil (Qian et al. 2018; Shetty et al. 2021). The application of biochar could have resulted in an increase in the cation exchange capacity (CEC) of the soil. This higher CEC might be responsible for the adsorption of Al on to its surface and making it unavailable for plants (Shetty et al. 2021).

Chlorophyll and other photopigments help to capture and convert sunlight into energy during photosynthesis and are therefore directly linked to plant growth. The observed behavior in chlorophyll concentrations may be explained by the presence of certain minerals in the soil conditions. While all nutrients showed strong correlation with chlorophyll levels, Al content had the highest degree of correlation (r = -0.95) (Online Resource 2). Al is reported to have inhibiting impacts on the uptake of nutrients such as calcium (Ca) and magnesium (Mg) by the plant roots (Rengel 1992). This was also observed to alter chlorophyll pigments in leaves, possibly due to interactions with photosystem II where Al can cause inhibition of the photosynthetic electron transport (Moustakas et al. 1995). Carotenoids are another important photopigment present in the leaves of plants, assisting chlorophyll for producing light energy; stabilizing the photosynthetic membranes in chloroplast; absorption of harmful free radical reactive oxygen species and organic pollutants; and securing the photosynthetic process (Dhami and Cazzonelli 2020; Kamran et al. 2021). Carotenoids were also most strongly correlated to Mn (r = 0.929) and Al (r = -0.920), while MSI also had the strongest correlation with Al (r = -0.888).

A considerable increase in chlorophyl a/b ratio was observed in both B6P6 and B6P0. Chlorophyll a is the main light harvesting pigment that converts radiant light energy to chemical energy while chlorophyll b is an auxiliary pigment which helps to broaden the light harvesting spectrum. Often an increasing chlorophyll a/b ratio is an indicator of plant stress, since chlorophyll b is less critical to the photosynthesis process, and it tends to reduce more significantly when the plant is under stress. However, the biochar application to the sandy soil increased levels of both chlorophyll a and b in this test, and so may rather be an indicator of stimulation and growth rather than stress. Farhangi-Abriz and Torabian (2018) also observed a significant increase in the total chlorophyll and a/b ratio of beans relative to the control treatment. One possible explanation is due to higher nutrient availability, such as K and Zn, and their uptake by plants under the addition of biochar and PM.

The hydration status of leaves relative to their turgid water level represented by LRWC is an indicator of the heat and drought stress on plants (Mullan and Pietragalla 2012). B6P0 showed the highest LRWC followed by B6P6 and B2P2. LWRI is linked to transpiration activity, MSI, photosynthesis, and ultimately biomass productivity (Arndt et al. 2015). In this study, LRWC followed a similar pattern to overall growth trends and photosynthetic pigmentation content of the leaves. Higher LRWC in amended soils is most likely due to an increased capacity of the soil to hold water, as evident in the water retention test (Online Resource 1). While B6P0 did not have the highest water retention in the soil, it did have significantly less plants than B2P2 or B6P6, giving more available water per plant.

Amongst the nutrients present in the soil, the largest difference in nutrient content were for the two nutrients present in the highest concentration, P and Al. While other nutrients did not differ by more than 18%, the largest soil P content was almost double than that of the control, while for Al it was 50% greater. For P, conditions that had the highest residual soil content also had the highest uptake in the plants. This indicates that the amended soils, particularly those containing biochar, likely released and greatly improved the use efficiency of P. The availability of nutrients may also allow microbial communities to grow in soil which ultimately benefit plants through symbiotic association (Xiang et al. 2017). For Al, all amended soils had higher Al than the control, despite the amendments themselves having lower Al content and plants incorporating less Al into biomass. This again indicates strong immobilization in the soil (Qian et al. 2018). Despite Al being available in low pH soils, Al can also become available in alkaline soils as reported by Brautigan et al., (2012). They concluded that the movement of Al in soil was based on charge and species of Al and the anionic form was dominant at high pH soil. These results indicated that, regardless of whether plants contained a higher or lower content of nutrients in the biomass, either biochar or biochar + PM is beneficial to retain or provide nutrients in the soil. Interestingly, the two elements best bound by biochar are both trivalent ions. Meta-analysis of phosphate availability in biochar studies demonstrated that biochar produced below 450 °C has higher plant available P than those produced at higher high temperatures (Glaser and Lehr 2019). This is possibly due to cleavage of organic bonds at low temperature releasing P while at higher temperatures increased mineral content again binds P. For Al, adsorption to biochar could be due to the bonding with alkaline elements or silicates present in the vicinity of biochar. However, it can also form surface complexation with organic oxygen containing groups through esterification reactions (Qian and Chen 2014), which were present from FT-IR.

Biochar dramatically improved basil growth over the control or pure PM addition to the sandy soil, while biochar + PM mix had the greatest productivity. As previously mentioned, changes in soil pH are not deemed to be a significant mode of action from biochar since PM did not realize the same improvements. Rather, two main modes of action are identified through the addition of biochar or biochar + PM. The first is increased nutrient retention in the soil through adsorption and immobilization. This reduces nutrient losses from soil and promotes greater root development from plants to access nutrients. This mechanism contributes to the similar plant growth and composition between the biochar only and biochar + PM mix condition. The second mechanism is a direct physical effect on pore size, creating reduced porosity and increased water holding capacity, which is greatest for the condition containing the largest portion of amending material (B6P6 – 12% total). This mechanism is important for maintaining non-stressed water conditions for the plan and promoting overall growth.

5 Conclusions

This study investigated the relative improvement in basil growth and development when using biochar, potting mix and their blend to amend a sandy soil, to determine whether biochar is a viable alternative to ameliorate poor soils in comparison to soil importation. The biochar produced at 360 °C from cabbage food waste markedly enhanced the growth of basil compared to the un-amended sandy soil (control). When compared with application of potting mix at a similar ratio, biochar outperformed potting mix in almost every aspect of plant growth, development and soil nutrient retention. These improvements corresponded to improved water retention, immobilization of Al in the soil and improved uptake of other nutrients by plants such as P, K, Zn, and Mn. However, the study also highlighted the stress imposed by high (6%) biochar loading, reducing germination. The study also confirmed the hypothesis that the combined application of biochar and potting mix would further improve plant growth due to their differing characteristics. This was primarily related to improvements in germination and water retention that led to an overall higher yield. However, most other measures were very similar between the biochar and the combined treatments.

A key finding was the beneficial role of biochar in reducing Al uptake by plants which describes its suitability in Al rich soils. In addition, biochar stimulated the significant uptake of beneficial nutrients such as P, K, Zn, and Mn by the plant roots and consequently enhanced the plant growth and soil water retention. Overall, the study demonstrated the value of biochar as a soil amendment and potential route to valorize food waste that provides greater benefits than importation of similar quantities of high-quality soil. However, it also shows the benefits of a mixed approach to soil amendment using both biochar and organic rich soils. These results contribute to our understanding of the benefits of biochar in the agriculture of desert areas, and the interactions between biochar and other organic soil amendments that could be used simultaneously. An important finding is that, despite biochar’s superiority in plant growth promotion, potting mix/soil importation may offer a more economical alternative to improve crop yield per dollar spent. This requires further detailed investigation, including long term studies considering frequency of replenishment.