Introduction
Biochar makes nutrients easily accessible for plant uptake1. It catalyses biotic and abiotic processes, notably in the rhizosphere, to boost plant nutrient delivery and absorption. Biochar has high adsorption capability, immobilises both phosphorus and nitrogen in soil, and promotes the availability of nutrients for crop development2. However, it is essential to balance biochar with plant nutrients, as an imbalance can lead to the absorption and unavailability of applied nutrients1. The accessibility of nutrients near the root system is crucial, which means that cations, such as calcium, potassium and magnesium, are essential plant nutrients that must be available near the root zone for transportation to the shoot3. Direct utilisation of the nutrient in the root or its transport via the xylem to active plant tissues is possible4. As a result, adding biochar to soil improves the availability of nutrients and nitrogen retention, increasing crop yield5. The ideal depth for incorporating biochar to maximise nutrient availability remains uncertain6. It is essential to balance biochar with plant nutrients to avoid nutrient imbalances that can lead to the adsorption and unavailability of applied nutrients7. The strength of soil is directly related to nutrient availability, and plants require several soil nutrients like nitrogen, phosphorus, and potassium. Therefore, incorporating an adequate amount of biochar into the soil can improve plant growth and yield by increasing the availability of essential nutrients while avoiding nutrient imbalances. Research has shown that biochar can potentially increase root biomass, volume, length, and diameter in vegetation grown in soil8. This increase in root growth can improve nutrient uptake and water absorption, resulting in higher crop yields. Additionally, biochar can improve soil physical properties, such as drainage and aeration, which can further benefit root health2. One area where biochar has shown particular promise is in improving root health in maize cultivation.
Applying biochar can alter the root morphology, leading to enhanced nutrient uptake, shoot growth, and increased growth and yield because biochar-amended soils showed a larger rhizosphere with biochar particles, which increased soil N retention, mainly soil-extractable nitrate–N9. Although the extent to which biochar also plays a critical function, there is less research on the proper depth of application of biochar in soil. The depth of biochar incorporation positively correlated with the growth of maize plants10.
As demonstrated in various studies, biochar application includes triggers that induce profound physiological adjustments in plants by managing soil characteristics and stress reactions. Biochar promotes root architecture in Zea mays, with increased root surface area (64%), volume (37%), and length (51%) as compared to the control, which enhances nutrient and water absorption efficiency11. According to recent studies, the biochar amendments at different soil depths have significantly impacted Zea mays root development and growth indices. For example, a long-term field trial conducted by Yan et al.12 found that biochar application improved maize root architecture as shown by total root length, root surface area, and root volume, which the authors attributed to improved soil physicochemical properties and increased microbial abundance and diversity in the rhizosphere. The main purpose of the direct discovery was to direct the expansion of performance factors and growth functions, such as the use of biochar that improves root penetration and promotes root growth where biochar was applied, primarily reducing soil bulk density and increasing porosity in the 0–10 cm layer of soil. Furthermore, biochar amendments decreased topsoil bulk density, contributing to root penetration. They increased the root weight density, root length density and root surface-area density occupied in soil, contributing to increases in maize biomass and grain yields13. Moreover, biochar application not only reduces soil compaction, but also improves soil pore structure, which can serve as passageways for deeper root penetration and elongation14 and thus enhances plant productivity, especially the 0.1–0.5 mm pore size, where root absorption of nutrients and water takes place. Our results indicate that biochar additions at adequate depths in the soil profile could markedly increase maize root distribution and overall healthy development by promoting soil physical structure and nutrient status.
Mechanistically, the porous nature of biochar and its cation exchange capacity improve soil aeration, nutrient holding capacity (Example N, P, K) and microbial activity, stimulating root development and nutrient uptake15. Such physiological adjustment supports biochar’s function in optimising plant resource utilisation, tolerance, and growth under stressful edaphic environments. Biochar is consistent with present-day green development ideals since it plays key roles in preserving ecosystem balance, reducing soil pollution, and promoting the sustainable growth of the agricultural environment. However, little information is available on the impact of biochar application at different depths on root health and crop growth. Using a new transparent rhizobox method to monitor real-time root growth continuously, this study examined the root growth dynamics in this crop, providing the first insights into the processes governing its response to suboptimal soil water availability (SWA). These transparent vessels allowed direct observation of root growth movement, direction and spread. This key improvement strategy comprehensively understands when and where roots form. It also aided scientists in understanding how roots react to biochar administered at different soil depths. The transparent rhizobox enabled real-time observation of root growth patterns such as direction, movement, and dispersion angle. This methodological improvement provided in-depth information on the spatial and temporal establishment of roots, giving an overall understanding of how roots interact when different depths of biochar are applied.
This study aimed to (1) investigate whether biochar application depth is a determining factor in enhancing maize root health, (2) promote plant growth, and (3) increase yield. (3) It also hoped to find out if the effectiveness of biochar is dependent on its soil application depth.
Results
Biochar characterisation
Thermogravimetric analysis (TGA) carried out over a 10–600 °C temperature range revealed that rice husk exhibited good thermal stability up to around 300 °C. A significant weight loss of 37.35% was observed between 300 and 350 °C Beyond 350 °C, the weight loss continued at a slower rate up to 600 °C, likely due to the decomposition of more thermally stable components such as lignin (Fig. 1).
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Fig. 1
Demonstration of thermal characteristics of feedstock assessed through Thermogravimetric Analysis.
Biochar analysis by FESEM, EDX
Surface morphology of fresh biochar (Fig. 2a–c) show that smooth and clear visible pore structure is there but in aged biochar (Fig. 2g–i) reduction pore size, weathering of pore wall and diminish the structural integrity of biochar due to physical, chemical and biological mechanism. When roots interact with biochar or root hairs enter the pore space of biochar, as shown in the biochar surface morphology. Energy Dispersive Spectroscopy of fresh rice husk biochar (Fig. 2d–f) at a magnification of 100 µm revealed that carbon was the most abundant element at 53.7% followed by oxygen at 19.9% > phosphorus 9% > potassium 6.6% > magnesium 5.8% > nitrogen 3.8% > calcium 1.2%. The elemental composition of the aged biochar sample (Fig. 2j–l) reveals that carbon was the most abundant element, comprising 37.4%, followed by oxygen, 36.4%, and silica, 12.6%. The remaining elements were in smaller quantities—iron 4.27% and aluminium 2.71%. Calcium and potassium were present at 1.18% and 1.1%, respectively, and magnesium was detected at 0.82%.
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Fig. 2
FESEM image of fresh biochar (a–c) aged biochar (g–i) mapping site fresh biochar (d), aged biochar (j) and EDS mapping of fresh (e) and aged biochar (k), different colour dotted shad represent different elemental presence. Fresh biochar elemental mapping and aged biochar elemental composition graph: fresh biochar (f), aged biochar (l).
Transition in root properties
Biochar incorporation had a significant difference in root traits (p < 0.05) as show in Fig. 3. As compared to the control (T1), a significant increase in root length was observed for biochar-amended treatments, i.e., T5 (48.2%) followed by T4 (40.4%) > T3 (24.7%) > T2 (12%). Root diameter was decreased with an increase in the depth of biochar application in the soil. The change in root diameter ranged from 0.045 to 0.038 mm. The maximum root diameter, i.e., 0.045 mm, was recorded in T1, followed by T2 = T3 (0.042 mm) > T4 (0.039 mm) > T5 (0.038 mm). An increase in root length or morphology showed that there was a significant increase in the root volume T5(42.7%), followed by T4(41.2%) > T3(26.75%) > T2(13.6%), and as compared to non-treated biochar treatment. Similarly, there was a significant increase in root fresh and dry biomass, maximum root fresh biomass recorded in T5 (55.8%), followed by T4 (44.9%) > T3 (7.1%) > T2 (3.0%). In comparison, the percent increases in root dry biomass were recorded as T5 (56.7%) followed by T4 (34.7%) > T3 (8.7%) > T2 (2.1%). Plants’ root growth angles became steeper under controlled conditions. The root angle (from the soil surface) ranged from 56.7° to 39.0°. For controlled plants, the maximum root angle was reported, i.e., 56.7° , which became steeper with increased depth of biochar amendment, i.e., 50.8°(T2) > 47.4°(T3) > 40.8°(T4) > 39.0°(T5). Furthermore, relative water content significantly increased in T5(13.1%) followed by T4(10.2%) > T3(6.7%) > T2(4.3%) as compared to control (T1).
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Fig. 3
Impact of biochar on root attributes. T1 (no biochar), T2 (Biochar amendment up to 5 cm depth), T3 (Biochar amendment up to 10 cm depth), T4 (Biochar amendment up to 15 cm depth) and T5 (Biochar amendment up to 20 cm depth). (a) root length, (b) root diameter, (c) root volume, (d) root fresh and dry biomass, (e) root spreading angle, (f) RWC% %. Different lowercases indicate significant differences between treatments (P < 0.05).
Identification of the compound exuded from the root
The GC–MS analysis revealed several common compounds across the treatments (Fig. 4). Methyl stearate was one of the most consistently abundant compounds. This fatty acid ester was present in all five treatments with relative abundances T1 (10.26%), T2 (8.67%), T3 (12.40%), T4 (12.93%) and T5 (14.65%). Another compound identified was Butylated Hydroxytoluene (BHT), a commonly used antioxidant. BHT was the most abundant compound in T1, accounting for 39.29% of the total peak area, and was also present in T5 at 11.10%. The depth of biochar incorporation appeared to influence the presence and abundance of certain compounds. For instance, 1-Cyclohexyldimethylsilyloxy-3,5-dimethylbenzene was the most abundant compound in T3 and T2, where biochar was incorporated at 10 cm and 5 cm depths, respectively. l-Norvaline, N-(2-methoxyethoxycarbonyl)- was the most abundant compound in both T2 and T5. Other notable compounds that were identified across multiple treatments include 2,8,9-Trioxa-5-aza-1-silabicyclo [3.3.3] undecane, 7,9-Di-tert-butyl-1-oxaspiro (4,5) deca-6,9-diene-2-one, Phenyl methylphosphonofluoridate, and 1-Ethylsulfanylmethyl-2,8,9-trioxa-5-aza-1-silabicyclo [3.3.3] undec. The relative abundance of these compounds varied depending on the specific treatment.
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Fig. 4
Effects of various biochar treatments on root chemical composition: T1 (no biochar), T2 (Biochar amendment up to 5 cm depth), T3 (Biochar amendment up to 10 cm depth), T4 (Biochar amendment up to 15 cm depth) and T5 (Biochar amendment up to 20 cm depth).
Microscopic study of root biochar interaction
Biochar provides an ideal environment for maize plant roots and hairs to penetrate and make robust connections, as shown in Fig. 5. These root hairs play a crucial role in increasing the overall surface area of the root system, thereby enhancing the plant’s ability to absorb water and nutrients from the surrounding soil.
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Fig. 5
The microscopic examination of the root system reveals a strong association between the root and biochar.
Shoot trait alterations influenced by biochar depth application.
The effects of biochar (applied up to different depths of soil) on the plant growth attributes are shown in Fig. 6. The plant height was significantly improved (p < 0.05) at 7 DAS and 14 DAS in T2 (7.1 cm, 12.2 cm), followed by T3 > T4 > T5. However, at 21, 28 and 35 DAS, the highest plant height was observed in T5 (19.8 cm, 31.5 cm, 35.9 cm), followed by T3 > T4 > T5 compared to non-biochar T1. Similarly, biochar incorporation up to varying depths affects maize stem girth. T2 had significantly increased stem girth at 7 and 14 DAS (5.05 mm and 7.06 mm, respectively). By 21, 28, and 35 days after sowing, T5 showed the maximum girth, i.e., 9.30 mm, 14.18 mm and 15.94 mm, respectively, compared to control T1. Incorporation of biochar up to varying depths also impacted morphological attributes like shoot fresh and dry biomass, and the number of leaves. There was a significant (p < 0.05) increase in both shoot fresh and dry biomass. The maximum fresh and dry biomass was observed in the T5 treatment, i.e., biochar incorporation up to 20 cm depth. The percentage increase in fresh biomass was 23.1%, 17.9%, 4.8%, 1.8%, and while the dry biomass increased by 7.1%, 5%, 0.7%, and 15% for T5, T4, T3, T2, respectively. The number of leaves was significantly higher in T5, followed by T4 > T3 by 40.7% > 29.6% and 25.9% respectively, compared to the control (T1). Along with the rise in leaf number, leaf area expanded significantly in T5 (50.5%), followed by T4 (42.9%), T3 (12.9%) and T2 (17.1%) as compared to the control T1.
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Fig. 6
Impact of biochar application at different depths of soil on: plant height (a), plant height (b), stem girth (c), fresh biomass (d), dry biomass (e), no. of leaves (f), leaf area. T1 (no biochar), T2 (Biochar amendment up to 5 cm depth), T3 (Biochar amendment up to 10 cm depth), T4 (Biochar amendment up to 15 cm depth) and T5 (Biochar amendment up to 20 cm depth). Different lowercases indicate significant differences between treatments (P < 0.05).
Dynamic shifts in physiological attributes
The effect of biochar amendment on plant physiological aspects is presented in Fig. 7. The leaf relative water content (RWC) in the shoot part of biochar-amended plants was significantly improved. RWC in plant shoot was increased by 39% in T5 followed by T4(29.7%), T3(21.2%) and T2(12.6%) as compared to non-treated biochar (T1). Biochar-amended treatment significantly impacted the membrane stability index (MSI). The highest percent increase in MSI was recorded in T5 (121.9%) followed by T4 (110.1%) > T3 (79.9%) and T2 (63.5%) while the membrane injury index (MII) decreases in T5 (5.8%) T4 (5.1%) T3 (3.7%), T2 (2.95%), as compared to control (T1). A significant increase was also recorded in chlorophyll a content, T5 (103%) > T4 (87.8%) > T3 (78.1%) > T2 (30.6%), and chlorophyll b content T5 (59.7%) > T4 (53.8%) > T3 (47.9%) > T2 (40.7%). The total chlorophyll content of plant leaves was also significantly higher than that of the control (T1) for T5 (53.6%) > T4 (46.6%) > T3 (41.5%) > T2 (23.2%). High chlorophyll content helps accumulate more macronutrients in the leaf as a sink. The concentration of nitrogen, phosphorus and potassium in the leaf is significantly higher in T5 followed by T4 > T3 > T2 as compared to control (T1), Highest nitrogen concentration is 24.8% followed by 22.6% > 18.9% > 8.2% same for phosphorus 28.4%, followed by 27.4% > 24.8% and 18.6% while potassium concentration is 65% > 58.3% > 42.2% and 27%.
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Fig. 7
Impact of biochar application at different depths of soil on; (a) RWC, (b) MSI, (c) MII, (d) chlorophyll a, (e) chlorophyll b, (f) total chlorophyll, (g) nitrogen, (h) phosphorus, (i) potassium. T1 (no biochar), T2 (Biochar amendment up to 5 cm depth), T3 (Biochar amendment up to 10 cm depth), T4 (Biochar amendment up to 15 cm depth) and T5 (Biochar amendment up to 20 cm depth). Different lowercases indicate significant differences between treatments (P < 0.05). Different lowercases indicate significant differences between treatments (P < 0.05).
Dynamics of soil physicochemical characteristics
Adding biochar also resulted changes in the soil’s physicochemical properties. The results of changes in physicochemical properties are presented in Fig. 8. It can be seen that soil pH significantly changed from 7.3 to (the control) to 7.5 for T2, to 7.8 for T3. No significant changes were reported to T4 (7.2) and T5 (7.2). Similarly, it can be seen in the table that the addition of biochar significantly improved EC values from 0.19 mmhos cm−1 (control) to 0.31 mmhos cm−1 (T3) to 0.29 mmhos cm−1 (T4 and T5). As compared to T1, a significant increase in nitrogen, phosphorus and potassium was recorded in biochar amended treatments, with maximum nitrogen in T5(20.9%), followed by T4 (16.5%), phosphorus in T5(103%), followed by T4 (86.2%) > T3 (75.5%). In comparison, the percent increase in potassium was recorded as T5 (55.5%) > T4 (39.1%) > T3 (29.1%). Similarly, there was a significant increase in both organic carbon and magnesium. The percentage increase in organic carbon was 75% > 61.1% > 33.3%, > 25%, and magnesium concentration was increased by 42.5% > 40.5% > 30%, > 19.8%, for T5 followed by T4, > T3, > T2 and respectively as compared to non-amended biochar treatment. The concentration of sulphur in the soil significantly increases after biochar application T5 (48.4%), followed by T4 (41.8%), and T3(34.1%), as compared to the control (T1). The average percentage of soil copper concentration increased (T5) 69.4%, followed by (T4) 59.7% > (T3) 37.1% and (T2) 33.9%, respectively. The study examined that Manganese and zinc concentration in soil significantly increased T5 (81%), T4 (71.4%), T3 (61.9%), T2 (76.2%), and zinc concentration T5 (123.3%) T4 (110%), T3 (46.7%), T2 (20%), as compare to control (T1) as shown in Fig. 9. The EDS in Fig. 2e,k indicates that no toxic elements or poisonous residues were detected on the biochar surface. It suggests that the biochar was chemically safe to be used in the experiment and did not leach any harmful substances that could damage the maize roots. Thus, there was no toxicity or physical harm to the root surface, implying that the biochar is appropriate for agricultural use. These observations confirm the non-toxic nature of the biochar and the safe biochemical interaction with maize root systems.
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Fig. 8
The changes in physicochemical properties in soils. pH (a), EC (b), organic carbon (c), nitrogen (d), phosphorus (e) and potassium (f). Different lowercases indicate significant differences between different treatments (P < 0.05).
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Fig. 9
The changes in physicochemical properties in soils. Sulphur (a), calcium (b), magnesium (c), iron (d), copper (e), manganese (f), zinc (g), boron (h). Different lowercases indicate significant differences between different treatments (P < 0.05).
Root and shoot trait variability through PCA
The PCA analysis showed that the first two principal components (PC1 and PC2) explain 98.65% of the total variation in the data (Fig. 10). The PCA biplot demonstrated a clear separation of the treatments into four distinct quadrants: T5 is associated with the highest plant height, plant fresh and dry biomass, stem girth, and root fresh and dry biomass—these traits clustered in the same quadrant, indicating a positive relationship between the aboveground and belowground growth. T4 was characterized by increased leaf area, root length, and root volume. These shoot and root traits were closely aligned in the PCA biplot. T3 and T2 intermediate biochar treatments are positioned in the third quadrant, indicating a less pronounced effect on the measured traits than the control (T1) and the highest biochar (T5) treatments. T1 is associated with increased root angle and diameter, which were positioned in the opposite quadrant from the other shoot and root traits. This suggests an inverse relationship between these root architectural traits.
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Fig. 10
Principal component analysis (PCA) of root and shoot traits at various biochar incorporation depths: T1 (control), T2 (5 cm), T3 (10 cm), T4 (15 cm), and T5 (20 cm). Above-ground traits are indicated in green, while root traits are shown in orange.
Discussion
Dynamic alterations in biochar from soil exposure
Biochar with ageing in the rhizosphere resulted in a considerable drop in micropore volume but increased pore size, indicating that biochar pores had been plugged. It might be due to adsorptive exudates from roots and microbial colonisation on the biochar surface16. It also discovered that organic material and bacteria closed the biochar pores throughout the composting process. Some studies also indicated that aged biochar releases a considerable quantity of dissolved organic matter after ageing, resulting in a significant rise in the specific area of surface and pore volume of biochar17,18.
Effect of biochar on root traits
Deep (up to 20 cm depth) mixing of biochar in the soil presents a promising avenue for increasing traits in maize plants. An increase in root length and volume is attributed to improved soil structure and nutrient availability. Biochar’s strong adsorption performance can immobilise nitrogen and phosphorus in the soil5. The increase in root volume and length is critical for plant development and production, allowing for the movement of nutrients and water from the deeper layer. Alongside benefits as biochar depth increases, there was a reduction in root diameter and angle compared to non-biochar treatment (T1) because biochar has been demonstrated to boost root metabolism and nitrogen absorption while also stimulating the development of roots8 Increasing root-microbial interactions and encouraging fine root growth19 implies that biochar improved the architecture of the roots of wheat seeking nutritional supplies, resulting in effective root uptake of soil nutrients20,21. It may be attributed to the biological N fixation of leguminous crops such as red clover, which can be promoted by biochar addition and potassium availability22. It might encourage more vigorous root growth by increasing biological N fixation and potassium availability in biochar. Along with root metabolism and the potential of the roots to acquire N, the root system can further restrict nitrogen leaching from the soil profile23. The increased specific area of biochar’s pore structure improved soil adsorption capacity for water and nutrients, as noted by5, enhancing roots’ relative water content. Adding biochar to soil in agriculture has been proven to boost the proliferation of beneficial bacteria, resulting in their dominance among the community of microbes24. Microorganisms require a carbon source and mineral nutrients to grow and function. Carbon may be used as a substrate for microbial operations, influencing populations of microbes by supplying energy in the form of carbohydrates. Biochar’s ash serves two purposes: it stabilises microbial metabolites by forming organic-mineral interactions and provides the essential inorganic nutrients for the growth of microbes10.
Biochar impact on root chemical properties
GC–MS analysis revealed that several compounds released from the root of maize were influenced by biochar incorporation compared to that of the control, as shown in Fig. 4. Especially, methyl stearate, a fatty acid aster, was observed in large quantities (Table 1). Some evidence suggests that this bioactive molecule helps the plant become more resilient to abiotic stress and is involved in lipid metabolism, which is essential for root development. Another compound, butylated hydroxytoluene, enhanced antioxidant enzyme activity, and also siloxane derivatives, such as 1-cyclohexyldimethylsilyloxy-3,5-dimethylbenzene, play a significant role in the adjustment of the rhizosphere against allelopathic effects involved in signalling and stress response mechanisms25. Amino acid derivatives, l-norvaline, N-(2-methoxyethoxycarbonyl), increased maize plants’ resistance to Cd and Zn toxicity by stabilising these metal ions in the soil and reducing their uptake into the plants, ensuring normal metabolic activities in their roots and shoots. Moreover, it helps in protein synthesis. This finding suggests that biochar incorporation influenced the nitrogen availability or changed the roots’ nitrogen assimilation pathways25,26. Biochar addition developed compounds such as Eicosane and Docosane, which are long-chain alkanes that help break down seed dormancy. Eicosane has been reported to have antitumor activity. It is also related to the formation of cuticular waxes on the root surface, protecting the roots from water loss and pathogen attack27.
Table 1. Represents the changes in compounds in roots after application of biochar up to different depths, analysed via gas chromatography and mass spectrometry.
Treatments | Peak | Retention time | Area% | Name | Molecular weight | Chemical formula |
|---|---|---|---|---|---|---|
T1 | 1 | 21.7 | 3.79 | Dodecane, 4,6-dimethyl- | 198 | C14H30 |
2 | 22.0 | 39.29 | Butylated Hydroxytoluene | 220 | C15H24O | |
3 | 27.4 | 3.45 | 3-Ethyl-2-pentadecanone | 254 | C17H34O | |
4 | 28.4 | 7.50 | 7,9-Di-tert-butyl-1-oxaspiro(4,5)deca-6,9-die | 276 | C17H24O3 | |
5 | 31.1 | 2.13 | 2-Methyltetracosane | 352 | C25H52 | |
6 | 31.3 | 2.74 | Octacosane, 1-iodo- | 520 | C28H57I | |
7 | 31.4 | 10.26 | Methyl stearate | 298 | C19H38O2 | |
8 | 31.5 | 20.61 | 2,8,9-Trioxa-5-aza-1-silabicyclo[3.3.3]undecane,1-ethyl- | 203 | C8H17NO3Si | |
9 | 32.7 | 5.08 | 3-Isopropyl-2,5-piperazine-dione | 156 | C7H12N2O2 | |
10 | 33.1 | 5.15 | 1-Cyclohexyldimethylsilyloxy-3,5-dimethylbenzene | 262 | C16H26OSi | |
T2 | 1 | 25.240 | 6.10 | Dodecane, 4,6-dimethyl- | 198 | C14H30 |
2 | 27.347 | 5.87 | 2-Methyltetracosane | 352 | C25H52 | |
3 | 28.389 | 8.37 | 7,9-Di-tert-butyl-1-oxaspiro(4,5)deca-6,9-dione | 276 | C17H24O3 | |
4 | 29.094 | 5.48 | Dibutyl phthalate | 278 | C16H22O4 | |
5 | 31.401 | 8.67 | Methyl stearate | 298 | C19H38O2 | |
6 | 31.482 | 26.56 | l-Norvaline, N-(2-methoxyethoxycarbonyl)-, | 303 | C15H29NO5 | |
7 | 32.664 | 7.14 | Glutarimide, N-(3-pentyl)- | 183 | C10H17NO2 | |
8 | 33.977 | 20.33 | 1-Cyclohexyldimethylsilyloxy-3,5-dimethylbenzene | 262 | C16H26OSi | |
9 | 36.373 | 5.53 | Hexacosane, 1-iodo- | 492 | C26H53I | |
10 | 41.835 | 5.97 | Octadecanoic acid, 2-hydroxy-1,3-propanediiyl di-ester | 624 | C39H76O5 | |
T3 | 1 | 12.793 | 23.59 | Phenyl methylphosphonofluoridate | 174 | C7H8FO2P |
2 | 22.321 | 6.70 | Eicosane | 282 | C20H42 | |
3 | 24.754 | 4.24 | Benzeneethanamine, 4-methoxy-N-[(pentafluorophenyl)methylene]-3-[(trimethylsilyl)oxy]- | 417 | C19H20F5NO2Si | |
4 | 25.087 | 2.46 | Isobenzofuran-1(3H)-one, 3-(4-tert-butylcyclohexyl)-3-phenyl- | 364 | C24H28O3 | |
5 | 25.457 | 4.42 | 1-tert-Butyl-1,1-dimethyl-N-(4-nitrobenzyl)silanamine | 266 | C13H22N2O2Si | |
6 | 28.486 | 12.40 | Methyl stearate | 298 | C19H38O2 | |
7 | 28.569 | 10.54 | 2,8,9-Trioxa-5-aza-1-silabicyclo[3.3.3]undecane, 1-ethyl- | 203 | C8H17NO3Si | |
8 | 31.064 | 28.25 | 1-Cyclohexyldimethylsilyloxy-3,5-dimethylbenzene | 262 | C16H26OSi | |
9 | 34.513 | 4.53 | Phenol, 4,4′-methylenebis[2,6-bis(1,1-dimethylethyl)- | 424 | C29H44O2 | |
10 | 37.600 | 2.88 | Isoferulic acid, 2TBDMS derivative | 422 | C22H38O4Si2 | |
T4 | 1 | 9.893 | 7.04 | 5-Methoxymethyl-2-furoic acid, trimethylsilyl ester | 228 | C10H16O4Si |
2 | 25.239 | 8.04 | Eicosane | 282 | C20H42 | |
3 | 28.398 | 10.60 | 7,9-Di-tert-butyl-1-oxaspiro(4,5)deca-6,9-diene-2,8-dione | 276 | C17H24O3 | |
4 | 31.407 | 12.93 | Methyl stearate | 298 | C19H38O2 | |
5 | 31.489 | 26.43 | 1-Ethylsulfanylmethyl-2,8,9-trioxa-5-aza-1-sila-bicyclo[3.3.3]undecane | 249 | C9H19NO3SSi | |
6 | 32.666 | 7.69 | (+)-2-(Diethylamino)butyl acetate | 187 | C10H21NO2 | |
7 | 38.583 | 5.62 | Nonahexacontanoic acid | 998 | C69H138O2 | |
8 | 41.056 | 7.10 | 6-O-Methyl-2,4-methylene-.beta.-sedoheptitol | 238 | C9H18O7 | |
9 | 41.990 | 5.59 | Cobalt, (2-methyl-.eta.-3-propenyl)-(pentame | 249 | C14H22Co | |
10 | 42.240 | 8.97 | Pentasiloxane, 1,1,3,3,5,5,7,7,9,9-decamethyl- | 356 | C10H32O4Si5 | |
T5 | 1 | 21.965 | 11.10 | Butylated Hydroxytoluene | 220 | C15H24O |
2 | 28.398 | 7.74 | 7,9-Di-tert-butyl-1-oxaspiro(4,5)deca-6,9-diene-2,8-dione | 276 | C17H24O3 | |
3 | 29.723 | 6.13 | Docosane, 7-butyl- | 366 | C26H54 | |
4 | 31.409 | 14.65 | Methyl stearate | 298 | C19H38O2 | |
5 | 31.490 | 27.83 | l-Norvaline, N-(2-methoxyethoxycarbonyl)-, hexyl ester | 303 | C15H29NO5 | |
6 | 32.675 | 8.11 | Glutarimide, N-(2-octyl)- | 225 | C13H23NO2 | |
7 | 32.870 | 5.43 | E-8-Methyl-9-tetradecen-1-ol acetate | 268 | C17H32O2 | |
8 | 33.099 | 8.99 | 1-Cyclohexyldimethylsilyloxy-3,5-dimethylbenzene | 262 | C16H26OSi | |
9 | 36.373 | 5.22 | Eicosane | 282 | C20H42 | |
10 | 44.207 | 4.79 | Distearin | 624 | C39H76O5 |
Effect of biochar on shoot traits
The current study suggests that biochar amendment promotes the growth of the above-ground portion due to increased available nutrients for plant uptake and the variable effect of biochar according to incorporation depth28. The depth of biochar inclusion was favourably linked with maize plant growth10. This may be due to increasing auxin and gibberellin levels by biochar application27,29 greater pH and K+, in particular, may trigger a series of signalling and functional changes in the plant, resulting in more significant cell growth mediated by auxin action30. In plant tissues that enhance cell elongation and division in leaves that directly lead to an increase in more specific leaf areas1, which captures more sunlight and makes more photosynthate that improves the growth of the aerial part of plants. biochar increases potassium availability31, a crucial element for plant water regulation, helping them maintain water content under stress. Biochar incorporation improves RWC in leaves32. Furthermore, biochar has a high cation exchange capacity33, which helps hold essential nutrients like calcium, magnesium, potassium, etc., enhancing plant availability. biochar helps in mitigating environmental stress, like drought and salinity34. Biochar incorporation at 20 cm depth increased the chlorophyll a, b and total chlorophyll1 due to increasing the N uptake from N retention with biochar (or from N adsorbed by biochar). There was a report in which biochar incorporation increased the MSI35. Applying biochar significantly decreased salinity effects, improving the growth of plants and physiological properties while also boosting the antioxidant defence system in opposition to reactive oxygen species (ROS). As a result, biochar was found to assist maize plants in enduring saline soil conditions. This is beneficial in raising the membrane stability index36.
Soil depth can boost shoot traits and methyl stearate content, which is mainly linked to various interacting physiological, biochemical and environmental factors sensitive to biochar addition at multiple depths. When applied to deeper soil layers, biochar alters the root environment by improving soil moisture content, availability of nutrients, cation-exchange capacity, and microbial activity. Such promotes root extension and function under improved conditions, which is beneficial for improving shoot growth through enhanced water and nutrient uptake. Deeper soils with biochar amendment have more water available as biochar acts as a sponge within the soil matrix, and less water is evaporated from the surface37. Available water is relatively higher in deeper soil reserves as this portion is less exposed to surface evaporation and is more adept at holding water38. Adequate water also guarantees a steady supply for the plant, which is needed to generate turgor pressure to assist with nutrient transportation and allow for both active photosynthesis and the expansion of cells in shoots. Moreover, deeper soils are often more available to provide nutrients like nitrogen, phosphorus, and potassium necessary for vegetative growth39. Biochemically, higher levels of methyl stearate were observed in deeper shoots, which might be associated with better lipid metabolism during optimal growth conditions. Methyl stearate is a methyl ester obtained from stearic acid, an unsaturated fatty acid that contributes to synthesising membrane lipids and energy storage compounds40. Optimal environmental conditions, supplied by deeper soils, induce less plant stress (wherein more metabolic resources are directed toward the biosynthesis of secondary metabolites and derivatives such as methyl stearate, derived from fatty acids). This compound can have multiple physiological functions, like helping with membrane stability, being used as a storage molecule, and fighting mechanisms of the plant. In addition, as we found in the previous sections, deeper soil layers preserve a nearly constant thermal environment to counteract temperature-induced fluctuations in enzymatic reactions to allow the stability and efficiency of metabolic pathways, such as those involved in fatty acid biosynthesis and methylation41.
Biochar effect on soil physicochemical properties, plant and nutrient uptake
When biochar gets into the soil, it will undergo a sequence of chemical and physical changes that change its characteristics and functions42. Biochar’s high pH is due mainly to inorganic minerals formed during pyrolysis, which increases soil pH43,44. Biochar had a positive effect on soil pH, EC and organic carbon content, increasing pH from 7.3 (T1) to 7.8 (T3) due to the alkaline nature of biochar. An increase in pH enhances the availability of specific nutrients and soil fertility45. Similarly, the EC value increased compared to the control, indicating that biochar decomposition and weathering increase the concentration of soluble salts2. Biochar application significantly raises the content of NPK as depth increases, since biochar may adsorb and store nutrients, prevent leaching, and make them accessible to plants46.
As shown in Fig. 7, biochar incorporation depth can significantly increase nitrogen and phosphorus uptake in the maize leaf. Our data was consistent with some articles which this may be because soil physiochemical properties change by biochar47 incorporation that promotes the hydrolysis reaction of urea and nitrogen transformation affected by urease activity48, and biochar also promotes phosphatase activity and increases the available phosphorus concentration in the soil49 while enhancing the nutrient-holding capacity50.
Biochar has been shown to enhance leaf development, primarily by supplying essential nutrients, particularly nitrogen51, 52–53. Analysis of the biochar used in this study revealed a high nitrate content, a key element for leaf growth54,55. Research indicates that biochar functions as an "N-trap" in soil, improving plant nitrogen availability56.
At different soil depths, the roots encounter salinity as one of the significant challenges. Applying biochar enhances plant resistance under saline conditions through physiological and biochemical processes. Studies have found that biochar increases chlorophyll production, preserves the leaf water content, and lowers proline, hydrogen peroxide (H₂O₂), and malondialdehyde (MDA) levels57,58. Moreover, biochar has the potential to mitigate salinity-induced toxicity by reducing the concentration of abscisic acid (ABA) and jasmonic acid (JA), while increasing the concentration of indole acetic acid (IAA), which promotes plant growth58.
Nitrogen, because of its mobility, has shown losses at different strata. And it has been found that the biochar also enhances the NUE in crops due to its porous structure, aeration, and large surface area, which facilitates the adsorption of NH4 + and reduces the inhibition of microbial denitrification59. Furthermore, biochar application also affects the volatilization of N losses from salt-affected soils. In addition, biochar increases stomatal conductance and enhances leaf gas exchange, thereby optimising photosynthesis and plant growth in soils60. It dramatically upregulates antioxidant enzymes (CAT, POD, SOD) and plays a crucial role in the ascorbate–glutathione (AsA-GSH) cycle, which is critical in balancing the redox state and preventing oxidative damage61.
Materials and methods
Fabrication of biochar and characterisation
Rice husk collected from the Lovely Professional University agriculture farm and biochar were prepared. Before making biochar from rice husk, rice husk’s thermal behaviour and stability were assessed using Thermogravimetric analysis (TGA 4000, Germany) across a temperature range of 10–600 °C. Pyrolysis of the rice husk was done at 450 °C, and the temperature was measured using a heat sensor thermometer. The produced biochar was sieved to separate ash content and further analysed for its major constituents. FESEM was also performed to elucidate the morphological changes in biochar samples after applying them to the soil. The samples were coated with gold for better conductivity in an ion coater (D II-29030SCTR, Japan). The biochar samples were examined, and images were captured under a SEM (FESEM, JSM-7610 F PLUS, Oxford Instruments X-Max N Japan) operated at 15–25 kv. At the same time, an energy-dispersive X-ray spectroscopy (EDS) analysis (OXFORD X-Maxn Japan) of the elemental distribution pattern was performed after the application in soil.
Experimental design and plant growth setup
The experiment was conducted in the transparent rhizobox mesocosms made up of a polyacrylic sheet with a thickness of 3 mm and a dimension of 20 × 4 × 25 cm (Length × Width × Depth) with 2000 cm3 capacity. The bottom of each rhizobox contained 3 mm diameter holes to drain surplus water, and inside borders were sealed with silicon to prevent water loss. Soil samples were collected from the university agricultural field. The soil texture was analysed using the international pipette technique. The sieved soil was mixed with equal fraction (i.e., 400 mg for each rhizobox @ 5 tons/ha) of biochar was added to the soil and filled up to the different depths in rhizobox i.e., 5 cm (T2), 10 cm (T3), 15 cm (T4) and 20 cm (T5) from the top surface to bottom. The control treatment was without biochar amendment, i.e., absolute field soil (T1). 1800 g of soil were filled in each pot, and the soil’s initial physical and chemical attributes were determined. The pH of the soil was 7.4, electrical conductivity (EC) 0.17 ds m-1, Organic carbon is 0.41%, nitrogen is 176kg/ha-1, phosphorus is 8.14 mg/kg, and potassium is 45 mg/kg. Physical characteristics indicated that the soil consisted of 74.1% sand, 13.9% silt and 10.9% clay, which placed it as sandy loam in terms of texture. Mazie seed SCH204 variety was obtained from Lovely Professional University’s agriculture farm, two seeds were sown in all rhizoboxes, and a single plant was maintained for further experimentation after germination. Three replicates of each treatment were maintained and arranged in a CRBD under high-tech polyhouse conditions. Plants were cultivated in the rhizobox for the growth period. The fertiliser recommended dose was applied in the following manner: NPK in a proportion of 120:60:60. Nitrogen was applied via urea, with 3.25-g urea in 3 split dose, first application of 1.25-g urea was used at 10 DAS, the second and third doses of 1 g urea application at 17 and 25 DAS, respectively. Phosphorus and SSP were applied as basal at sowing time, 4.5 and 1.2 g, respectively.
Sample preparation and GC–MS analysis
The maize plants were uprooted when they reached the desired vegetative growth stage (35 DAS), while ensuring that the roots remain intact. The roots were washed with distilled water to remove all soil particle residues without destroying the fine root hairs, as exudate is secreted in this region. Sampled roots were air-dried in ambient conditions (15–20 °C) for 15–20 min after washing to reduce excess moisture on the surface. After this, the roots were pulverised using a sterilised mechanical blade and mixed to make them as homogeneous as possible.
Five grams of sampled plant roots were put in a clean, level flask and immersed in 80 ml of methanol. The extraction was done using the hot extraction technique. The extract-containing flask was left for 24 h before being filtered with Whatman filter paper No. 42 to produce a clear, concentrated extract of up to 5 ml. The methanolic extract was subsequently assessed by Gas chromatography-mass spectrometry (GC–MS) at Lovely Professional University’s Central Instrumentation Facility in Punjab, India. Compound detection through the methanolic extract was done using a GC–MS system Model GC–MS-TQ8040 NX, Tokyo, Japan. The equipment includes an Rxi 5 ms fused silica capillary column consisting of 5% diphenyl/95% dimethyl polysiloxane, along with an AOC-20i + s autosampler with a diameter of 0.25 mm, length of 30 m and film thickness of 0.25 μm. A sample size of 2 μl was introduced using an injector. Helium, known for its properties, served as the carrier gas. The mass spectrometer operated at an ionization energy level of 70. The flow rate was recorded at 16.3 ml/min with a column flow rate of 1.21 ml/min and a linear velocity flow control at 39.9 cm/s. The initial oven temperature was set at 50 °C, followed by a ramp to reach 250 °C. Over 5 min and then further ramped to reach 280 °C in 22 min before holding for 69.98 min in ACQ mode. The scan range spanned from 40 to 700 m/z, lasting for half a second at a temperature of 260 °C, and a 20:2 split ratio. The entire GC–MS process took 65 min to complete its analysis run, expressing the percentage amount of each component based on peak area.
Plant attributes
Chlorophyll was extracted from a 100 mg sample using 20 ml of 80% acetone. After centrifugation for 10 min at 5000 rpm, the supernatant was transferred to a volumetric flask, and the extraction was repeated until the residue became colourless. The extract’s absorbance was measured at 645 and 663 nm using a spectrophotometer, and the chlorophyll content was calculated using a formula62.where V is the Final volume of the extract, W is the Fresh weight of the leaves, and A is = absorbance at the specific wavelength. The value is expressed as mg/g fresh weight.
The membrane stability index and membrane injury index were assessed by soaking 200 mg of fresh leaves in 10 ml of double-distilled H2O in two sets. One pair was heated in a water bath at 40 °C for 30 min, before being tested for electrical conductivity (C1). The second set was heated in a water bath at 100 °C for 10 min before conductivity (C2) testing. The calculation is done by using the following formula63.
Root diameter is measured using a digital vernier calliper at 5 cm from the root length, and root angle was measured using Image J software with a reference scale20.
Soil analysis
Following harvesting, each rhizobox was dismantled and properly sampled. The root systems were removed from the soil and gently shaken to extract the ‘rhizosphere’ dirt. For ‘rhizosheath’ soil, the roots were weighed and shaken by hand to extract the soil, which was then collected. The soil-free root samples were washed with a fine screen and stored. Biochar particles were manually extracted from three randomly selected rhizospheres. Soil samples were taken from each box and tested for pH, electrical conductivity (EC), organic carbon (OC), nitrogen (N), phosphorus (P), potassium (K), sulphur (S), calcium (Ca), magnesium (Mg), copper (Cu), zinc (Zn), manganese (Mn), and boron (B).
Statistical analysis
Statistical data analysis was carried out using SPSS Statistics 22.0, United States. The impacts of the biochar amendment treatments on parameters were evaluated using one-way ANOVA. Duncan’s test assessed differences between treatment means at p < 0.05. Origin Pro 2024 was used to do PCA (principal component analysis) and generate graphics.
Conclusion
The present study indicates that applying biochar up to a depth of 15 and 20 cm significantly impacts root and shoot traits. The optimum depth of biochar incorporation provided an ideal rhizosphere environment. Biochar is rich in nutrients and transports more essential nutrients. Due to easy nutrient availability, maize plant roots performed a smoother growth and reduced root spreading angle, allowing easy nutrient uptake and reduced energy expenditure in root development. Biochar increases electrical conductivity, transporting essential nutrients such as Ca, Mg and N to the root system. Additionally, biochar incorporated up to 15–20 cm depth maintained more soil moisture in the complete root zone, which increased RWC in both root and shoot. In contrast, treatments without biochar incorporation led to deeper and wider root systems searching for nutrients, indicating a less efficient nutrient acquisition process and potentially higher metabolic costs for the plant. Furthermore, biochar’s porous and rough surface aids nutrient retention and reduces leaching, thereby maintaining soil fertility over time. Overall, the application of biochar not only enhances soil pH and EC but also supports sustainable agricultural practices by improving plant growth and nutrient management. These findings underscore the appropriate depth for incorporating biochar, which ultimately has the potential to optimise plant health and productivity in various agricultural systems.
Acknowledgements
This research is the outcome of the Aakash Project (Project No. 14200133), which is financially supported by the Research Institute for Humanity and Nature, Japan (RIHN: a constituent of NIHU). The author acknowledges the cooperation of the present leader, Dr. Prabir Patra and all members of the Aakash Project. The author also acknowledges the support of CIF, LPU, for providing the necessary instrumentation for sample analysis. Vishnu D. Rajput acknowledge the support by the Strategic Academic Leadership Program of the Southern Federal University (“Priority 2030”).
Author contributions
All authors contributed to the study conception and design of the experiment. Material preparation, data collection, and analysis were performed by Gaurav Sharma, Diptanu Banik, Prasann Kumar, Eiji Nishihara, Kzuyuki Inubushi, Shigeto Sudo, Sachiko Hayashida, Parbhir Patra, Tatiana Minkina, Vishnu D. Rajput and Chandra Mohan Mehta. The first draft of the manuscript was written by Gaurav Sharma and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.
Data availability
The datasets used and/ or analysed during the current study can be obtained from the corresponding author.
Declarations
Competing interests
The authors declare no competing interests.
Publisher’s note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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Abstract
Biochar application in the soil has shown its potential for improved plant growth, root structure, and nutrient availability. However, uncertainties remain regarding the optimal depth for biochar application and its interaction with roots, which significantly influence plant growth and development. This transparent rhizobox trial consists of five treatments: control treatment (T1) with recommended dose of fertilizer, and four biochar addition treatments with different depths viz. 5 (T2), 10 (T3), 15 (T4) and 20 cm (T5). FESEM, EDX-Spectroscopy was performed to elucidate the change in morphology and element distribution pattern of biochar after application in soil. Fresh biochar has 53.7% carbon and 19.9% oxygen, however, aged biochar shown 37.4% carbon and 36.4% oxygen content. The T5 exhibit the best outcomes, with the most significant increment in maize root traits over the control treatment (T1). In particular, T5 recorded a maximum improvement in root length (+ 48.2%), root volume (+ 42.7%) and root dry biomass (+ 56.7%) compared to the control treatment when biochar was applied at 20 cm soil depth. Shoot traits at 20 cm biochar incorporation revealed an increase in shoot fresh biomass (+ 23.1%), shoot dry biomass (+ 15%), leaf area (+ 50.5%) and number of leaves (+ 40.7%) as compared to the control treatment. As compared to the control, a considerable rise in soil nitrogen, phosphorus, and potassium was observed in biochar amendment at 20 cm depth, with the highest nitrogen in T5 (20.9%), phosphorus in T5 (103%), and the percentage increase in potassium in T5 (55.5%). One of the most consistently prevalent molecules examined by GC–MS was methyl stearate, a fatty acid ester detected in all five treatments. Methyl stearate content increased as the depth of biochar increased: T1 (10.26%), T2 (8.67%), T3 (12.40%), T4 (12.93%), and T5 (14.65%). Overall, the findings of this study suggest that uniform application of biochar in the top soil layer significantly enhances the above- and below-ground attributes of plants.
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Details
1 Lovely Professional University, Department of Agronomy, School of Agriculture, Phagwara, India (GRID:grid.449005.c) (ISNI:0000 0004 1756 737X)
2 Tottori University, Tottori, Japan (GRID:grid.265107.7) (ISNI:0000 0001 0663 5064)
3 Tokyo University of Agriculture, Department of Agricultural Chemistry, Faculty of Applied Biosciences, Tokyo, Japan (GRID:grid.410772.7) (ISNI:0000 0001 0807 3368)
4 NIAES-NARO, Tsukuba, Japan (GRID:grid.410772.7)
5 Research Institute for Humanity and Nature, Kyoto, Japan (GRID:grid.410846.f) (ISNI:0000 0000 9370 8809)
6 Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Yokosuka, Kanagawa, Japan (GRID:grid.410588.0) (ISNI:0000 0001 2191 0132)
7 Southern Federal University, Academy of Biology and Biotechnology, Rostov-on-Don, Russia (GRID:grid.182798.d) (ISNI:0000 0001 2172 8170)