Abiotic stresses are the dominant constraints to successful crop production in the modern era. Over the past few decades, researchers have devised various techniques to ease the effects of abiotic stresses on crop plants. Biochar is a vital eco-friendly biostimulant that mostly increases crop production and alleviates the adverse effects of different abiotic stresses. It improves crop yield as a fertiliser and soil quality as a soil conditioner. Biochar amendment in the soil is gaining popularity these days. This is because it improves the physio-biochemical and biological properties of soil. Biochar enhances abiotic stress tolerance as well as growth and yield of plants by modulating ionic homeostasis, photosynthetic apparatus, antioxidant machineries, reducing heavy metal accumulations and oxidative damages. However, the stress-relieving roles of biochar have not been thoroughly assembled. This review summarises current reports of biochar application and discusses the potential roles of biochar amendment in soil for crop growth and production under stress and non-stress conditions. This review also covers the possible mechanisms for how abiotic stress mitigation is accomplished in plants and the limitations and prospects of biochar application in agriculture.

The Egyptian soil contains low organic matter and high calcium carbonate with a pH of 7.5–8.2, which reduces the availability of phosphorus (P) improve the efficiency of applying P fertilisers in alkaline soils. Therefore, the aim of this study is to evaluate the effect of biochar (B) at 0 t ha⁻¹ and 10 t ha⁻¹ with different P fertiliser rates (i.e. 0% P, 50% P, 100% P and 150% P of the recommended dose) on wheat yield (Triticum aestivum L.) and soil fertility in clay texture soil during the 2015–2016 and 2016–2017 growing seasons. The results show a significant increase in all of the following: soil availability and plant uptake of NPK; flag leaf area (FLA); the number of fertile tillers (NFT); the number of grains in spike (NGS), and the grain yield of wheat plants treated with P fertiliser alone, or when P fertiliser combined with biochar addition. FLA increased by 81.82% and 72.27% in the plots treated with biochar during the 1st and 2nd seasons respectively. Wheat uptake of total nitrogen–P–potassium (NPK) increased as a result of increasing the concentration of inorganic P in the studied soil. It is noteworthy that adding biochar to P fertiliser supplements P-fertiliser such as in the treatment of 10 t ha⁻¹ biochar at 50% P, where the highest grain yield was recorded compared with adding 100% P and 150% P of the recommended dose. The results indicated that integrating biochar and P fertiliser can be a practical approach to improve wheat production and soil fertility.

As a multi-beneficial amendment, biochar is very useful to be applied for improving soil health and crop productivity. Therefore, this study was carried out to assess the influence of wood biochar and mineral nitrogen (N), phosphorus (P) and potassium (K) fertilisers viz, [(control; 100% NPK (120:90:60 kg ha⁻¹); 75% NPK + 5 tonne biochar; 50% NPK + 10 tonne biochar; 25% NPK + 15 tonne biochar and 20 tonne biochar ha⁻¹)] on wheat yield and soil properties under different management practices [(raised bed (more than 30 cm above the ground) and flat-bed)]. Split plot two factors randomised completed block (RCB) design with three replications were used where management practices were placed to main plot, while treatments were assigned to subplots. Maximum spike length, grain per spike, 1000 grain weight, grain and biological yield were obtained with application of 75% NPK + 5 tonne biochar ha⁻¹ under both raised and flat-bed, which were statistically at par to 50% NPK + 10 tonne biochar ha⁻¹. The grain and biological yield observed at 75% NPK + 5 tonne biochar and 50% NPK + 10 tonne biochar ha⁻¹ were significantly higher than that of 20 tonne biochar ha⁻¹. However, maximum soil organic matter, extractable P and K contents with slight increases in soil pH and EC was observed at 20 tonne biochar ha⁻¹. Moreover, almost all agronomic parameters were significantly better in raised bed compared to flat-bed sowing. Hence, the present study suggested that 75% NPK + 5 tonne biochar ha⁻¹ is suitable for improving wheat yield and soil properties.

ContextAcidic soil stress is a silent threat to the plant biome including rice.

Aim. To develop a feasible method for ameliorating acidic soil stress that neutralises the acidity and benefits the plants to restore the necessary nutrients in the soil environment in an eco-friendly approach.

Methods. A traditionally prepared organic biochar from stem, peel and suckers of banana, i.e. kolakhar was used individually and also in a combination with commercial biochar (biokhar) to evaluate the growth of five traditional rice varieties with contrasting characters for tolerability of Al toxicity and P deficiency.

Key Results. The rice seedlings treated with kolakhar showed better responses in biomass, photosynthetic efficiency and antioxidant protection mechanisms. Enhanced production of ascorbate peroxidase, guaiacol peroxidase etc. in seedlings growing under kolakhar treated soil reveals their possible mechanism developed to cope with the stress. The final pH of all the treated soils with ameliorants was found to change to ∼6.0, nearly similar to normal soil from 4.2 to 4.5. The amount of phosphorous was highest in soil treated with kolakhar 250 mg kg⁻¹. Kolakhar showed the highest reduction of available Al from 2.107 to 0.018 mg kg⁻¹ after 30 days of treatment. Above all, the histochemical staining elucidated that the Al uptake was more profound in the controls. In contrast, much reduced Al uptake was observed in roots under treatment with ameliorants, which could be because of the decreased Al content in the soil as determined by inductively coupled plasma optical emission spectroscopy (ICP-OES) analysis.

Conclusions. These findings have paved the way to overcome the severe issues of acidic soil stress, associated hazards of Al accumulation and other stresses.

Implications. Kolakhar could be exploited for ameliorating soil acidity through low cost and eco-friendly way.

Sunflower (Helianthus annuus L.) growth and yield are severely compromised by boron (B) deficiency in alkaline calcareous soils of Pakistan. Biochar produced through low-temperature pyrolysis helps to maintain acidic functional groups in such soils. The aim of this study was to enhance B availability in biochar-amended alkaline calcareous soils in order to improve morpho-physiological and yield attributes of sunflower. In a pot experiment, sunflower was grown with three fertiliser treatments: diammonium phosphate (DAP) alone; simple B + DAP; and DAP coated with biochar and B (BC-BDAP). All treatments were grown in soil receiving no biochar, and the BC-BDAP treatment was also applied in soil amended with wheat straw biochar at 2% and 4%. Preparation with wheat straw biochar improved soil properties such that soil with 4% biochar showed highest levels of organic matter (0.97%), total nitrogen (0.08%), available phosphorus (6.21 mg/kg), extractable potassium (93.4 mg/kg) and available B (0.41 mg/kg). Furthermore, compared with DAP alone and B + DAP, BC-BDAP in 4% biochar-amended soil enhanced the concentrations of shoot B (by 64% and 35%) and root B (by 41% and 26%), respectively. This indicated sufficient B availability in soil for root absorption, which subsequently increased sunflower achene weight per plant (by 33% and 17%), protein content (by 29% and 18%) and oil content (by 17% and 10%). Thus, application of BC-BDAP fertiliser in 4% biochar-amended soil can be an efficient strategy for enhancing B availability in alkaline calcareous soils and increasing sunflower growth and yield.

Context. Biochar application not only remediates soils contaminated by heavy metals but also improves soil fertility and plant growth in salt-affected soils.

Aims. The aim of the study was to investigate the effects of water source salinity and biochar on soil properties and growth of a marigold (Tagetes erecta) crop.

Methods. This pot study used a factorial completely randomised design with four levels of salinity of the water source (0.04, 2.01, 3.32 and 5.16 dS m⁻¹) and five biochar doses (0, 2.5, 5.0, 7.5 and 10 t ha⁻¹). Salinity treatments contained different proportions of treated tannery effluent, and biochar was derived from water hyacinth (Eichhornia crassipes). Soil physico-chemical properties, nutrient contents and enzyme activities, and plant biomass, were measured during crop growth and post-harvest.

Key results. Water salinity level and biochar dose both had significant effects on soil properties. Higher salinity of water and biochar dose increased soil pH, whereas electrical conductivity decreased with biochar application in soils receiving higher salinity water. Soil organic carbon and available nutrients were significantly affected by salinity level and biochar dose. Irrespective of salinity level, biochar application at 10 t ha⁻¹ increased soil organic carbon and available nitrogen, phosphorus and potassium by 40.5%, 23.1%, 15.2% and 30.5% in post-harvest soils. Enzyme activities and plant growth were significantly reduced at higher salt levels, whereas application of biochar improved these parameters.

Conclusions. The results indicate that application of biochar at 10 t ha⁻¹ significantly promotes nutrient availability and enzyme activities in soils with various levels of salinity. Biochar application enables plant growth primarily by trapping the soluble salts on the pore space available in its surface.

Implications. Biochar application may help to mitigate nutrient deficiency and crop failure under salt-stress conditions.

Context. Organic amendments including biochar can improve crop production under salt stress. However, it is still not clear whether biochar enriched compost would enhance legume performance under salt stress after fresh application and in succeeding crops.

Aim. The aim of the study was to examine the effect of biochar enriched compost in reducing the salinity stress after fresh application at increasing rates and in the succeeding crop.

Methods. In a pot trial, biochar–compost was applied at four different rates (0, 1, 2, and 3%) while mungbean was grown under five different salt stress conditions (0, 2, 4, 8, and 12 dS m⁻¹). In the field trial, the residual effect of different organic amendments (control, compost, cow urine, compost with cow urine, biochar–compost, and biochar–compost with cow urine) was evaluated under three different salt stress conditions (0, 3, and 6 dS m⁻¹). Soil properties, plant performance, and nutrient uptake were determined.

Key Results. Results revealed a significant biochar × salt treatment interaction in our pot culture. Biochar–compost application can minimise salt effects at a higher application rate resulting in better plant performance; however, these effects are minimal when salt was added at higher rates. We also observed a significant residual effect of biochar compost on biomass production (51.03%), seed yield (79.48%), and K⁺ uptake (77.95%) than the control treatment. We believe that biochar–compost buffered Na⁺ while improved plant water, and nutrient availability and uptake. In addition, biochar–compost might have increased nitrogen acquisition through enhanced biological nitrogen fixation.

Conclusions. Biochar enriched compost enhances the yield of legume grown under salt stress.

Implications. Our results suggest that biochar–compost can be one of the sustainable means for alleviating soil salinity.

Cropping of acid sulfate soils requires effective treatment of their inherently low pH. We evaluated the efficacy of applications of two levels of lime (0 or 2 Mg/ha), two levels of organic fertiliser (0 or 5 Mg/ha), and three levels of biochar (0, 10 or 30 Mg/ha) in a factorial design for ameliorating acidity in an acid sulfate soil, and measured the subsequent growth and yield of baby corn (Zea mays L.). Lime increased soil pH(H₂O) from 3.75 to 4.12, salinity from 1.72 to 1.95 dS/m, and cob yield by 30%. None of the amendments significantly altered total organic carbon or total nitrogen concentrations in the soil. Biochar additions increased cob yields by an average of 28% on both unlimed and limed soil. Addition of organic fertiliser increased cob yield by 45% on unlimed soil but had no significant effect on yields on limed soil. The yields obtained with liming were similar to the highest yields achieved with biochar or organic fertiliser applied either separately or in combinations. Overall, cob yields were increased by 19% with addition of organic fertiliser. The yield increseas from additions of biochar or organic fertiliser were associated with improvements in nutrient supply. However, the increases in cob yield were associated with reduced cob protein, probably resulting from poor availability of nitrogen late in the season. We conclude that biochar and organic fertiliser applied in relatively large quantities can be viable treatments for cropping acid sulfate soils.

Context. Soil enzyme activities are key regulators of carbon and nutrient cycling in grazed pastures.

Aims. We investigated the effect of biochar addition on the activity of seven enzymes involved in the carbon, nitrogen and phosphorus cycles in a Sil-andic Andosol and a Dystric Cambisol under permanent pastures.

Methods. The study consisted of a one-year field-based mesocosm experiment involving four pastures under different nutrient and livestock practices: with and without effluent under dairy cow grazing on the Andosol, and with either nil or high phosphorus fertiliser input under sheep grazing on the Cambisol. Soil treatments were: (1) willow biochar added at 1% w/w; (2) lime added at the liming equivalence of biochar (positive control); (3) no amendments (negative control).

Key results. Compared with the Cambisol, the Andosol had higher dehydrogenase, urease, alkaline and acid phosphatase and, especially, nitrate-reductase activities, aligning with its higher pH and fertility. In both soils, biochar addition increased the activity of all enzymes, except for acid phosphatase and peroxidase; lime addition increased peroxidase and nitrate-reductase activity.

Conclusions. The increased enzyme activity was strongly positively correlated with soil biological activity following biochar addition. Biochar caused a 40–45% increase in cellulase activity, attributed to increased root biomass following biochar addition. The response in acid and alkaline phosphatase activity can be attributed to the impact of biochar and lime addition on soil pH.

Implications. The results provide more insights in realising the potential benefits of biochar to the provision of ecosystem services for grazed pastures.

Biochar has received significant attention for its potential in bioremediation of polluted soils. However, the impact of biochar particle size is yet to be investigated. We evaluated the role of various particle sizes (<3, 3–6 and 6–9 mm) of commercially available hardwood Acacia arabica biochar applied at the rate of 10 g kg⁻¹ soil in the immobilisation of heavy metals 12 months after application. Heavy metals cadmium (Cd), nickel (Ni) and lead (Pb) were spiked in designated pots from their respective sources CdSO₄, Ni(NO₃)₂ and PbNO₃ according to their permissible limits (0.2–2, 8.1 and 10–15 mg kg⁻¹, respectively). Biochar particles of sizes <3, 3–6 and 6–9 mm significantly mitigated contamination of Cd by 35%, 10% and 9%; Pb by 61%, 60% and 35%; and Ni by 64%, 45% and 3.2%. Relative to the control, application of biochar particles of sizes <3, 3–6 and 6–9 mm significantly increased soil porosity by 10.3%, 4.2% and 3%; saturation percentage by 100%, 42% and 27%; pH by 0.53%, 2.6% and 4%; and organic matter by 33.6%, 19.7% and 16.8%. Soil bulk density decreased by 12%, 5% and 2.3%; and electrical conductivity by 19%, 20% and 24%. The contamination factor for Cd was >1 (in the moderate contamination category), whereas Pb and Ni had contamination factors <1 (in the low contamination category), under biochar application. The heavy metal spiking effect was also significant and ranked as Pb > Cd > Ni for soil pH, electrical conductivity and porosity; Cd > Ni > Pb for organic matter; and Ni > Cd > Pb for bulk density and saturation percentage. Smallest sized biochar (<3 mm) maximally stabilised heavy metals in the soil and improved soil physicochemical properties. Therefore, heavy metal polluted soils should be treated with fine (<3 mm) biochar for maximum immobilisation for heavy metals and improvement in soil physicochemical properties.

Context. The application of soil amendments to immobilise pesticide residues is a promising technology for meeting human health requirements, environmental protection and cost-effective remediation.

Aims. This study aims to evaluate the effect of rice straw biochar (RB), rice straw compost (RC) and their mixtures at rates of 0.5% and 1.0% in pots, on the immobilisation of pesticides (such as atrazine, glyphosate and chlorpyrifos) in contaminated soil, using spectroscopy analysis under grown canola (Brassica napus L.) plants.

Methods. Determination of immobilisation of pesticide residues by GC or HPLC and chemical properties of RB and RC and adding them at differentrates to the contaminated soil.

Keyresults. The results showed that the addition of RB or RC alone or their mixtures led to a significant increase in the exchangeable Ca²⁺ contents, organic matter (OM), cation exchange capacity (CEC), uptake of N, P and K and dry weight of canola plants. Pesticide concentrations decreased with increasing OM, CEC, and exchangeable Ca²⁺ with the soil amended by RC and RB. The data indicated that adding RB at levels of 0.5% and 1.0% resulted in reductions in chloropyrifos by 43.2% and 63.1%, glyphosate by 32.8% and 77.3%, and atrazine by 21.9% and 72.2%, respectively, as compared to the control. Addition of (RC + RB) at 0.5% gave the highest pesticide immobilisation, followed (RC+RB) at 1.0%.

Conclusion. These results indicate that the dominant mechanisms of pesticides immobilisation in the alkaline soils amended with RC and RB by FTIR and XRD analysis were π–π interaction, pore filling, hydrophobic effect, H-bonding, degradation as well as improvement of soil properties and dry yield of canola plants.

Implications. Our results suggested the possibility of adding recycled rice straw in the form of compost or biochar to the contaminated soil to improve its properties, immobilise pesticides and increase its production capacity.

Many cost-effective and environmentally friendly strategies are applied to improve soil fertility, reduce soil pollution, and reduce the human health risks of consuming metal-contaminated vegetables. We evaluated the effects of three soil amendments, biochar, slag and ferrous manganese ore (FMO), at application rates of 3% and 6%, on the bioavailability of heavy metals in a contaminated soil, their bioaccumulation, and antioxidant enzyme activities in water spinach (Ipomoea aquatica) plants. We also measured a range of soil physicochemical and biological properties, as well as plant biomass. Application of biochar at 6% was the most effective treatment for improving the fresh biomass of plants, with an increase of 32.31% in the roots and 47.98% in the shoots relative to the unamended soil. Compared with slag and FMO, biochar was most effective in improving soil physicochemical and biological properties. All amendments significantly reduced the bioavailability of lead and cadmium. We observed significantly positive correlations among bio-concentration factor, translocation factor and bioaccumulation coefficient. Correlation analysis also demonstrated that bulk density was positively correlated with soil available water content, but negatively correlated with soil organic matter, total porosity and hydraulic conductivity. Soil macro- and micronutrients were found to have a positive correlation with soil physical, chemical and biological properties. In conclusion, biochar, slag and FMO significantly affected the physical, chemical and biological properties of soil, and metal bioavailability and fertility status, safeguarding soil health and ensuring the healthy growth of the plants.

China is the major producer of chestnut, with 1.84 million tons of chestnut production, resulting in an enormous waste of chestnut shells. In the current study, shell biochar (SBC) was produced using the inside shell covering fruit, and the outside shell with thorns was used to produce thorn biochar (TBC). Both types of biochar were characterised through Brunauer–Emmett–Teller (BET) analysis, scanning electron microscopy (SEM) and Fourier transform infrared (FT-IR). These analytical results showed a more obvious smooth surface and micro-pore structure in SBC. The vibration of C=O/C=C and C–O (phenolic) showed a significant difference between the two types of biochar. Sorption experiments indicated that the adsorption capacity of the different types of biochar for cadmium (Cd) did not differ significantly, whereas the adsorption capacity of TBC for lead was better than that of SBC. In the pakchoi cultivation experiment (28 days), the application of TBC (1.5%) promoted plant shoot weight, root weight, shoot length and root length by 465%, 143%, 109% and 97% respectively. The application of biochar effectively increased soil pH and reduced the bioavailability and migration of heavy metals. Besides, membrane integrity and chlorophyll content were enhanced because of the alleviation of oxidative stress. Noticeably, application of TBC (0.1% and 1.5%) reduced the Cd concentration in the root by 40–60%, and enhanced accumulation of Pb by 75–191%. Overall, our study demonstrated that 1.5% TBC has promising potential for remediating Cd-contaminated soil. Our study has demonstrated the remediation potential of chestnut and provided a clue for sustainable management of chestnut shell waste for further development of chestnut resources.