Agricultural Waste-Based Biochar for Agronomic Applications

: Agricultural activities face several challenges due to the intensive increase in population growth and environmental issues. It has been established that biochar can be assigned a useful role in agriculture. Its agronomic application has therefore received increasing attention recently. The literature shows different applications, e.g., biochar serves as a soil ameliorant to optimize soil structure and composition, and it increases the availability of nutrients and the water retention capacity in the soil. If the biochar is buried in the soil, it decomposes very slowly and thus serves as a long-term store of carbon. Limiting the availability of pesticides and heavy metals increases soil health. Biochar addition also affects soil microbiology and enzyme activity and contributes to the improvement of plant growth and crop production. Biochar can be used as a compost additive and animal feed and simultaneously provides a contribution to minimizing greenhouse gas emissions. Several parameters, including biochar origin, pyrolysis temperature, soil type when biochar is used as soil amendment, and application rate, control biochar’s efﬁciency in different agricultural applications. Thus, special care should be given when using a speciﬁc biochar for a speciﬁc application to prevent any negative effects on the agricultural environment.


Introduction
The high increase in world population has resulted in a rise in agricultural activities of more than three-fold in the last five years [1]. Technological advancements and the expansion of agricultural soils to meet the increased worldwide demand for food will inevitably result in a continuous growth of agricultural activities [2,3]. Agricultural activities worldwide generate significant amounts of greenhouse gases; agriculture contributes, respectively, by 52 and 84% of the global anthropogenic emissions of methane and nitrous oxide [4]. Agriculture is often accompanied by negative effects on the environment and ecosystems. New practices are thus required to ensure the sustainability of the agricultural sector, including the sustainable management of agricultural residual materials [5]. This is consistent with the idea of the circular economy (grow-make-use-restore) [6], which, in contrast to the linear economy (take-make-use-dispose) [7], aims to eliminate wastes from a system by converting them into high added-value products [8]. In other words, the concept of the circular economy seeks to recover resources instead of importing new resources from outside of the system [9]. The worldwide challenges related to climate change, biodiversity loss, increased demand for raw materials, waste production and pollution are pushing organizations to shift their activities from a linear to a circular economy. In this context, several applications in many areas such as dairy products [10], agri-food [11], the use of fertilizers [12,13] and biochar [14,15] have been presented.
Biochar has been gaining increasing attention recently as an organic material that can be effectively involved in agriculture to enhance soil fertility, stimulate plant growth, and increase agronomic productivity [16,17]. Biochar is a carbon-based solid generally produced by pyrolysis of organic wastes under complete or partial exclusion of oxygen.
Agricultural residues generated on farms as by-products of crop production are widely used for biochar preparation. The recovery of biomass harvested from agriculture is more efficient in developed countries as compared to developing countries, where the insufficient government policies and the lack of investment in waste management technologies still limit the appropriate management of these residues. The conversion of the large volume of agricultural residues produced annually to biochar could be a promising alternative to exploit their pronounced and manifold potential [18][19][20].
Biochar has many different aspects of application in agriculture, the main examples of which aim to increase soil fertility, increase nutrient uptake, and improve the water retention capacity of the soil. This will simultaneously improve crop productivity, mitigate plant stresses related to both salinity [16,21] and toxic contaminants [22,23], and reduce dependence on fertilizers [24,25]. Biochar application directly affects soil physicochemical characteristics such as pH, nutrient availability, electrical conductivity, porosity, surface area, bulk density, and cation exchange capacity (CEC) [26,27], and influences soil microbial structure by providing additional habitats to the microbial populations [28]. Biochar was also reportedly able to remediate organic and inorganic pollution in the soil by complexation or through sorption [29,30]. Biochar has stable aromatic organic carbon. This carbon is hardly degraded and therefore does not return to the atmosphere [31,32]. This will, in fact, minimize CH 4 and N 2 O emissions to the atmosphere and consequently reduce climate change [33]. The features of biochar essentially depend on both the type of feedstock and the pyrolysis conditions. Consequently, many biochar types with many different properties may be produced from different types of biomasses and at a broad range of pyrolysis temperatures [34]. The high variation in the chemical and physical characteristics of biochar, despite its highly documented benefits for soils, still imposes some limitations on its applications in agriculture [35][36][37].
The goal of this review is to emphasize the correlation between biochar characteristics and its possible advantages for soil structure, crop productivity, pollutant removal, animal growth and greenhouse gases emission mitigation from the soil. This review also highlights the risks and key limitations of biochar with respect to soils and agricultural ecosystems.

Physiochemical Characteristics of Biochar
Physico-chemical characteristics of biochar are controlled by the type of feedstock and pyrolysis conditions [38,39]. Surface area, porosity, density, water retention capacity, and hydrophobicity are the main physical properties of biochar [40]. Biochar is porous in nature and generally characterized by its low bulk density (less than 0.6 g/cm 3 ) [26]. The pyrolysis temperature has a strong impact on biochar porosity; volatile substances are released at high temperatures, which supports the formation of pores [41]. Somerville et al. claimed that a higher porosity of wood-derived biochar is obtained, from 50 to 70%, when pyrolysis is performed at temperatures from 300 to 850 • C [42]. Biochar is mainly microporous, with more than 80% of its total pore volume being micropores [40]. The biochar surface area increases similarly to porosity with rising pyrolysis temperature. For most biomasses and under suitable pyrolysis conditions, the surface area can reach values between 100 and 800 m 2 /g, while for biochars derived from sewage sludge, the surface area shows values below 100 m 2 /g [39]. The biochar's water-holding capacity is also enhanced at higher pyrolysis temperatures. However, in some works it was observed that on the biochar surface the functional groups decrease at higher temperatures, which alters the biochar's affinity to water [40]. Surface functional groups are responsible for biochar reactivity and form the interface where different biological and chemical reactions occur. Among many functional groups found on biochar surface, such as aliphatic, aromatic, hydroxyl, epoxy, acyl, ether, amido, sulfonic, and azyl groups, phenolic-OH, carboxyl, carbonyl, and ester groups are the most abundant. The association/dissociation of these groups with protons results in biochar alkalis, hydrogen bond-induced sorption, hydrophilicity/hydrophobicity, surface charge alterations and CEC [43]. The dissociation/associations of protons provided by biochar functional groups as a function of pH allow biochar-incorporated soils to resist the variation in pH and to increase their buffering capacity for longer periods [44].
The pH value of biochar is generally varying from 5.9 to 12.3, with an average of 8.9 [29]. The biochar pH value depends strongly on the pyrolysis temperature. With the increase in temperature (>350 • C), alkali salts start to separate from organic materials and basic cations increase in biochars, which increases the biochar pH [45]. In contrast, at low pyrolysis temperatures (200-300 • C), acidic biochar is produced due to the decomposition of cellulose and hemicelluloses, which yields organic acids [46]. In addition to pH and CEC, the elemental composition and the atomic ratios of H/C and O/C are the main chemical properties of biochar. Biochar is determined by its stable C existing in the inorganic phase in the form of carbonate and bicarbonate, and in the organic phase as aliphatic carbon, aromatic carbon, and functionalized carbon [32,43]. The elemental composition of biochar is constituted by major elements (C, H, O, and N) and other elements (P, S, Si, Cu, Fe, Zn, and Mn) present in variable percentages depending on the nature of feedstock and the pyrolysis conditions [47,48]. These elements make the surface of the biochar heterogeneous and reactive [49,50]. Higher pyrolysis temperatures intensified polymerization reactions and reduced the atomic ratios of H/C and O/C [38,48]. Biochars typically have a lower molar H/C ratio than that of feedstocks [51,52], indicating a high content of aromatic structures and a potential recalcitrance of the biochar. The N in biomass is generally present in the form of proteins, amino acid, pyrrolic, and pyridine. Peptide-N bonds transform during pyrolysis into N-heteroaromatic carbon compounds, and some amide-N reduces at elevated pyrolysis temperature. Organic phosphorus is converted to inorganic form, which is not volatilized at a pyrolysis temperature below 700 • C. The phosphorus content is enriched within the biochar and could serve as a source of phosphorus for the plant [53]. Generally, the content of phosphorus, as well as that of other elements such as Ca, Mg, K, Fe, Si, S, Zn, Cu, and Mn, increases as the pyrolysis temperature increases; however, these elements' bioavailability is reduced due to their insertion into highly aromatic structures within biochar. Contrary to this, at lower pyrolysis temperatures, an improvement in the bioavailability of these nutrients is observed. Naeem et al. [54] showed that exchangeable K in maize straw-derived biochars produced at 300 and 400 • C was more available to plant roots than that in biochars pyrolyzed at 500 • C. In addition to pyrolysis temperature, the nature of feedstock also affects the chemical properties of biochar. For instance, biochars derived from wood-based biomass are reported to be more resistant to biodegradation than biochars obtained from crop residues and animal manures [27,55]. Contrary to this, biochar produced from manure contains more nutrients [56,57]. Elemental composition and some other characteristics of biochar derived from different feedstocks and at different pyrolysis temperatures are listed in Table 1. The main properties of biochar and various forms of its application in agriculture are summarized in Figure 1.

Impacts on Soil Structure and Composition
Biochar has been described as an attractive material that can be applied in agriculture to improve agricultural soil properties and crop productivity. The main impacts of biochar on soil characteristics and agricultural products yield are listed in Table 2 and Figure 2. Bamboo branch 0, 5, 10, 20 t/ha Bamboo Increase in soil moisture, soil microbial biomass and influence on the mean soil respiration rate - [74] Olive stone, 0.5-7.5% (w/w) Sunflower Decrease in soil bulk density and Increase in seed germination, [75]

Impacts on Soil Structure and Composition
Biochar has been described as an attractive material that can be applied in agriculture to improve agricultural soil properties and crop productivity. The main impacts of biochar on soil characteristics and agricultural products yield are listed in Table 2 and Figure 2.  Olive stone, almond shell, wheat straw, pine woodchips, and olive-tree pruning 0.5-7.5% (w/w) Sunflower Decrease in soil bulk density and increase in soil field capacity Increase in dissolved organic C, available P and decrease in soil nitrate concentration Increase in seed germination, plant dry biomass and crop production [75] Poultry waste 0-1% (w/w) Brassicacampestris Improvement in soil physical and chemical conditions Enhancement of the availability of nutrients especially nitrogen Decrease in metals uptake in Brassica campestris and improvement of growth Increase in photosynthetic and accessory pigments production [76] Hardwood and softwood 0-5% (w/w) Potato -Mitigation of salinity stress Increase in shoot biomass, root length and volume, tuber yield, photosynthetic rate, midday leaf water potential, and increase in K + content in xylem [77] Corn cob 10-20 t/ha Soybean -Mitigation of water stress effects on soybean seedling especially at an application rate of 20 t/ha Enhancement in seedling growth Mitigation drought stress impacts Improvement in germination rate, shoot length and chlorophyll and carotenoid contents [78] Virgin pine wood 30 t/ha Maize No effect on the soil food web because the soil biological community is already degraded Mitigation of salinity stress and improvement in maize grain yield and plant performance in saline croplands Increase in maize leaf sap nitrogen, phosphorus and potassium and decrease in sodium and chloride Improvement in leaf bioactivity related to osmotic stress [80] Pellets of coniferous wood Reduced conductivity in biochar-ameliorated soils Reduced Na uptake of plants [81] Rice straw 22.5 t/ha Rice -Increase in total C and total N contents -Increase in C/N ratio and moisture of rhizosphere soil -Increase in soil pH and water retention ability Enhancement in rice yield and N retention in a paddy field [82] Poultry manure 1% (w/w) Barley -Improvement in petroleumhydrocarbonsdegradation -Improvement in soilmicrobial respiration Increase in dry weight yield [83] Maize stubble 3% Maize -Improvement in chlorophyll content and shoot to root biomass Decrease in contaminant retention by shoots [84] .  Biochar improves the soil texture and porosity, increases the pH buffering capacity of soils, increases CEC and helps in the binding of important anions and cations [85]. The increase in the soil pH subsequent to the addition of biochar improves the retention of basic cations in acid tropical soils and consequently improves the crop nutrition [86]. Biochar improves the soil texture and porosity, increases the pH buffering capacity of soils, increases CEC and helps in the binding of important anions and cations [85]. The increase in the soil pH subsequent to the addition of biochar improves the retention of basic cations in acid tropical soils and consequently improves the crop nutrition [86]. Similarly, Atkinson et al. [87] confirmed that the increase in the soil pH following biochar addition enhances the availability of phosphorous and potassium. Alkaline biochars are more suitable for acidic soils [88], while low-pHbiochars could be effectively applied to alkaline soils to lower their pH [89]. The soil CEC increases after biochar addition due to the enhancement of the oxidation of aromatic carbon [90]. The increase in CEC helps in improving nutrients retention in the soil and in mitigating leaching process, which increases the fertility of the soil [91]. The physico-chemical properties of soils in humid areas of Asia varied significantly following biochar addition, where pH value increased from 3.9 to 5.1, CEC from 7.41 to 10.8 cmol/kg, base cation percentage from 6.4 to 26, and bulk density decreased from 1.4 to 1.1 mg/m 3 [92]. In a study performed by Huang et al. [93], authors recorded an increase in the key indicators of soil quality by about 33% for soil organic carbon, 6% for pH, 10% for total nitrogen and 43% for N-use efficiency. Application of biochar also promoted the formation of macro aggregates in soils, which decreased the soil erosion potential [92]. Soils behave differently following biochar addition. Despite the good number of studies reporting the positive impact of biochar addition on fertility of tropical and subtropical soils and plant growth and yield [86,94], other investigations showed that the improvements in crop production were only observed in highly degraded and nutrient-poor soils [95], while richer soils showed less tendency to take benefits from biochar addition. Nevertheless, soil amendment with biochar is still an effective method for C sequestration in soils [73]. In addition to biochar properties, soil type and composition and the method of biochar application, i.e., single application of biochar or combined application with compost, are essential factors that define the effects of biochar on soil agronomic properties.
Despite the favorable effect of biochar on soil properties, some noticeable unfavorable changes in soil structure and composition following the addition of biochar have also been reported. Mukherjee et al. [96] applied oak-derived biochar pyrolyzed at 650 • C to silt loam soil in a field experiment of 16 weeks and observed that the resistance to water penetration in soil was lowered by 10-18%. Authors reported that short-term application of biochar could not lead to a significant effect on the physico-chemical properties of soils, whereas greater changes are mainly related to biochar ageing. Moreover, biochar in certain cases may contain some potential contaminants such as Zn, Cu, Cd, Pb and Cr when prepared from toxic solid wastes [97]; this can lead to undesirable effects on soil composition and soil microbial activity.

Impacts on Water Retention
Many investigations reported that biochar amendment improves soil water retention capacity [98][99][100] and reduces the water stress of plants during prolonged dry periods caused by global climate changes in the future [101][102][103][104]. The highly porous structure of biochar improves soil physical properties and interstitial spaces within the biochar-soil matrix and allows the reorganization of pore size distribution, which enhances the soil's water retention capacity [24,69]. Biochar has a hydrophilic surface because of the presence of oxygen-containing functional groups on its surfaces [105], which enables water to infiltrate into its intrapores [106]. An increase in the number of oxygen-containing functional groups on the biochar surfaces induces an enhancement ofthe biochar's hydrophilicity [105]. Another consideration that must be examined during biochar amendment is its effect on plant-available water content. Generally, biochar improves plant-available water, but the improvement significance is quite dependent on soil texture; the water uptake by plants increases by 45% for coarse-textured soils, by 21% for the medium-textured soils, and by only 14% for fine-textured soils, while others reported different values between 4 and 130% [26] and an average of 15% [107]. Although a high number of investigations reported the beneficial effect of biochar on water content in soils, other studies reported contrary results. Jeffery et al. [108] applied biochar produced from herbaceous plant cuttings to a sandy soil at a rate of 1-50 t/ha and reported no significant enhancement in soil water retention ability, even with the increase in its porosity following biochar addition. The authors attributed that to the high hydrophobicity of biochar, which allowed for preventing the infiltration of water into soil intrapores and then negatively affected its water retention ability. Additionally, not all studies reported the favorable effect of biochar addition on plant growth and yield. In the work performed by Herath et al. [109], the application of biochar derived from maize stover at a rate of 11.3 Mg/ha for 295 days resulted in no effect on available water capacity; this tendency is probably associated with the blockage of soil micropores by some components contained in the applied biochar, such as ash.

Impacts on Nutrients Availability
Biochar can incorporate many macronutrients and micronutrients into soil that can be used by plants [110]. The amount of incorporated carbon, which will be used by microbial communities or sequestered in soil, depends strongly on pyrolysis temperature and therefore varies from biochar to biochar. During pyrolysis and depending on carbonization temperature, some elements, such as C, N, H, O and S, are partly lost in the form of volatile substances, while other elements, including P and K and other metals, are more concentrated in the biochar than in the feedstock, which increases the fertility of soil [111][112][113]. Many investigations reported that biochar-amended soil has a high ability to capture ammonium and nitrate [113,114], and to prevent nitrate leaching [115][116][117]. Wang et al. [118] showed that biochar amendment significantly increased rice N-uptake and reduced leaching of N-forms from the soil.
The effect of biochar on the concentration of N-forms in soils depends on many factors, including the pyrolysis temperature, the residence time of biochar in soil, the biochar application rate and the soil pH [119]. Nguyen et al. [119] showed that the concentrations of ammonium and nitrate are reduced following biochar amendment by about −11% and −10%, respectively. The significance of reduction depends, according to the authors, on the residence time of biochar in the soil, and decreases as the biochar ages. Comparably, Liu et al. [120] reported that nitrate concentration decreased by 12% after biochar application. By contrast, Borchard et al. [121] observed an averaged decrease in nitrate leaching by −13% and N 2 O emissions by −38% in the majority of biochars that were applied to soils. This reduction is, however, dependent on the soil type and the residence time of biochar in the soil. Similarly, Hagemann et al. [113] reported that biochars could prevent nitrate loss from agro-ecosystems and act as a slow-release fertilizer, which can reduce global N fertilizer demands. Following biochar addition, not only N but also P leaching is reduced. According to Liang et al. [122], the conversion of manure into biochar allows the formation of less soluble (Ca, Mg) 3 (PO 4 ) 2 , which is preserved in the biochar produced. Similarly, Hong and Lu [123] observed an enhancement of P availability in soil after biochar addition, which may help in reducing the demand for chemical fertilizers in biochar-amended soils.

Impacts on Soil Microbiology and Enzymes
Soil microbial populations play a primordial role in the decomposition of organic substances, in nitrogen fixation and in nutrient cycling. In many investigations, biochar application to soil has been proven to increase microbial biomass and to affect nutrient cycling, enzyme activities and crops health [124].
Soil microbial populations are affected, as biochar has an influence on soil chemical and physical properties including pH, nutrient availability, and water retention capacity [125]. Biochar provides space for colonization, forms habitats for different microorganisms [126], and potentially protects them from desiccation and predation. Biochar amendment is accompanied by a general change in the use of carbon sources and can therefore lead to substantial changes in the microbial functional diversity and activity and in the taxonomical composition of the microbial community [127]. Liao et al. [128] recorded in a maize/legume intercropping system that biochar addition not only affectedthe bacterial community structure and the diversity of the rhizosphere, but also made plant-derived C assimilable to more microbial species following biochar addition. Li et al. [129] reported an increase in the bacterial abundance by about 161% when straw-and wood-derived biochars were applied to soil; this increase was especially remarkable for Gram-positive bacteria. In a study performed by Han et al. [130], among 45 bacterial genera, 21 genera were affected by biochar in cotton soils. The results show an increase in the abundance of Sphingomonas and Pseudomonas genera, while the abundance of nitrite-oxidizing bacteria was reduced [130]. Nitrospira is one of the nitrite-oxidizing bacteria, which is, in addition to other contributors in the N cycle, reported to be influenced by biochar application [131]. Li et al. [132] observed at a rate of 4% biochar addition an increase in the diversity of genes (nirK and nirS) related to denitrifying bacteria. Rafique et al. [133] observed that biochar in the presence of phosphate-solubilizing bacterium Lysinibacillusfusiformi can also improve the retention of phosphorous and nitrogen by maize plants by about 61.5% and 23.1%, respectively. Biochar was also proved for its ability to suppress soil pathogens and diseases. Elad et al. [134] observed a significant decrease in the intensity of the disease caused by foliar fungal pathogens in pepper and tomato upon biochar application. Similarly, Harel et al. [135] obtained results for strawberries. Jaiswal et al. [127] reported that biochar allowed the suppression of Fusarium crown and tomato root-rot, which leads simultaneously to an improvement in tomato plant growth. In another study, Huang et al. [136] showed that biochar decreases the sensitivity of rice plants to root knot nematode infections. Biochar has been also reported to enhance plant defense systems against fungi and to significantly reduce the colonization of roots by arbuscular mycorrhiza fungus [137] and to suppress the infection of tomato plants by Fusarium oxysporumf.sp. lycopersici [138]. Although a high number of studies have examined the impact of biochar on the microbial population in the soil, some points nevertheless need to be investigated. The interrelation of soil microorganisms with biochar and the special scale of this interaction have to be studied. The long-term impact of biochar and biochar ageing on soil microorganisms should be further clarified to prevent any possible negative impact. The large-scale study of the biochar impact on the microbial population of the soil under field conditions is still missing [139].Enzymes in the soil play a crucial role in decomposing organic substances and make both energy and NH 4 available to plants. Since biochar application stimulates changes in soil structure and composition, the production of enzymes by microbial communities will also change; the production of some enzymes will increase, while others will decrease [125]. Luo and Gu [140] claimed that biochar amendments in mangrove sediments induced an increase in enzymes such as phenol oxidase and β-glucosidase, while other enzymes such as peroxidase, N-acetyl-glucosaminidase, and acid phosphatase decreased. The influence of biochar on the activity of soil enzymes can vary. This depends on the soil type and the presence of substances that interact with biochar and can thus make it inaccessible to soil enzymes [141]. As compared to activated carbon exhibiting much higher surface area, biochar amendments may preserve most of enzyme'sactivity when adsorbed on its surface due to substrate availability [125]. The effect of biochar application on soil enzyme composition has been proven; however, the mechanisms controlling biochar interactions with enzymes and the response of each enzyme in soil to biochar application still have to be clarified in further investigations.

Impacts of Biochar on Plant Growth
Biochar addition stimulates soil fertility and then improves seed germination and crop productivity. Uzoma et al. [142] recorded an increase in maize grain yield by about 98% at biochar application rates of 15 and 20 t/ha. Similarly, an increase of 12 to 14% was observed in rice yield in a paddy rice soil in China after the addition of biochar at application rates of 10 and 40 t/ha without fertilizer addition [91]. Houben et al. [143] reported a three-fold increase in biomass production of rapeseed crop in a heavy metal-contaminated soil after 10% biochar addition. Hossain et al. [144] reported an increase in cherry tomato yield by about 64% during a pot experiment, with simultaneous enhancement in P and K uptake and soil chemical properties. An increase in pH, N, P, K, Ca and Mg concentrations in a low-organic carbon and -fertility soil in the southeastern United States was observed by Gaskin et al. [145] after addition of peanut hull biochar. In the same study, authors recorded a simultaneous significant response of corn yield following biochar application during the experiment lasting for two-year. The depth of roots and the existence of biochar in the root zone play a primordial role in defining the influence of biochar addition on plant growth. Jones et al. [146] reported that a deep rooting of more than 1 m is responsible for the insignificant productivity of maize as compared to pasture grass crop characterized by shallow rooting zones (<30 cm).
Although biochar has been shown to have a positive effect on soil, this cannot always be confirmed for plant growth and yield [98]. Chan et al. [147] reported an increase in radish dry matter after the addition of both biochar and nitrogen fertilizer, but no increase in yield was observed when biochar was used alone, even at the highest application rate of 100 t/ha. Borchard et al. [148] observed neither a positive nor a negative effect on maize yield after biochar application, and also Xu et al. [149] observed lower biomass yield of Suaeda salsa in saline sodic soil. According to some results in the literature, biochar can act positively on same plant species, while negative or no significant effect can be recorded for other species. Bass et al. [98] observed a reduction of 18% in banana yield, while no remarkable effect was reported for papaya yield. Hagemann et al. [150] also reported that the application of biochar did not induce any effect on sunflower biomass yield. Similarly, Graber et al. [151] showed that biochar addition can increase the flower and fruit yield of pepper plants, but no simultaneous increase in tomato yield was observed.

Heavy Metals
Biochar has been widely reported as an efficient material for the retention of both organic and inorganic contaminants [152]. Biochar's interface with pollutants in the soil can be controlled by many mechanisms. According to the assumptions reported in the literature, electrostatic interaction and precipitation in the case of heavy metals, and surface adsorption, partition and sequestration for organic pollutants are the main potential mechanisms controlling the biochar remediation efficiency [152,153]. Eco-toxicology of contaminants in soil is mainly related to their bioavailability and their solubility in water, rather than their concentration [154]. Biochar addition to soil systems may affect the mobility and the bioavailability of metals either by direct interaction between metals and biochar or indirectly by inducing variations in soil properties [155,156].The high variation in the characteristics of biomass used as a feedstock for biochar production and the diverse preparation methods and conditions allow the production of biochars with different characteristics, such as pH, CEC, organic carbon content, porosity, surface functional groups and specific surface area. Biochars will thus behave differently in the soil matrix, and then its interaction with metals and its impact on metal mobility and bioavailability will vary depending on the biochar nature (Table 3). Beesley et al. [157] studied the retention of inorganic contaminants on hardwood-derived biochar added in some specific sites in the UK in a 60 day-study. The results confirm the efficiency of biochar to reduce the concentration of total and bioavailable Cd and Zn in pore water, allowing the remediation of contaminated soil. Similarly, Beesley and Marmiroli [158] described a significant decrease in the concentration of inorganic contaminants (As, Cd, Zn) due to biochar adsorption, which significantly reduces their leachate. Kosseva [159] investigated the effect of biochar added to sandy soil on Cu toxicity in quinoa plants (Chenopodium quinoa). The author observed the same biomass production for quinoa plants in soil treated by 4% biochar as in the control with the maximum Cu concentration. Reduction in the exchangeable Cd by 28.6% in soil and decrease in the Cd bioaccumulation factor from 0.36 to 0.32 [171] Chicken manure, rape straw Cd 1%, 5%, 10%,15% (w/w) Reduction in Cd concentration in soil by 11.64-16.3% at an application rate of 15% Reduction in Cd uptake by maize shoots at application rates of 1% and 5% [172] Rice straw Cd 0, 10, 20 t/ha Cd immobilization in soil increased with increasing amounts of applied biochar [173] Miscanthus Ni 3, 4, 5% Reduction in the concentrations of Ni in spinach tissues and in soil with the increase in biochar rate, however no significant changes were observed below 3% rate [174] 3.6.2. Pesticide The ability of biochar to lower the bioavailability of pesticides in soils has been widely reported [29,175]. Clay et al. [176] showed that biochar application can significantly increase herbicide sorption but reduce herbicide efficacy. The adsorption of pesticide on biochar mainly depends on the properties of biochar used for remediation. Biochars prepared at higher temperatures present generally higher affinity to adsorb organic pesticide due to electrostatic attraction between organic pollutants and charged surfaces of biochars [29,177,178]. Another aspect that has to be considered with respect to biochar addition to remediate pesticide in soils is the release of pesticides previously adsorbed onto biochar, which can impact its bioavailability in soils and in surface-and groundwater as well [179]. Thanks to biochar properties, not all adsorbed pesticides can be readily desorbed [179][180][181]. Khorram et al. [182] showed in a one-step desorption experiment that some amounts of fomesafen adsorbed on soils amended with rice hull biochar at an application rate of 2% were desorbed; this could be due, according to the authors, to the relatively low surface area of rice hull biochar promoting the detachment of weakly adsorbed molecules of fomesafen molecules from biochar micropores [182]. Deng et al. [183] observed a significant reduction in the bioavailability of atrazine in soil amended with cassava wastes-derived biochar, due to its irreversible chemical binding or sequestration in biochar pores. Wang et al. [184] studied the effect of pyrolysis temperature on the desorption rate of herbicide terbuthylazine in a soil amended with sawdust biochar; the authors showed that the desorption rate is lower and slower in the presence of biochar produced at 700 • C as compared to that produced at 350 • C. The effect of biochar addition on pesticide sorption-desorption behavior in soils may also be affected by biochar ageing. Zhang et al. [185] showed that biochar ageing decreases biochar's specific surface area, which negatively affects the biochar's sorption capacity. Similarly, Lou et al. [186] reported that ageing of rice straw biochar in soil significantly decreased its sorption capacity to pentachlorophenol. Khorram et al. [187] found that despite the decrease in the sorption capacity of rice hull biochar after 6 months of its application, its ability to sequesterfomesafen is still higher as compared to non-amended soil. In the study performed by Jones et al. [146], the authors showed that the immobilization of triazine pesticides such as simazine varied little as a function of biochar ageing, suggesting the persistent of biochar and its long-term efficiency in soil.

Effect of Biochar on Greenhouse Gas Emissions
Agricultural activities contribute significantly to the release of greenhouse gases into the atmosphere. Natural processes through photosynthesis can participate in CO 2 sequestration; however, the long-term efficiency of this process is limited, as the unstable fraction of C can be decomposed and returned to the atmosphere as CO 2 within months to years [188]. An efficient long-term C sequestration could be only possible by converting biomass into more stable forms of carbon [188,189]. Biochar as a carbon material has been widely reported for its high potential for long-term carbon sequestration. The average residence time of biochar in soil can reach thousands of years [190,191]. This is due to its recalcitrant character, which makes its organic fraction unavailable for soil microorganisms and allows for the long-term sequestration of carbon in soils. The results of a life cycle assessment for CO 2 sequestration in Miyako Island revealed that if all the available sugarcane bagasse, which is around 12,000 t/year, is converted to biochar, a total of 1200-1800 t CO 2 can be sequestrated each year [154]. Spokas et al. [192] showed a reduction in CO 2 emission in a silt loam soil after the addition of biochar derived from wood chip at a rate of 20% (w/w). Similar results were also found by Liu et al. [193] in a biochar amended paddy soil. Contrary to this, Major et al. [194] claimed to find an increase in the CO 2 emissions after addition of biochar derived from mango (Mangifera indica L.) trees. The same tendency was also reported by Bell and Worrall [195], who observed increased CO 2 emissions from unvegetated arable plots amended with lump-wood-derived biochar; however, no significant increase was recorded for planted plots.
Biochar was also reported for its ability to mitigate CH 4 and N 2 O emissions from soils [196][197][198]. The efficiency of biochar in mitigating CH 4 and N 2 O emissions is strongly related to soil type, cultivated crops and origin of biochar [124,199].In a 2-year experiment performed by Dong et al. [200], biochar produced from rice straw was reported to be more capable to reduce CH 4 emissions from a paddy field than bamboo-derived biochar. The results show a reduction in CH 4 emissions of up to 86.43% during the rice growing cycle and an annual decrease between 20 and 51% after four years. Similar results were also described by Chen et al. [201]. Ibrahim et al. [202] recorded an increase in CH 4 emissions with a simultaneous decrease in CO 2 release after application of peanut shell and sewage sludge-derived biochars in bean plant growth. Biochar can also affect the denitrification process as reported by Cayuela et al. [203]. The results show a reduction in N 2 O emission by about 10-90% in 14 different tested soils [203]. Biochar addition promotes the transfer of electrons to denitrifying microorganisms, which accelerates the process of N 2 O reduction. According to Harter et al. [204], the efficiency of biochar in mitigating N 2 O emission is related to its ability in affecting the relative abundance and the taxonomic composition of N 2 O-reducing functional microbial in soil. In the study of Brassard et al. [205], biochars with relatively low N concentrations were reported as being more favorable in reducing N 2 O emissions. In the same line, Harter et al. [206] observed an increase in the transcription of bacterial N 2 O reductase following the addition of biochar in water-saturated soils, with a simultaneous increase in the abundance of N 2 fixation bacteria. Many investigations addressed their research to study the impact of biochar on greenhouse gas emissions. The results presented so far are inconsistent, and sometimes biochar is reported as negatively affecting greenhouse gas emissions, depending on the type of biochar and soil properties [199,207]. In contrast, several studies have reported positive results, providing sufficient evidence that biochar can effectively reduce greenhouse gas emissions and mitigate climate change [154]. In a field experiment in Australia performed by Scheer et al. [208] with biochar derived from cattle waste, the authors observed no significant changes in greenhouse gas emissions in biochar-amended red Ferrosol soil as compared to the control. In an earlier investigation, Zhang et al. [209] showed that biochar addition to paddy fields in China resulted in positive effects on soil properties and significant reduction in N 2 O emissions, while CO 2 remained unchanged and CH 4 was only reduced in the second paddy cultivation cycle.

Improving Animal Growth through Biochar
Scientists and a growing number of farmers conducted experiments where biochar, as non-toxic and edible material, was included into animal farming systems [210]. Since 2010, this practice has begun to be adopted by agricultural practitioners, who found good results in terms of improving productivity. Since 2012, the biochar market in Europe has grown considerably with the use of biochar as an additive to animal feed in Germany and Switzerland [211]. To frame this market, the European Biochar Foundation introduced in 2016 a new certification standard particularly for biochar used in animal feeding [212].
Schmidt et al. [213], in their interesting review, evaluated more than 100 scientific publications on the use of biochar for animal feeding to various animal groups. Most of the reported results were promising and exhibited for all investigated animal groups positive (but not always significant) effects on parameters such as digestion, toxin adsorption, blood values and cell numbers in milk. The authors also claimed the positive effect of biochar on livestock weight gain by increasing the pH, since an alkaline pH in the rumen could prevent acidosis, which is known to affect livestock weight gain. Similarly, O'Toole et al. [214] and Toth and Dou [215] reported that the oral ingestion of biochar improves animal health by improving nutrient intake and toxins adsorption. Devi et al. [216] reported an enhancement in growth rate of catfish following the addition of biochar at 2% in animal feed ingredients. Lecroy et al. [217] also recorded an enhancement in piglet growth after dietary biochar powder use as a feed additive.
In other studies, the addition of bamboo biochar enhanced the feed conversion efficiency of flounder [112] and wheat straw-derived biochar reduced abdominal fat deposition and enhanced the growth of poultry [218]. Biochar used as a feed additive can also provide resistance to various diseases and anti-bacterial effects [219] and can enhance the immune function and anti-stress properties of piglets [220]. Despite the large quantity of investigations demonstrating that biochar used as animal fodder has no negative effect, there are still open questions on its long-term impacts, for instance its effect on the resorption of liposoluble feed ingredients and potential interaction with themycotoxin fumonisin that should be considered before the unrestricted recommendation of its adoption.

Biochar in Composting
The mixed use of biochar and compost to soils may significantly affect soil quality and crop productivity [221]. The addition of biochar combined with compost can be performed in two ways: one is that biochar is mixed with compost after the composting process is finished, and the other is that biochar is added to compost at the early stage of the composting process [222]. The first method was reported as enhancing soil nutrient content and crops productivity [89,223], while other authors showed that biochar-based compost is more beneficial for soil and plants. Glaba et al. [224] found that the application of biocharbased compost improves the water retention capacity of sandy soils to a greater extent than compost without biochar. In another study, Kammann et al. [225] described that the application of compost, which is based on biochar, to soils protects nitrate content in biochar from leaching due to ion-water bonding in biochar micro-and nanopores. The addition of biochar during composting allows for improving the properties of the final product due to the biochar porous structure, which increases pile porosity and then enhances O 2 supply and avoids anaerobic fermentation [226]. Sánchez-García et al. [227] showed that biochar adjusts the pile density of the compost and prevents the formation of agglomerates larger than 70 mm. Godlewska et al. [228] reported that the use of biochars with alkaline pHs increases compost pH value. Contrary to this, Zhang et al. [229] showed that biochar addition to wheat straw and pig manure composts promoted the decomposition of bulk organic matter in the mixture and the release of inorganic and organic acids, which decrease the pH of the final composts. Biochar addition can also promote the creation of micropores for lactic-acid bacteria, which leads to a decrease in the pH of the compost [230].
Biochar has been reported to accelerate the humification process during composting [231,232]. Its addition promotes the C transformation from raw materials to humic substances, mainly humicacids, fulvic acids and humans [233]. A compost rich in humic substances can provide a variety of agronomic benefices to soil, including water and nutrients, which enhances plant growth [234]. The time required to enter the thermophilic phase is also reduced after biochar addition and the temperature of composting is increased, which improves the humification efficiency [235]. Moreover, biochar reduces the leaching of organic C, P and N and promotes nutrient retention during the composting process. Zhang and Sun [236] observed an increase in nutrient content during green waste composting after the addition of 20% biochar combined to 35% mushroom compost (w/w). Similarly, Zhang et al. [229] recorded an increase in water-soluble nutrients (PO 4 3− , K + and Ca 2+ ) of the compost product after addition of 10-15% (w/w) of wheat straw derived-biochar. In the study performed by Jain et al. [235], the total nitrogen of the final compost increased by about 45% after the addition of biochar at a rate of 5% (w/w) as compared to the control. The ratio of C/N (w/w) is one of the most important parameters controlling the composting efficiency of organic wastes, and its optimal value should be about 25:1 [237]. As biochars contain high content of C, its addition could increase the C/N ratio of raw feedstocks rich in N. Simultaneously, composting can increase the biochar O/C ratio following the acceleration of the biochar surface oxidation [238]. Addition of biochar combined with compost can bring the same benefits to soils as chemical fertilizers. Biochar recycles P and C, and compost recycles P, N, C, and K; a combined application of both materials promoted the recycling of macro and micronutrients necessary for plant growth [239]. The biochar application rate significantly influences the composting process. A high loading rate could induce desiccation in composting due to excessive aeration. Steiner et al. [240] reported that 10% to 15% loading rates could be considered as optimal, while other values of 20% [240] and 27% [241] have also been reported.
Since biochar starting feedstock and pyrolysis temperature are proved to influence biochar properties, these parameters can also influence the quality of the final composting product. Most of the biochars used in composting are produced from woody biomass (e.g., oak wood, beech wood, bamboo) and crop residues (e.g., wheat straw) by pyrolysis at temperatures ranging from 400 to 700 • C [229]. The joint application of biochar and compost and their interaction during composting can improve the effect of both materials [237]. Composting promotes the formation of carboxylic and phenolic groups on the biochar surface, which increases its reactivity [238], while biochar can affect the microbiological structure of the compost by affecting moisture, aeration, pH, and nutrient contents of the compost [242,243]. The biochar porosity and its high surface area enhance qualitatively and quantitatively the composition of the microbial communities [244]. Liu et al. [245] found an increase in microbial growth by 15% CFU/g after addition of biochar to compost. Biochar addition was reportedly able to increase the abundance of N 2 O-consuming bacteria, which reduces N 2 O emissions from biochar-amended compost during the maturation phase [246] and to reduce the abundance of pathogenic microorganisms during composting of organic wastes [247]. Furthermore, some studies reported the positive effect of biochar application on the enzyme's activity during composting. Jindo et al. [248] found that urease, phosphatase, and polyphenol oxidase activities increased by 30-40% in the biocharamended compost. Similarly, Du et al. [249] reported that biochar addition to sewage sludge compost increased the activity of enzymes such asaryl-sulfatase, b-glucosidase, phosphatase, and dehydrogenase.
Biochar addition during composting also reacts with heavy metals and contributes to their immobilization, which accordingly reduces the risks of applying compost to soils. In the study performed by Zhou et al. [250], passivation rates of 95%, 66% and 69% were reached for Cu, Pb and Cd, respectively, when biochar and commercial humic acid were added to pig manure compost. Similar results were found by Liu et al. [251], observing a reduction in the bioavailability of heavy metals, particularlyAs and Pb, in compost based on sewage sludge. Wang et al. [252] reported that the addition of biochar mixed with zeolite induces an increase in the binding ability and affinity of humic acid and Cu and Zn forming humic acid-complexed Cu and humic acid-complexed Zn, which enhanced the immobilization of heavy metals. The addition of biochar during the composting process has been also proved to control the GHG emissions from compost [232,253,254]. The release of NH 3 and NH 4 + during the composting process as a result of organic matter mineralization under increased temperature, pH and aeration, could be reduced after the addition of biochar due to its porous structure and its high adsorption capacity [253]. According to Wang et al. [254], the addition of 10% tobacco stalk biochar combined with 10% zeolite and 2% wood vinegar (w/w) to pig manure compost allows for a reduction in NH 3 loss by about 64.45%, and a decrease in CO 2 , CH 4 and N 2 O emissions by 33.90%, 50.39% and 79.51%, respectively. Opposite results have been found, however, such as by Czekala et al. [255], who observed an increase in CO 2 emissions due to the increase in pile temperature after biochar application.

Limitations
Although the majority of research studies reported beneficial aspects of biochar use as a soil amendment, others reported insignificant or sometimes negative results, giving rise to some apprehension regarding its large-scale and long-term application. The availability of biochar could be a limitation to its application; biochar may not be sufficiently available to reach significant positive results. Furthermore, the positive impact of biochar has been reported to be soil-specific; its application may not be beneficial for all types of soil [256,257]. Other problems related to biochar application were also reported. SafaeiKhorram et al. [258] recorded an increase in weed growth by 200% during lentil culture after biochar application at a rate of 15 t/ha. The effect of biochar on crop productivity varies depending on the plant species and the target part of the plant. Vaccari et al. [259] observed no fruit yield of tomato plants after the addition of biochar at 14 t/ha, while an increase in vegetative growth was recorded. A delay in flowering for plants was also observed because of biochar addition to the soil [260]. Biochar may also be selective, and in some cases its addition does not import any immobilization effect towards some contaminants in soil [261]. The higher ash content of biochars produced at high temperatures may also induce detrimental effects on plants growth [262]. Another limitation of biochar amendment is its ability to adsorb essential nutrients for plant growth and then act as a competitor, which can be counterproductive to crop productivity [263,264]. For instance, the addition of biochar and phosphorous fertilizer in saline sodic soil promotes the precipitation/sorption reactions of phosphate and thus reduce the availability of phosphorous for plants [149]. Addition of biochar in soils may also cause a disturbance in organic matter decomposition, which may decrease the abundance of fungi species such as Ascomycota and Basidiomycota [265]. Other inconsistent effects may also occur depending on the biochar starting feedstock. In the study performed by Gonzaga et al. [266], soils treated by coconut husk biochar at 30 t/ha allowed for an improvement of the Zea mays biomass by about 90%, while no significant effect was observed with orange bagasse biochar applied at the same rate. The application of biochar derived from some nutrient-poor biomasses may also limit their efficiency in improving crop productivity, which may require its combined application with compost or some chemical or organic fertilizers. Additionally, some feedstock used for biochar preparation may contain contaminant substances, which in turn affect plant growth [267]. Unless a good number of studies prove that these limitations can be overcome, the future of biochar for agricultural applications may be uncertain [154]. Crucial examination of all factors should be performed to prevent any potential detrimental effects of biochar that can limit its large-scale application. More research is after all needed to explore all mechanisms and to assess the long-term risk of biochar agronomic applications.

Conclusions
This review deals with the beneficial role of biochar in different agricultural applications. Biochar'sphysico-chemical properties, which depend on many variables such as biomass source, pyrolysis conditions, biochar ageing, determine its various agronomic functions. Biochar can act as a soil amendment, as a co-substrate in composting and as a feed additive to improve livestock productivity. The effect of biochar amendments in mitigating greenhouse gas emissions has also been proven. However, and based on literature findings, benefits of biochar application in agriculture are often limited to specific conditions. For instance, biochar can significantly improve soil properties and crops yield for certain soils and certain plants, while showing no significant effect or sometimes negative effect on other soils. This can be due to the large variety in biochar properties but also to the variety in soil types, crops specificity, climate conditions and the difficulty to predict the real interaction between biochar and soil nutrients and contaminants. This makes it difficult to decide on the appropriate biochar for the appropriate agricultural application. Hence, the real challenge is not the feasibility and efficacy of biochar application, but rather the selection of the appropriate biomass and pyrolysis conditions to produce specific biochars for specific purposes. Moreover, most of the studies presented so far were performed under laboratory conditions, which make the underlined conclusions unsuitable to cover all biochar effects and to define all limitations that could be related to its application. More experiments are therefore needed to evaluate the longevity of biochar's impact in agriculture under realistic conditions.