New Frontiers for Raw Wooden Residues, Biochar Production as a Resource for Environmental Challenges

Abstract

Biochar has gained significant interest in the agroforestry sector, mainly because of its ability to improve soil quality and sequester carbon in the atmosphere. Among the feedstocks of possible use for biochar production is biomass, understood as products and residues of plant origin from agriculture and forestry. The quality of the biomass used for biochar production is important because the physicochemical characteristics of the final product depend on it. This review examines the use of biochar produced from forest wastes and its impact on agriculture, forest ecosystems and the environment in general. This work demonstrates that the use of biochar not only improves agricultural productivity and production, but also that the sustainable management of the environment and forests and contributes to forest fire risk mitigation. The authors, examining the physico-chemical properties of biochar produced by forest waste, noted that the most critical variable is the process (pyrolysis temperature, residence time and heating rate), but the type of biomass used as a raw material and the forest species used also have a significant impact in determining the characteristics of the final product.

1. Introduction

In recent decades, the attention of major global and European institutions has increasingly focused on the need to address environmental and climate challenges. One of the main causes of environmental and air pollution is the production of greenhouse gases: it causes, for example, rising earth temperatures; the melting of glaciers and rising sea levels; climate change; heat waves, periods of drought and increase in desert areas as well as the increase in extreme natural phenomena such as floods, storms, hurricanes and fires.

The agricultural sector is the fourth largest greenhouse gas emitting sector (after industry, buildings and transport). In 2021, it is estimated to have emitted more than 100 million tons of CO₂ equivalent into the atmosphere in Europe, about 10% of the European total. In the face of these issues, as mentioned, major European institutions have been active in enacting acts to reduce air and environmental pollution. In particular, the Green Deal, which can be considered the main act in environmental matters at the European level, sets important goals that must be achieved in the short term: above all, the achievement of climate neutrality by 2050. This is complemented by sub-strategies, such as the RePowerEU and Farm to Fork (F2F), which set limits for all productive sectors, including agriculture, from the perspective of greenhouse gas emissions [1].

In addition to the strategies, there is a need to find new tools aimed at mitigating climate change and promoting sustainable environmental and forest management. Biochar is such a valuable tool for mitigating global warming [2] that it has been called an environmental remediation material [3]. For these reasons, the biochar industry has captured the interest of scientists, farmers and environmentalists as a possible solution to address environmental and climate challenges [4,5].

Biochar is a carbon-rich substance produced by the anoxic pyrolysis of agricultural [6], food [3] and forestry waste [7,8]. Due to its microporous structure and high carbon content, biochar can be used as a soil conditioner to improve soil quality or as a replacement for activated carbon [9], sequester carbon [10] and absorb and remove different types of pollutants such as heavy metals, organic pollutants, and other inorganic pollutants [11]. However, the impact this has on soil properties may vary depending on the feedstock used [12], pyrolysis conditions [12,13] and the amount of application [14,15].

Pyrolysis, torrefaction, hydrothermal liquefaction, and gasification are well-known thermochemical technologies that transform biomass into high-value biofuels and chemicals [16,17,18]. Among these, pyrolysis is emerging as the most important thermochemical process [19], especially in the application area of organic waste treatment (e.g., biomass such as wood, garden waste, and residues from the food chain, etc.), capable of being highlighted as a negative emission technology (NET) by the Intergovernmental Panel on Climate Change [20] to achieve carbon neutrality. The composition and yields of the final product are essentially determined by the temperature at which the process is carried out [21]: high temperatures (>700 °C) provide the production of Syngas as the main product, lower temperatures (500–700 °C) favor the production of Bio-oil, while under conditions of temperatures <500 °C, biochar is obtained.

Syngas is the gaseous by-product of the reaction and can be burned in thermal reactors or upgraded to methanol, dimethyl ether (DME) and advanced hydrocarbon fuels [22]. Bio-oil is the liquid by-product of the process and can be upgraded to transportation fuels, and biochar is a carbon-rich solid material.

In the partial or total absence of oxygen, the thermal decomposition of plant-derived biomass, known as pyrolysis, can be managed to generate a range of products, including CO₂ and, in varying proportions, combustible gases such as H₂, CO, CH₄, volatile oils, tarry vapors, and a high-carbon solid residue commonly called char. Thus, it can be argued that the pyrolysis process offers significant opportunities for the energy valorization of biomass.

This review aims to examine the characteristics of biochar derived from various sources of forest waste, highlighting the variables that most influence its final characterization (such as production process, analysis methodology, and type of feedstock). Through a literature review, we aim to explore the challenges and opportunities that biochar, exclusively produced from forest waste, offers at the environmental level, delving into the critical issues and possibilities for the use of this important material. The present work aims to fill any gaps in knowledge currently present in the literature caused by the lack of consideration of the forestry component to produce quality biochar.

A literature search was conducted in English, and relevant publications were downloaded if they were open-access. The search engines used were Scopus, Elsevier Science Direct, and Google Scholar. The following criteria were used to determine which papers would be included in this review: (i) full-text accessibility; (ii) filter time: 2017–2024. We searched for the following search elements in the title, abstract or author keywords: “biochar,” “forest waste,” “biomass pyrolysis,” and “environmental applications.”

To further investigate the physical chemical characteristics and the elemental analysis of biochar produced by different types of forest waste, ten articles selected from the literature were examined and discussed in detail, identified through the search engines ScienceDirect and Google, using as keywords “forest waste as raw material”, “biochar” and “biomass pyrolysis”, “surface”, “physico-chemical properties”, and “proximal analysis”.

2. Standards of Biochar Production

Given the diversity of feedstocks, conversion processes, and post-processing techniques for various end uses, biochar exhibits a wide variation in its physical and chemical properties [13]. For these reasons, several standards have been developed globally with the aim to mitigate potential health and environmental risks associated with the production and use of biochar, while indicating the minimum thresholds for biochar properties to be met to produce a certified biochar.

Currently, the most widely used standards for biochar production and use are the International Biochar Initiative [23] and the European Biochar Certificate [24]. These have established standards for biochar production and characteristics, distinguishing it from competing products (e.g., low-carbon fly ash or carbon adsorbents). The following table (

Table 1. Thresholds for specific properties established by two widely used biochar standards: IBI and EBC.

Criterion		IBI Standard	EBC Standard
Feedstock		Biomass, not including municipal solid waste, hazardous materials, or hazardous wastes	All biomasses included in the EBC-Positive list may be used individually or in combination as feedstock to produce EBC biochar.
Carbon content (%)		10% minimum in 3 classes: Class 1: ≥60 %Class 2: ≥30% and <60 %Class 3: ≥10% and <30%	35–95% of dry matter
Molar H:C organic ratio		<0.7	<0.7
Surface area (m2 g−1)		No minimum	No method provides absolute values for the specific surface area
Heavy metals		Range of maximum allowed thresholds	Thresholds of heavy metals related to EBC-Agro (Class III)
	Pb	121–300 mg kg−1	150 g t−1
	Cd	1.4–39 mg kg−1	1.5 g t−1
	Cu	143–6000 mg kg−1	100 g t−1
	Ni	47–420 mg kg−1	50 g t−1
	Hg	1–17 mg kg−1	1 g t−1
	Zn	416–7400 mg kg−1	400 g t−1
	Cr	93–1200 mg kg−1	90 g t−1
	As	13–100 mg kg−1	13 g t−1
Polycyclic Aromatic Hydrocarbons (PAHs)		6–300 mg kg−1	6.0 ± 2.2 g t−1
Polychlorinated Biphenyls (PCBs)		0.2–1 mg kg−1	0.2 mg kg−1
Dioxins and furans		17 ng kg−1	20 ng kg−1

) shows the specific thresholds set by the two standards.

3. Biochar: A Potential Ally for Improving Soil Health

The concept of soil health refers to the ability of soil to perform several important functions, both agronomically and environmentally [25]. These include, for example, the promotion of agricultural productivity, the ability to respond effectively to management and environmental inputs, and resistance to biotic and abiotic stresses [26].

Unfortunately, in recent times, we are witnessing rapid soil degradation and accelerated land erosion. These processes represent urgent environmental challenges, with serious consequences on ecosystem functions [27,28], such as a decrease in the productivity and sustainability of agricultural lands [29].

Among the main causes of soil degradation are the decline of organic matter and the loss of nutrients through the leaching process. The former contributes to erosion, compaction, salinization and nutrient deficiency, while the latter reduces soil fertility and accelerates acidification, thus increasing dependence on chemical inputs such as fertilizers. This negative cycle has detrimental consequences on biodiversity, environmental health, and costs to farmers [30].

Biochar is a carbonaceous material produced through the pyrolysis process, and it is used as a soil amendment [31]. Its application as a soil amendment [32] is considered an effective strategy to combat soil degradation and support the health of agricultural soils on a global scale [33]. It is a potential “geoengineering” tool for climate change mitigation, as it promotes soil carbon sequestration [34]. However, it should be stressed that the use of biochar as soil improver must comply with certain characteristics and requirements established by current legislation (Legislative Decree no. 75/2010, supplemented by Ministerial Decree 6 July 2023), and this could represent, in certain situations, a possible limitation of its use.

One of the main ways biochar can affect erosion rates is by improving soil structure, namely, the size, stability and arrangement of soil aggregates [35]. This, in turn, can change the runoff time (infiltration capacity), duration and amount of runoff, as well as soil erodibility, thus contributing to erosion reduction.

Indeed, the use of biochar in agriculture alters soil properties by improving soil organic carbon (SOC) content, bulk soil density, and nutrient retention [13,36], improving soil water-holding capacity, changing soil biological community composition and abundance [37,38], and increasing plant tolerance to biotic and abiotic stresses [39]. More generally, all of this can be translated into improving soil quality, increasing agricultural yield and thus improving agricultural production [40,41], through the application of biochar [42].

Studies conducted by [43,44] showed several positive effects of biochar application in agricultural soils. Specifically, in the first study, the application of biochar derived from willow and acacia biomass significantly reduced the leaching of various nutrients, such as NO₃-N, P, K, Ca, Mg and Na. Also, in the second study, it is observed that the addition of biochar reduced the total runoff volume, suggesting an increased ability of biochar-treated soil to retain water. However, interestingly, this study [44] showed a different response in terms of soil loss depending on the concentration of biochar applied. Lower concentrations (1% and 3%) inhibited soil loss, while higher concentrations (5% and 7%) promoted soil loss. This suggests that there is an optimal point of biochar application in terms of concentration, beyond which undesirable effects on soil stability might occur.

Bruckman et al. [34] observed that the use of biochar (10 t ha⁻¹) as a soil conditioner positively influenced soil respiration, showing a potential priming effect up to 15 months after application. What is more, the addition of biochar led to increased levels of stable carbon in the soil organic horizon.

The results of the studies clearly show that biochar application can lead to a few significant benefits to soil physical properties. In particular, ref. [35] shows that biochar tends to reduce soil bulk density by 3% to 31%, increase porosity by 14% to 64%, and improve wet aggregate stability by 3% to 226%. These positive effects contribute to increased water availability in the soil, which increases from 4% to 13%, thus improving plant growth conditions.

However, it is important to note that the effects of biochar on aggregate stability in dry soil may be mixed, suggesting that the interaction between biochar and soil moisture may influence the observed results. In addition, the saturated hydraulic conductivity of soil can be affected by biochar application, with a decrease in coarse-textured soils and an increase in fine-textured soils. This suggests that the effect of biochar on hydraulic conductivity also depends on the intrinsic characteristics of the soil [35].

The study conducted by [45] provides additional valuable information on the effect of biochar on soil water retention, emphasizing the importance of several parameters in determining this effect. The research shows that the application of oxidized biochar to hot air (250 °C) post-pyrolysis can increase water retention in a sandy soil. However, it is highlighted that the ability of biochar to retain water depends on a complex combination of factors, including the biochar’s internal porosity, its surface chemistry and the raw material source used to produce it, along with the production temperature. Specifically, biochar increases soil porosity due to its particularly porous internal structure. This increase in soil porosity, in turn, increases the surface area of the soil, promoting better water percolation and a greater ability to retain moisture in the soil.

Previous studies [46,47] have shown that pyrolysis temperature and air oxidation during the biochar production process can significantly affect its internal porosity and surface chemistry. For example, a biochar produced at higher temperatures (600 °C, using pine wood, pine bark, and poplar wood as feedstock), and with adequate air oxidation can show a more microporous and less hydrophobic structure [48,49], with higher surface functionality than a biochar produced at 350 °C, which contribute to a higher water-holding capacity in the soil.

Additional studies that support this thesis are cited below. According to [50], the use of biochar produced from acacia whole tree green waste has been shown to increase soil porosity in a Planosol-type soil by creating lodging pores and improving the stability of soil aggregates. The addition of biochar produced from wood charcoal to Antrosol- and Ferralsol-type soils also showed several benefits, including an increase in soil carbon content, an increase in soil pH, an increase in phosphorus availability, and a reduction in the leaching of nutrients such as nitrogen, calcium, and magnesium with a reduction in soil aluminum content [51,52]. These results indicate that biochar application can positively influence several aspects of soil fertility and nutrient management, contributing to greater agricultural sustainability.

Overall, these results support the idea that the application of biochar can be an effective strategy for improving soil physical properties. However, biochar also has effects on the chemical properties of soils, contributing to the improvement of degraded and nutrient-depleted soils. Several studies have shown that the addition of biochar can increase soil pH, improve cation exchange capacity (CEC), increase soil water-holding capacity, change the bulk density of soil, and increase exchangeable base cations [53]. In addition, the use of biochar can improve nitrogen fertilizer use efficiency [54,55,56], contributing to more sustainable nutrient management in agricultural systems.

The persistence and stability of biochar in soil are important considerations in assessing its long-term impact on the environment and the effectiveness of soil management practices. Several researchers have suggested that biochar can persist in soil for longer periods than other forms of soil organic matter, as indicated in studies conducted by [55,57]. However, the precise residence time of biochar in soil is still debated and varies depending on a number of factors, including its composition, soil temperature and moisture, and agricultural management conditions. This has important implications for the value of the technology in terms of carbon exchange [5]. Understanding the decomposition kinetics of biochar in soil is critical for assessing its potential contribution to carbon sequestration and for developing sustainable soil management practices [4]. Further research is needed to elucidate the mechanisms and factors that influence biochar decomposition in soil in order to optimize its application and maximize its long-term benefits.

The application of biochar can have a significant impact on soil biological functionality by affecting habitat for microorganisms and nutrient availability. A study by [58] found that biochar application can alter soil biological functionality by providing a favorable habitat for microorganisms due to its highly porous nature. In addition, biochar can alter substrate availability and enzyme activity around the biochar particles themselves, promoting increased microbiological activity.

However, the effects of biochar application on crop production can vary depending on the specific characteristics of the soil and the crop grown. A study by [59] found that soil characteristics and crop type can determine the impact of biochar on crop-specific production compared to the type of feedstock used to produce the biochar itself. For example, the ineffectiveness of slow pyrolyzing wood biochar in enhancing the activity of phosphorus solubilizing microbes (PSMs) for phosphorus mobilization was observed in phosphate-rich soils, while it significantly improved crop yield in phosphorus-deficient soils.

In addition, biochar application can affect the availability of nutrients, such as phosphorus, in the soil in complex ways. A study by [60] showed that the application of pine biochar at 2% and 4% resulted in a significant decrease in the abundance of arbuscular mycorrhizal fungi (AMF) in roots by 58% and 73%, respectively, but not in soils, which were accompanied by a significant decrease of 28% and 34% in soil phosphorus availability.

4. The Uses of Biochar in Forestry

Second only to oceans, forests represent the largest sink of carbon on earth, most of which is stored in the soil [61]. However, these invaluable ecosystems are facing ever-increasing pressures, including increased demand for production and the growing effects of climate change-induced natural disturbances. The crucial role of forests in climate change mitigation is also underscored by the Paris Agreement and is recognized by the European Union as a key step in achieving its climate goals.

Sustainable forest planning and management combined with the responsible utilization of woody biomass for energy and renewable products are the most relevant contribution the forest ecosystem can make to today’s climate and environmental challenges [62].

The current management of waste biomass usually leads to two outcomes: pile burning, which has negative implications for air quality, soil health, and carbon emissions [63,64] and fire risk, or on-site decomposition, which also emits carbon and contributes to forest fuel loads [65]. Both scenarios represent missed opportunities for carbon sequestration.

Despite compelling evidence pointing to the potential improvements in soil health and carbon sequestration offered by biochar [66,67], its scale of production and use remains surprisingly limited. However, the recognition of the climate and soil benefits of using this waste biomass for biochar production is well documented [68]. In fact, the reuse of forest harvest for biochar production offers a unique dual opportunity to mitigate fire risks and provide a sustainable, abundant, and low-cost source of biomass for biochar production [68]. Integrating biochar into land management strategies offers a versatile solution to several pressing environmental challenges [69].

Lehman et al. [5] demonstrated the potential to transform bioenergy into a carbon-neutral industry by using biochar in forest ecosystems. They suggested that the ability of forest ecosystems to sequester carbon can be further developed through pyrolysis of plant biomass into biochar. Although half of the carbon in waste is released into the atmosphere as CO₂ by pyrolysis, pyrolysis converts woody materials with twice the carbon content, and biochar locks the rapidly decomposable carbon from plant biomass into a very durable form. In addition, the storage capacity of biochar is not limited in the same way as biomass sequestration [5]. On the one hand, the use of pyrolyzed material provides an effective means for storing carbon, while on the other, its integration in the soil promotes the growth of plant species. This natural symbiosis promotes the absorption of CO2 from the atmosphere, thus contributing to the mitigation of climate change. It is important to note, however, that the transport of forest residues to biochar production sites can generate atmospheric emissions and have a negative environmental impact. This critical phase of the process deserves more detailed attention in the scientific literature.

4.1. Biochar’s Effects on Tree Growth

There are few studies in the literature that analyze the effect of biochar on plant growth in forestry. However, some studies have been conducted that assess the importance of considering specific environmental and soil conditions, plant species, and type of biochar used in evaluations of the effects of biochar on plant growth. The authors of [70] evaluated the effects of the growth of spruce seedlings in alluvial loamy soil enriched with biochar in different proportions were evaluated. Although there was no clear trend in growth, the addition of biochar had no negative effects, even at higher dosages.

Lin et al. [71] conducted a mesocosm experiment to evaluate the effects of biochar on the growth of young trees. Two 2-year-old pine trees (Pinus elliottii Engelm.) were transplanted in soils, taken from a 100 cm depth in a forest environment. The application of biochar derived from chicken manure (2.4 kg/m²) increased primary net production (NPP) and above-ground biomass plus litter by 180%.

However, ref. [72] showed that the use of biochar did not improve the growth of Douglas-fir seedlings when grown in containers with biochar-modified peat-based growing media in greenhouses. Although these studies have evaluated the effects of biochar on seedlings or saplings, few studies have directly investigated the effects of biochar in forest settings [73].

4.2. Biochar: A Forest Fire Prevention Tool

Agroforestry by-products or waste from uses such as branches, logs, and sawdust represent not only a huge potential for biochar production, but also a tool to combat fire risk. In fact, forest residues or wood waste generated during normal commercial logging operations pose a forest fire risk that needs to be removed or properly treated [68].

Forestry legislation at the national level, the European Forestry Strategy, and local administrative acts, stipulate the obligation of the removal of utilization residues on agroforestry stands in order to minimize the fire risk and consequent damage to the ecosystem. Usually, people opt for the quickest and least economically costly way, that is, burning the residues in more or less massive piles. This requires an additional cost for forest enterprises. This includes the cost of using harvesting machinery [74,75], transport and biomass handling [76]. Nonetheless, it is a source of air pollution [77]. Moreover, the burning of large piles can alter the soil, thus reducing site productivity for residual trees for decades.

While the production of biochar, which sees the removal of forest residues from the cutting, prevents the risk of fire, the addition of biochar, which is a high-carbon material, can improve fire resistance itself [78]. During the pyrolysis process, oxidation damages the polymer chains in the structure of cellulose, hemicellulose and lignin (the main components of forest residues), creating charred residues. The final product, biochar, is a stable, carbon-rich, dense and compact organic material, less prone to flame propagation, which can act as a physical barrier, limiting combustion penetration and external heat flow [79], thus acting as a fire inhibitor [80].

4.3. Effects of Biochar on Forest Litter

The application of biochar to forest soils results in the release of this substrate over the litter layer, which is the third-largest carbon reservoir in European forests (ca. 8.4%) [81] after soil carbon (ca. 54%) and biomass above ground (approx. 29%). As a result, changes in the decomposition rates of the litter layer may occur because of biochar applications which can, in turn, produce a potential advantage in terms of carbon sequestration when decomposition rates are reduced, or a potential disadvantage when these rates accelerate.

Studies have shown, for example, that biochar applications (10 t ha⁻¹) have increased the rate of decomposition of fresh and old Quercus serrata L. leaves which fall above and below the biochar application layer [82]. However, it has generated positive or negligible effects on the decomposition of both the shoots and the leaves of Phleum pretense L. buried in sandy and sandy medium-fine soils (3 cm from the soil surface) [83]. Negligible effects of biochar applications (about 20 t ha⁻¹; different origin derived from wood) were also observed for Triticum aestivum L. litter buried in silty–silty soils (−15 cm from the ground surface) [84].

Vannini et al. [85] suggest that the application of biochar to beech forest soils not only appears to have negligible effects on the early decomposition rate of high-quality litter, but it could also reduce carbon loss during plant material degradation. This suggests that biochar amendments may not affect the turnover of organic matter in forest soils in the short term but may contribute to increasing the carbon stock in the soil of deciduous hardwood forests in the long term.

5. The Uses of Biochar in Agriculture

The use of biochar in agriculture offers the opportunity to combine carbon capture with important agronomic and environmental benefits [86], provided the type of biochar, the application rate and method are carefully selected according to the specific objectives of the application, the environmental characteristics of the site and the requirements of the crops [28,87].

The application of biochar produced from forest waste to soil has been shown to significantly influence the response of different crop varieties. For example, biochar derived from mango wood (0.8–16 t ha⁻¹), combined with corn stalks (2.6–91 t ha⁻¹) and applied to maize, led to an increase in biomass from 30% to 43% and a yield of 22%, improving soil pH, cation exchange capacity (CEC), nutrient availability and water retention [88].

Similarly, the application of biochar produced from acacia bark (10 L m⁻²) to maize and peanut crops almost doubled the yield of maize and peanuts due to the increased nitrogen content and exchangeable bases, and the low aluminum content in the soil.

In other cases, such as in the biochar treatment of teak and rosewood (4–16 t ha⁻¹) on soils cultivated with rice and sorghum, an increase in yield was observed up to 2–3 times, thanks to improved crop response to NP fertilizer [89,90]. Also, the combination of biochar derived from forest residues (30, 60 t ha⁻¹) with wheat chaff favored seed germination (4–9%), improved yield up to 30%, and supported crop growth for two consecutive seasons [91,92].

The integration of biochar derived from pine sawdust (0.5, 10, 15 t ha⁻¹) in a tomato crop has led to a significant increase in growth, yield and crop quality compared to the use of pine sawdust alone [93]. Similarly, the addition of wood biochar increased wheat yield by up to 30%, keeping the nitrogen content of cereals constant and maintaining a sustained yield for two consecutive seasons without additional biochar additions in the second year [92]. These results indicate a long-term beneficial impact of biochar on soil yield and fertility.

The study conducted by [94] showed that the addition of 20 t of biochar per hectare did not lead to a significant increase in maize grain yield in the first year. However, in the following three years, there was a significant increase of 28%, 30% and even 140% compared to the control. This suggests a long-term beneficial impact of biochar on soil yield and fertility, indicating that its effect may manifest over time, providing lasting benefits.

In addition, the yield responses of corn, black-eye beans, and peanuts to the application of charred bark of Acacia mangium Willd. at the rate of 37 t ha⁻¹ were observed mainly in sites with less fertile soils, with an increase of up to 200% in yield on less fertile soils when the biochar was applied together with the fertilizer. This increase could be attributed to the increased availability of nitrogen and phosphorus, the colonization of mycorrhizal fungi and the reduction in exchangeable aluminum ions [95].

On the other hand, the effects of biochar on crop growth are highly variable when observed in individual cases, with responses ranging from increased growth to decreased growth [96] or no response [97]. Jeffrey et al. [42] also revealed that biochar has no effect on crop yield in temperate latitudes but causes an average yield increase of 25% in the tropics. In terms of soil fertility benefits resulting from the incorporation of biochar, it was found that biochar derived from manure and legumes is superior to that derived from wood. The choice of raw material to produce biochar and the conditions of pyrolysis can significantly affect the physical and chemical properties of the soil [98,99]

Singh et al. [100] noted that wood-derived biochars had a higher total carbon content and a lower ash content than manure-derived biochars. Moreover, the biochars derived from wood showed a lower content of elements such as nitrogen, phosphorus, potassium, sulfur, calcium, magnesium, aluminum, sodium and copper, as well as a potential cation exchange capacity and a lower number of exchangeable cations than manure-based biochars, with an intermediate leaf biochar. The differences in biochars generated by various crop residues have also led to different remedial effects on an acidic medium. In general, it was observed that the biochar from leguminous plants increased the pH and the calcinating effect on acidic soils due to its greater alkalinity than the biochar from non-leguminous plants [101].

6. Forest Waste As Feedstocks: Key Types and Characteristics of the Biochar Produced

Feedstocks derived from forest waste can include a wide range of organic materials, such as wood chips, bark, sawdust, pruning residues, twigs, fruits, leaves, etc. [102]. These materials undergo the pyrolysis process to produce biochar, a charred material with different characteristics and beneficial properties for the soil and the environment.

This section focuses on the in-depth analysis of the distinctive characteristics of biochar, highlighting the variations that these can undergo in relation to several factors. Among these, the type of forest waste used as raw material, the production process (pyrolysis temperature and residence time) and the analysis methodology used emerge as fundamental factors in the determination of physical properties-biochar chemicals, influencing the effectiveness and applicability of the product.

In the following sub-paragraphs, the authors examine in detail 10 selected studies, highlighting similarities and differences in the physico-chemical properties and elementary composition of biochars produced by different types of forest waste.

6.1. Physico-Chemical Characterization of Biochar Produced by Different Forest Waste

In

Table 2. Physico-chemical properties of biochar: type of forest waste and pyrolysis temperature. MT = macrotype, BF = biochar feedstock, Sp = species, PT = pyrolisis temperature (°C), PY = product yield (%), SSA = specific surface area (m²g⁻¹), VM = volatile matter (%), A = ash (%), CEC = cation exchange capacity (cmolkg⁻¹).

MT	BF	Specie	PT (°C)	PY (%)	pH	SSA (m2/g)	VM (%)	A (%)	CEC (cmol/kg)	References
Hardwood	Bark	Oak	450			1.88	22.8	11.09		[103]
Hardwood	Sawdust	Eucalyptus	350	42.5	5.9		36.9	0.9		[12]
Hardwood	Sawdust	Eucalyptus	450	36	8		28.5	0.7		[12]
Hardwood	Sawdust	Eucalyptus	750	28.2	9.7		6.5	1.1		[12]
Hardwood	Chip	White Poplar	300	53	9.1		16.76	32.94		[104]
Hardwood	Chip	White Poplar	400	29	6.1	3.75	27.9	39.45		[104]
Hardwood	Chip	White Poplar	500	31	9.5	8	24.84	43.04		[104]
Hardwood	Chip	White Poplar	600	26	7.5	120.31	21.74	50.93		[104]
Hardwood	Chip	White Poplar	700	22	10.9	228.5	13.67	52.17		[104]
Hardwood	Chip	White Poplar	800	20	9.9	137.9	15.68	26.8		[104]
Hardwood	Wood	Hardwood	450		5.6	0,4		38.6		[105]
Hardwood	Wood	Oak	400			2.73	15.6	2.92		[103]
Hardwood	Wood	Mulberry	350	37.5	10.2	16.6		7.5	23.3	[106]
Hardwood	Wood	Mulberry	450	32.7	11.1	31.5		7.7	22.1	[106]
Hardwood	Wood	Mulberry	550	26.2	10.6	58		9.8	19	[106]
Hardwood	Wood	Mulberry	650	22.8	10.6	24.5		9.8	21.8	[106]
Softwood	Bark	Pine	350	59.6	7.8		38.5	8.3		[12]
Softwood	Bark	Pine	450	49.3	8.3		29.3	7.9		[12]
Softwood	Bark	Pine	750	38.9	9.9		6	14.5		[12]
Softwood	Chip	Pine	500			6.2	22.4	2.6		[99]
Softwood	Needles	Pine	450	40.96	9.64	5.76				[107]
Softwood	Pinecones	Pine	350	49			76.9	12.1	24.8	[108]
Softwood	Pinecones	Pine	450	39				15.1	22.7	[108]
Softwood	Pinecones	Pine	550	28				20.5	21.8	[108]
Softwood	Sawdust	Pine	350		5.75 ± 0.02	3.39 ± 0.79			56.13	[109]
Softwood	Sawdust	Pine	450		6.31 ± 0.04	179.77 ± 2.35			52.43	[109]
Softwood	Sawdust	Pine	550		6.66 ± 0.08	431.91 ± 5.46			47.43	[109]
Softwood	Sawdust	Pine	650		6.84 ± 0.03	443.79 ± 0.98			39.22	[109]
	Bark		400		8.9				23	[110]
	Forest residue		350	38			84.5	1.4	24.2	[108]
	Forest residue		450	30				1.6	19.5	[108]
	Forest residue		550	23				2.3	15.2	[108]

below, the physico-chemical properties of the different biochars present in the articles examined are summarized. In particular, the physico-chemical characteristics have been categorized according to the macrotype (MT) of the forest species used as waste raw material and in more detail on the specific type of waste used as raw material to produce biochar (BF) and the process temperature (pyrolysis temperature—PT). The characteristics and physico-chemical properties examined are the percentage yield of the product (PY), pH, specific surface area (SSA), volitive substances (VM), ash (Ash), and cation exchange capacity (CEC).

Table 2 shows, in general, that the pyrolysis temperature shows a strong correlation with changes in the structure and physico-chemical properties of biochar [111,112,113,114]. In particular, a higher pyrolysis temperature tends to increase the surface area, carbonized fractions, pH and volatile matter, while decreasing the cation exchange capacity (CEC).

Residence time, in addition to the pyrolysis temperature, is a crucial variable in the process. The analysis of the studies presented in Table 2 reveals a considerable variation in the time of residence of the material, ranging from less than 5 s [105] to 4 h [106]. This variability makes it difficult to compare the physico-chemical characteristics of the examined biochars [115]. However, most of the studies considered in Table 1 are in a residence period of between 10 and 40 min.

It is, however, essential to note that not only the decomposition process has effects on the characteristics and yield of the biochar, but that precise variations depend on the raw material used and the species. Then, we will discuss the nuances of change between the different forest waste used raw materials to produce biochar [116].

6.1.1. Specific Surface Area

During the biomass pyrolysis process, water losses due to dehydration and the release of volatile components from the carbon matrix contribute to the formation of biochar pores [117]. Pore size distribution is a significant parameter in characterizing the structural heterogeneity of biochar pores. The value of the surface area is strongly associated with its porosity, of which micropores make the greatest contribution [118]. There are several methods for the analysis of the surface area of biochar, but both the European Biochar Certificate and the International Biochar Initiative recommend the Brunauer–Emmett–Teller method (BET) to analyze the nitrogen adsorption isotherm [119].

All 10 studies examined use BET analysis for the evaluation of SSA (m²g⁻¹). As a general rule, it is observed that the specific surface area (SSA, expressed in m²g⁻¹) tends to increase with the increase in temperature (°C) during the pyrolysis process. This phenomenon constitutes a significant variable for the specific surface of the biochar produced, with the same methodology of analysis. However, comparing some different forest waste used in the studies examined, it can be noted that the variability of detail also depends on the size of the starting raw material.

To investigate further, the authors elaborated Figure 1, in which three different types of waste were compared: chips, sawdust and wood. As already underlined, in general, the specific surface area tends to increase with the increase in temperature; however, it emerges that for wood, the increase is almost imperceptible, while for sawdust (sawdust), the increase almost follows a logarithmic trend. Pariyar et al. [109], in their study on sawdust, also show that the increase in the pyrolysis temperature from 350 °C to 450 °C produced an increase of 50 times the SSA.

It can therefore be observed that SSA depends both on the biochar production process and on the analytical methodology used. However, it is equally important to recognize that the type of raw material used has a significant impact on the variability of the specific surface.

6.1.2. Biochar Yield, Volatile Matter and Ash

Biochar yield, as observed in the 10 studies considered, is calculated through equations that generally include biomass flows (e.g., initial biomass of raw material, dried biomass and biochar mass).

In general, it is observed that the yield of biochar PY (%) tends to decrease with increasing temperature. This phenomenon can be explained by the fact that, as the temperature increases, a greater production of volatile substances occurs due to the decomposition of the raw material.

As indicated by [108] and reported in Figure 2, the percentage yield of two biochars produced, respectively, by pinecones (a) and forest residues (b) at three different temperatures (350 °C, 450 °C and 550 °C) showed a similar trend although the biochars were produced by different feedstocks. This was confirmed by a comparative analysis of all the studies considered. The decrease in biochar yield is constant even considering different types of biomass depending on temperature and related elements. This suggests that the predominant variable is the process adopted (pyrolysis temperature, residence time, heating rate).

For determining volatile matter (VM) and ash content (Ash), the biochar was again subjected to a decomposition process at temperatures ranging from 550 °C to 750 °C for a maintenance time of 4–6 min; the estimation of the percentage value was obtained through equations referring to mass fractions or through approximate analyses using ASTM D2866-11 (American Society for Testing and Materials) for the ash content and ASTM D5832-98 for the content of volatile substances, as indicated by [109].

In general, both volatile substances and ash content decrease with increasing temperature, but it is not clear whether this depends more on the process adopted, the analysis methodology or the type of feedstock.

Enders et al. [120] confirmed that biomass type has less impact on the volatile substances of biochar than pyrolysis temperature, although woody materials resulted in biochars with greater variation in volatile matter than biochar produced from manure.

However, current data from [109] indicate a change in the content of volatile biochar substances by pyrolysis temperature, even when it is the same biomass. This shows that the pyrolysis temperature is, however, a significant variable in determining the content of volatile substances and ash.

6.1.3. pH and Cation Exchange Capacity

The pH and cation exchange capacity (CEC) are considered fundamental chemical characteristics for biochar characterization. These properties are important because they affect the interactions of biochar with soil, water and nutrients. However, from the data collected, it is not possible to identify with certainty the behavior of these two properties in relation to the production process or the type of biomass used as feedstock.

6.2. Elemental Composition and Carbon Content

The elemental characterization of biochar provides crucial information on the chemical composition of the pyrolyzed material, characterizing the percentage mass fraction of carbon (C), hydrogen (H), nitrogen (N), oxygen (O), and sulfur (S). On the basis of the ten studies examined in the literature,

Table 3. Elementary biochar analysis: pyrolysis temperature and effects on the type of raw material used. MT = macrotype, BF = biochar feedstock, Sp = species, PT = pyrolisis temperature (°C), C = carbon content (%), H = hydrogen content (%), N = nitrogen content (%), S = sulfur content (%), O = oxygen content (%), H/C ratio and O/C ratio.

MT	BF	Specie	PT (°C)	C (%)	H (%)	N (%)	S (%)	O (%)	H/C	O/C	References
Hardwood	Bark	Oak	450	71.25	2.63	0.46	0.02	12.99			[103]
Hardwood	Sawdust	Eucalyptus	350	70.4	3.81		0.02	24	0.65	0.26	[12]
Hardwood	Sawdust	Eucalyptus	450	78.6	3.42		0.01	16.6	0.52	0.16	[12]
Hardwood	Sawdust	Eucalyptus	750	90.9	1.52		0.04	5.6	0.20	0.05	[12]
Hardwood	Chip	White Poplar	300	51.75	3.91	2.58	41.76	1.21	1.21	0.48	[104]
Hardwood	Chip	White Poplar	400	50.31	3.09	2.67		43.93	0.93	0.39	[104]
Hardwood	Chip	White Poplar	500	56.89	2.86	2.82		37.43	0.82	0.39	[104]
Hardwood	Chip	White Poplar	600	55.21	1.96	2.26		40.57	0.29	0.34	[104]
Hardwood	Chip	White Poplar	700	50.44	1.51	2.07		45.97	0.25	0.45	[104]
Hardwood	Chip	White Poplar	800	43.83	2.9	2.17		51.1	0.26	0.42	[104]
Hardwood	Wood	Hardwood	450	53.4	2.3	0.07		5.7	0.52	0.08	[105]
Hardwood	Wood	Oak	450	82.83	2.7	0.31	0.02	8.05			[103]
Hardwood	Wood	Mulberry	350	67.9	4.53	2.16	0.20	25.2	0,80	0,28	[106]
Hardwood	Wood	Mulberry	450	70.8	3.32	1.92	0.15	23.8	0.56	0.25	[106]
Hardwood	Wood	Mulberry	550	77	2.41	1.68	0.15	18.8	0.38	0.18	[106]
Hardwood	Wood	Mulberry	650	80.1	1.63	1.58	0.13	16.6	0.24	0.15	[106]
Softwood	Bark	Pine	350	67.6	3.73		0.01	28.7	0.66	0.32	[12]
Softwood	Bark	Pine	450	75.2	2.74		0.02	24.7	0.44	0.25	[12]
Softwood	Bark	Pine	750	86.3	1.16		0.04	19.1	0.16	0.17	[12]
Softwood	Chip	Pine	500	88.9	3.14	0.45	0.002	4.97	0.43	0.04	[99]
Softwood	Needles	Pine	450	39.58		19.93		22.02			[107]
Softwood	Pinecones	Pine	350	62.7	4.9	1		19.2			[108]
Softwood	Pinecones	Pine	450	62.9	3.2	1		17.8			[108]
Softwood	Pinecones	Pine	550	63.4	1.9	1		13.2			[108]
Softwood	Sawdust	Pine	350	52.28	5.17	0.15	0.00	30.5	1.19	0.44	[109]
Softwood	Sawdust	Pine	450	58.2	4.23	0.16	0.00	25.11	0.87	0.32	[109]
Softwood	Sawdust	Pine	550	59.19	3.97	0.51	0.00	20.73	0.80	0.26	[109]
Softwood	Sawdust	Pine	650	62.87	3.44	0.18	0.00	12	0.66	0.14	[109]
	Bark		400	80		0.5					[110]
	Forest residue		350	68.4	4			26.2			[108]
	Forest residue		450	62.7	2.9			32.8			[108]
	Forest residue		550	56.5	2.1			39.1			[108]

summarizes the elementary biochar analysis.

It is important to note that, as regards carbon, reference is made to the percentage of total carbon (C_tot), as this was the only figure available in the literature. However, both IBI and EBC use the fraction of organic carbon (C_org) in their qualification standards.

Fixed carbon (C in %) is the mass fraction of solid carbon in biomass that remains in the char in the pyrolysis process after devolatilization. It is generally determined by equations [12,109] that consider the volatile substance (VM), the ash content (Ash) and the percentage of moisture (M) or determined using standard analysis (e.g., Flash EA 1112, Thermo Finnigan—[105]; ASTM D 3172 and 3176—[99]; energy-dispersive X-ray spectroscopy analysis—[107]).

In general, it is observed that the mass fraction of carbon increases with increasing temperature for all types of forest waste examined.

However, in [104], pyrolyzing white poplar chips from 300 °C to 800 °C demonstrated that the carbon fraction increases to the temperature of 500/600 °C and then looks similar (if not slightly lower) to that of the initial biomass when reaching 700/800 °C.

This finding is not confirmed by [12], who, pyrolyzing at 750 °C pine bark and eucalyptus sawdust, observes an increase in carbon content (%). Given the limited size of the sample considered, these results indicate that the carbon content in the biochar varies based on both the production process and the type of raw material used.

As for hydrogen (H%), the content decreases with increasing temperature in all types of feedstocks used. However, for the other elements (nitrogen and sulfur), it is not possible to identify with certainty how the production process or the type of biomass used as feedstock influence the amount.

According to [106], the content of hydrogen and oxygen with associated atomic ratios (H/C, O/C) decreased with temperature rise for all biochars. This is confirmed by [109], who state that the atomic ratios of hydrogen–oxygen H/C and oxygen–carbon O/C undergo a sharp decrease as the pyrolysis temperature increases.

7. Conclusions

This review aims to investigate the impact of the use of biochar produced by forest waste by examining effects in the agricultural system, in the forest ecosystem and, more generally, in the environment. In the first case, the application of biochar produced from forest waste improves agricultural productivity, the ability to respond effectively to environmental management and inputs, as well as resistance to biotic and abiotic stresses. In the second case, woody biomass used to produce energy and renewable products represents the most important contribution that the forest ecosystem can provide to today’s climate and environmental challenges. The use of forest waste to produce biochar offers a dual opportunity: on the one hand, it provides a sustainable, abundant and low-cost biomass source for the production of biochar; on the other hand, the removal of biomass from forest stands promotes the reduction in the risk of forest fires and therefore has a positive impact on the emission of pollutants into the atmosphere.

The authors investigated which variable was most effective in defining the physico-chemical characteristics of biochar and, based on the scientific literature cited, the most critical variable is the process (pyrolysis temperature, residence time and heating rate). However, the type of biomass used and the forest species used have a significant impact on determining the characteristics of the final product. Furthermore, the authors also noted that there are more studies in the literature using pine as the main waste material than other forest species.

Further studies should be conducted to understand whether pine is used mainly because it is the most available material, or whether it lends itself better than others to being transformed into biochar. In this case, it would be interesting to understand which type of waste (e.g., bark, strobili, wood, needles) has better characteristics after the pyrolysis process.

In addition, it would be interesting to deepen the life-cycle assessment (LCA) of the biochar produced by forest waste, analyzing the environmental impact of production and the long-term implications on soil and forest health. Further scientific studies should be conducted in this regard.