Effects of Hydrothermal Carbonization Conditions on the Characteristics of Hydrochar and Its Application as a Soil Amendment: A Review

Abstract

Hydrochar (HC) has received increasing attention due to its potential impact on soil amendment. The process parameters of hydrothermal carbonization (HTC) have a significant impact on the characterization of HC, and HC as a soil amendment has effects on soil properties. Therefore, this work summarizes the effects of feedstock type, temperature, residence time, and solid–liquid ratio on the characteristics of HC, and analyzes the effects of HC on soil by HC addition. The feedstock type determined the characteristics of the HC. Temperature had the greatest effect on HC properties, while residence time had a similar but smaller effect than temperature had. The residence time did not affect the characteristics of the HC when HTC was carried out to a certain extent. Solid–liquid ratio determined the characteristics of HC, but research in this area is lacking. Moisture is a crucial factor in plant growth. Compared to other soils, HC significantly increased the water-holding capacity (WHC) in sandy soils. HC is rich in nutrients and micronutrients, making it potentially useful as a fertilizer. The toxicity of HC may inhibit plant growth but may be ignored for poor soil. This review aims to provide recommendations for HC as a soil amendment and to identify areas where further research is needed.

1. Introduction

In the last decade, HC has received increasing attention for its ability to be used as a soil amendment to improve soil quality and promote plant growth [1,2,3], as illustrated in Figure 1. HC is an environmentally friendly and cost-effective carbon material [4]. The application of HC to soil completes the circular nutrient cycle by returning nutrients from renewable and low-cost biomass to the earth [5]. HC contains major elements such as C, N, P, and other minerals that can increase soil fertility and promote carbon sequestration [6]. HC offers significant advantages in agriculture and the soil health by directly and indirectly providing essential nutrients and enhancing soil physical-chemical properties and microbial communities [7]. Additionally, HC has lower GHG emissions compared to other traditional methods, such as composting and anaerobic digestion [8], and can remediate polluted water and soil through adsorption and degradation [9]. The production of HC from waste biomass as a soil amendment is a promising and sustainable strategy for resource recovery and soil improvement [10]. Although HC has been used as a soil amendment, its effects are not yet fully understood.

HTC, produced HC, is a type of hydrothermal reaction using biomass as the feedstock and water as the solvent and reaction medium [11]. The elevated temperatures (180–250 °C) and pressures employed in the HTC process have been shown to facilitate the ionization of water, thereby forming acidic hydronium ions (H₃O⁺) and hydroxide ions (OH⁻). The role of H₃O⁺ in the HTC process is of particular significance, as it promotes the saturation of organic material, enhances the cleavage of hydrogen bonds, and inhibits radical condensation [12]. During HTC, biomass first hydrolyzes into intermediates, which is followed by dehydration and decarboxylation, and finally, produces HC due to polymerization and condensation. At the same time, part of the biomass undergoes dehydration and decarboxylation to form fragments. The fragments are condensed and re-polymerized to grow on the outer side and form HC [13]. During the growth process, various functional groups attach to the surface of the HC, resulting in significant changes in HC surface properties. There are two primary types of functional groups: oxygenated acidic functional groups and alkaline functional groups [14]. The strength of HTC increases the number of surface functional groups of HC. However, after reaching a certain level, it actually removes the surface functional groups.

The characteristics of HC are determined by the process parameters of HTC, particularly feedstock type, temperature, residence time, and solid–liquid ratio. Changing the reaction conditions of HTC could alter the morphological structure, physical-chemical properties, nutrient form, and effectiveness of HC [15,16]. HTC has the ability to provide suitable conditions for many reactions that cannot occur under normal conditions [17]. By controlling reaction conditions such as temperature, residence time, and solid–liquid ratio, it is possible to regulate HC composition, morphology, size, and other characteristics [18].

The pH of HC is milder than that of feedstocks, and the solubility of organic components is enhanced. HC has a larger specific surface area (SSA) and richer surface functional groups than feedstock [19,20]. SSA is a crucial factor in determining the impact of HC on soil organic levels [21]. HC contains O- and N-containing functional groups that can enhance soil carbon sequestration and cation exchange capacity (CEC), and prevent nutrient loss [22]. HC has hydrophilic characteristics due to the polar functional groups and well-developed porosity on the surface of the HC, which is expected to enhance the nutrient exchange between the HC and the soil [23]. The diverse characteristics of HC offer many possibilities for soil improvement. HC as a soil amendment can increase the SSA of the soil and provide nutrients [24], and HC produced from biomass can be used as a fertilizer to promote plant growth and sustainably improve soil quality [25]. HC contains a significant amount of nutrients, which are released gradually, making it an ideal slow-release fertilizer [26].

However, the growth of plants is influenced by various factors, such as the type of HCs, soil, and plants, as well as the duration of HC retention in the soil. The modification of feedstock and HTC process parameters may affect the presence of hazardous chemicals on the surface of HC [27]. These substances include furfural, polycyclic aromatic hydrocarbons (PAHs), organic acids and phenols, polychlorinated dibenzodioxins, and dibenzofurans, which may pose a threat to plant and soil health [28]. The direct use of HC may adversely affect seed germination and plant growth.

Previous articles have summarized the application areas of HC, including adsorbents, carbon materials, fuels, and soil conditioners [29]; the HC skeleton, formation mechanism, and structural properties [30]; the effect of HC on soil properties [31]; nutrient migration and transformation during HTC [32]; and energy recovery rates [33]. However, there is a lack of review articles on the effects of HC feedstock and HTC process parameters on soil amendment and plant response. This review aimed to (1) determine the effects of HTC process parameters on HC characteristics; (2) analyze the effects of HC as a soil amendment on plants and soils; and (3) propose measures to mitigate the negative effects of HC on plants. The study results provide a foundation for the practical application of HC in agricultural production.

2. Process Parameters on HC Characteristics

2.1. Feedstocks

The type of feedstock contains different contents and structures of components, which determines the characterization of HC. The environmental benefit of HC depends on the feedstock type used, and therefore, the source of the feedstock must be carefully considered [34]. As shown in Table 1, biomass waste such as fruit peels [35], sludge [36,37,38], kitchen waste [39], livestock manure [6], and municipal waste [40] can be served as feedstock to prepare HC. Biomass waste is abundant, renewable, and cheaper.

HC contains high C content and a porous structure that can improve soil aeration and WHC. Chen et al. applied watermelon peel to produce HC and discovered that the C content of watermelon peel HC increased by 39–75%, while the H and O contents decreased by 6–11% and 30–71%, respectively, compared to the feedstock. As a result, the H/C and O/C ratios decreased by 36–49% and 49–83%, respectively [35]. The high degree of carbonation of HC improved its stability. Peanut straw HC has a hydrophilic functional group with a small aromatic structure and is easily soluble in aqueous solutions with its higher pore volume, pore size, and SSA [41]. Cow manure and pig manure have high ash and C content, making HC highly resistant to degradation and conducive to carbon sequestration and N fixation [42].

HC generally contains nutrients such as N, P, K, Ca, and Mg and can be used directly as a slow-release fertilizer for plants. The bagasse and wine pomace HC contains N, P, C, Mg, K, Ca, and other elements and organic functional groups, thus giving the HC great potential for soil quality improvement [43]. Manure-sourced HC not only acts as a soil conditioner but also has the properties of organic fertilizers and provides a significant boost to N and P in the soil, as well as reducing the risk of non-point source pollution [44,45]. The application of cow manure HC significantly increases soil organic carbon and dissolves organic carbon (DOC) contents. Furthermore, the use of cow manure HC has resulted in a higher grain yield compared to the direct return of cow manure to the field [46]. Positive correlations between sludge HC and organic carbon and total N confirm that HC can act as a slow-release fertilizer, directly providing organic matter and N compounds and improving soil nutrient efficiency [47]. Effectiveness measurements of P on maize and tomatoes have shown that sewage sludge HC is a good source of P [48].

Plant-based HC, with its low nutrient content, cannot be used directly as a fertilizer, but it improves the effectiveness of fertilizer by reducing the amount of fertilizer lost through surface runoff. Nutrients can also be adsorbed into the pores on the surface of HC and slowly released into the soil over time for plant uptake [49]. Manure HC has a higher nutrient content than plant-based HC [9]. Sewage sludge has gained attention due to its high N and P potential. However, it is a high-risk feedstock that may contain high concentrations of heavy metals and PAHs, requiring treatment before being added to the soil [28]. The addition of sewage sludge HC to soil promotes plant growth by slowly releasing nutrients [50].

Table 1

Feedstock and process parameters for the production of HC.

Feedstock	Temperature (°C)	Time (h)	Solid-Liquid Ratio	Reference
Microcrystalline cellulose, hemicellulose lignin	225, 245, 265	20	1:3	[51]
Microcrystalline cellulose	220, 260, 300, 340, 380, 420	0.5	1:10	[13]
Cabbage slurry	190, 260	1, 6, 12	Self	[52]
Corn straw	240	2	1:1, 1:2, 1:3, 1:4,1:5, 1:10	[53]
Corn straw	160, 190, 220, 250, 280	3	1:5, 1:10, 1:15, 1:20, 1:25	[54]
Date palm leaflets	250	3	1:3	[55]
Garden, park waste	180	1	1:4	[27]
Maize silage feedstock	190, 210, 230	6	1:6	[56]
Orange peel	200	2, 4, 8, 12, 24	Self (60%)	[57]
Rice straw	200, 250, 300	2	1:10	[22]
Rice straw, corn straw, poplar wood, Enteromorpha	300	2	1:6	[21]
Straw	150, 300	0.5, 2, 8	1:30	[58]
Spent mushroom substrate	220	1	1:10	[59]
Sweet grasses, sedge, herb, rushes	180, 200, 220, 240	0.25, 1, 5, 12	1:2.5	[60]
Watermelon peel	190, 260	1, 6, 12	self	[35]
Wetland plant samples	200, 220, 240, 260	2	1:10	[61]
Wheat straw	260	2	1:10	[62]
Wheat straw	260	1	1:10	[63]
Cow manure	180, 260	1	1:10	[46]
Cow manure	200, 220, 240, 260, 280	4	1:20	[64]
Cow manure, corn stalk, Myriophyllum aquaticum	220	1	1:9	[10]
Cow manure, reed straw	260	2	1:10	[65]
Cow manure, reed straw	260	1	1:10	[66]
Cow manure, sesbania straw, reed straw	220	1	1:10	[67]
Cow manure, pig manure	180, 220, 260	1	1:10	[44,45,68]
Yak dung	150, 250, 300	0.5	1:11	[69]
Agro-industrial sludge	190	2	Self	[70]
Agro-industrial thickened digested sludge	190	1	1:20	[71]
Dairy Processing Sludge	160, 180, 200, 220	1	Self (80–90%)	[36]
Digestate sludge	200	3	self	[72,73]
Freshwater sludge	140, 160, 180, 200	4	1.5:20	[74]
Municipal sludge	220	1	Self	[37]
Paper sludge	200	2	Self	[75]
Paper sludge	180, 210, 240, 270, 300	0.5	1:15	[76]
Sewage sludge	190	4	self	[50]
Sewage sludge	120, 150, 180, 210, 240, 270, 300	0.5, 1, 2, 3	Self (85)	[16]
Waste activated sludge	250	2	Self (85–95%)	[47]
Waste activated sludge	200, 250, 300	0.5,1, 2	1:10	[48]
Cattle paunch waste	150, 180, 200	1, 4	1:4, 1:9	[77]
Grape pomace	175, 200, 225, 250, 275	0.16,0.5, 1	1:4	[78]
Grape pomace	180, 220, 250	0.5, 1, 3, 8	1:1	[79]
Kitchen waste (cellulose, skeleton, protein, and starch)	180, 220, 260	1	1:10	[39]
Vinasse and sugarcane bagasse	230	13	1:20	[43,80]

Note: The term ‘self’ refers to the hydrothermal reaction of moisture inherent in the reactants themselves, without the addition of water or other media.

2.2. HTC Parameters

As illustrated in Figure 2, the characteristics of HC depend on the temperature, residence time, and solid–liquid ratio of the HTC. It is important to note that all of the hydrothermal carbonization reactions discussed in this review were carried out under autogenous pressure, with no additional pressurization. As Petrović et al. have previously observed, the conversion of biomass to hydrochar is optimized by the application of pressure [81].

2.2.1. Temperature

Temperature plays a key role in determining the characteristics of HC, which is related to the potential application of HC as a soil conditioner.

Temperature changes affect HC yield, stability, nutrient availability, pore volume, pore size, and functional groups. Biomass is composed of a highly complex array of components, the majority of which consist of carbohydrates, lignocellulose, proteins, and lipids [82]. Cellulose is a complex carbohydrate composed of a multitude of glucose molecules that are interconnected via β-(1-4)-glycosidic linkages [83]. Hemicellulose is a complex, heterogeneous polymer composed of multiple C₅ and C₆ sugar monomers, includingxylose, glucose and galactose. It possesses a reduced resistance to both hydrothermal extraction and hydrolysis, and it can be efficiently extracted under elevated temperature and pressure conditions [84]. Lignin constitutes an essential component of plant cell walls, and its highly complex polymeric structure is more irregular than that of hemicellulose [85]. Wu et al. prepared HC at different reaction temperatures (200–280 °C). It was found that the maximum yield of HC was 58.34% at 200 °C [64]. The increase in temperature generally decreased the yield [52], promoted the degradation and synergistic effect of the feedstock, and led to the secondary decomposition of HC [78,86]. Meanwhile, the reaction temperature had a significant effect on the surface morphology of HC. Thus, temperature affects the ingredients in the feedstock to varying degrees. Ghanim et al.’s study on the production of HC from rice straw at 150–300 °C showed that the cellulose and hemicellulose contents decreased with increasing HTC temperature, but the lignin content increased and then decreased [58]. The cellulose (in terms of glucose), hemicellulose, and lignin contents varied significantly at temperature (175 °C). Hemicellulose started to degrade at about 175 °C and was completely degraded at about 225 °C, whereas the cellulose content was completely degraded at temperatures above 250 °C [87]. The cellulose content of the HC was also found to decrease. For all residence times, the lignin content of HC increased as the treatment temperature increased from 150 °C to 250 °C [68]. This could be due to the high thermal stability of lignin and the removal of hemicellulose and cellulose fractions. However, at higher treatment temperatures, there was a slight decrease in lignin content, possibly due to lignin degradation.

During the HTC process, the C, N, and ash contents of HC increased while the H and O contents decreased. This is primarily caused by dehydration and decarboxylation reactions [56,88]. As the temperature of the HTC process increased, the biomass underwent further carbonization [89], which led to a decrease in O/C and H/C ratios and O-containing functional groups [51]. Additionally, while the content of C and ash increased, the pore size grew, and the stability of HC improved [13]. The dairy processing sludge HC produced at 190 °C could retain more available N, P, and K than those produced at a higher temperature (260 °C), and dairy processing sludge HC produced at low temperatures (160 or 180 °C) had a high content of available P [36]. At temperatures below 200 °C, HTC tends to produce N-containing HC, whereas temperatures above 200 °C tend to produce P-containing HC [33]. The wetland plant samples of HC showed an increase in the conversion of water-soluble P to more stable P fractions with increasing temperatures. The results showed that HTC reduces the risk of P leaching and provides a more persistent source of P to the soil [61].

Cabbage slurry HC produced at lower temperatures (190 °C) has a relatively high ion exchange capacity and a large amount of nutrients, making it suitable for plant nutrition [52]. At temperatures below 220 °C, HTC facilitates the decomposition of feedstocks and enhances the dissolution of substances in the sludge, resulting in the formation of surface pores. However, temperatures exceeding 220 °C negatively impact the basic morphology and structure of HC, thereby inhibiting the formation of porous structures [90]. The biowaste HC was produced in the temperature range of 150–200 °C to provide a porous surface structure and a high proportion of O-containing functional groups, which enables the HC to improve the WHC, nutrient retention capacity, and CEC of the soil [27]. Dehydration had a greater impact on WHC than decarboxylation. Charring biomass for a short time at higher temperatures or for a long time at relatively mild temperatures did not significantly affect WHC, because the dehydration was almost complete under these conditions, whereas more intense conditions mainly induced decarboxylation [60].

An increase in temperature generally results in a decrease in HC yield and an increase in C, N content, pore size, and ash content. This leads to a decrease in O/C and H/C, which enhances the stability and adsorption capacity of HC. However, excessively intensive HTC would lead to a decrease in HC functional groups and nutrient content, making it unsuitable for soil amendment application.

2.2.2. Residence Times

Residence time is considered one of the most significant factors affecting the characteristics of HTC. In general, the longer the residence time, the higher the reaction completion.

The effect of residence time on HC yield is similar but smaller than that of temperature [91]. Increasing the residence time at a constant temperature in the HTC results in a decrease in the yield [92]. Chen et al. prepared HC from watermelon peel by varying the residence time. They found that the yield of watermelon peel HC and its elemental content depended largely on the reaction temperature rather than the residence time [35]. Gao et al. prepared microcrystalline cellulose HC at 250 °C and the results showed that longer residence time did not significantly affect the yield of HC. However, as the residence time of HTC increased, the yield of HC from rice straw and pig manure gradually decreased, while the ash content increased [93]. When the reaction conditions exceeded 250 °C and the reaction time was 4 h, the HC yield and ash content stabilized. This could be attributed to the fact that HC was essentially fully hydrolyzed after 4 h of reaction at 250 °C so that the effect of increasing the reaction time on the yield and ash content decreased [94].

An increase in residence time caused the biomass to generate a small number of hydroxyl groups, which dehydrated the cellulose and promoted the formation of organic acids [93]. Generally, the number of O-containing functional groups (such as -OH and -COOH) on the surface decreases as the residence time increases. This results in a decrease in volatiles, which makes the HC highly hydrophobic [33]. The condensation polymerization of N-containing heterocycles was facilitated by extending the residence time to 2–4 h. This allowed the migration of N from the liquid phase into the HC. It also favored the migration and transformation of P in the liquid phase into the HC [86].

At high HC temperatures, the effect of residence time on HC yield and functional group number is insignificant. Migration of nutrients from the liquid phase to the HC may be facilitated by prolonged residence time.

2.2.3. Solid–Liquid Ratio

The solid–liquid ratio is one of the factors that influences the characteristics of HC. According to previous studies, solid–liquid ratios are widely studied in the vicinity of 1:10. Si et al. prepared HC from corn straw by varying the solid–liquid ratio, and the results showed that when the solid–liquid ratio was reduced from 1:10 to 1:2, the total DOC of HC increased by 55.0%, while there was no significant change in elemental content, thermal stability, carbon sequestration potential, SSA, pore volume, and type of functional groups [53]. When the solid–liquid ratio was reduced to 1:1, the SSA and pore volume decreased significantly due to incomplete carbonation of HC. The influence of the solid–liquid ratio on HC yield was relatively modest, especially at shorter reaction times (1 h). However, the influence became more significant when the reaction time was increased to 4 h. Saverettiar et al. prepared HC from cattle paunch waste with similar HC yield at different solid–liquid ratios when the reaction time was 1 h, after 4 h of HTC. When the feedstocks had high solid–liquid ratios and were subjected to 4 h of HTC treatment, more solid products were obtained [77]. By reducing the solid–liquid ratio, the HC production is reduced [95]. The nutrient content in HC exhibits a trend of increasing and then decreasing as the solid–liquid ratio decreases. A decrease in the solid–liquid ratio leads to a more complete HTC [54]. Increasing the solid–liquid ratio can alleviate the adverse effects of increased reaction time and temperature on HC [89].

The effects of changing the solid–liquid ratio in the HTC process has been poorly studied. Altering the solid–liquid ratio can influence the yield, stability, elemental content, and functional groups of HC, and the target characteristics of HC can be obtained by altering the solid–liquid ratio.

3. Impact of HC on Soil

The response of plant growth effectiveness varied greatly depending on the characteristics of HC, soil conditions, and plant type [96]. Thus, the addition of HC has a positive or negative effect on plant germination and growth. As Figure 3 illustrates, the management of soil physical characteristics and chemical characteristics is essential to control soil properties.

3.1. WHC

Water supply is crucial for plant growth. Therefore, increasing WHC is the primary factor for the improvement of plant growth in coarse-grained soils such as grey and sandy soils [72]. Melo et al. discovered hydrophilic functional groups present on the surface of sewage sludge HC [50]. This suggests that amending the soil by adding sewage sludge HC can improve water utilization, reduce water stress on plants, and increase the capacity of the soil to retain rainfall, hence enhancing the WHC of the soil and increasing plant yield. Kalderis et al. found that adding orange peel HC to compacted clay soil resulted in improved physical properties. At a 5% addition rate, bulk density was significantly reduced, and aerated porosity and hydraulic conductivity were improved [57]. The addition of HC increases the porosity and reduces the weight capacity of various soils (clay soils, sandy soils, and loamy soils), promotes the formation and stabilization of soil aggregates, improves the WHC of the soil, and thus increases the effective water capacity of the plant [9]. Mau et al. found that adding the poultry litter HC as an amendment to sandy soil resulted in a reduction of soil bulk weight, an increase in soil porosity, and an increase in WHC. However, when HTC temperature was increased, the amendment effect of HC on soil WHC was diminished [97].

The addition of HC to soil reduces its bulk density and increases its total pore volume, allowing the soil to retain more water and achieve a high WHC. This is why WHC improvement was prominently observed in sandy soils with low WHC. The addition of HC to soil with high organic matter content resulted in no significant improvement in soil physical properties. The hydrophilic and hydrophobic properties of HC also play an essential role.

3.2. Nutrients

HC contains a significant amount of nutrients that can be used for plant growth [33]. HC can be used as organic mineral fertilizer, and appropriate application of HC (depending on soil requirements and tillage type) improves soil properties [98]. Hou et al. found that the addition of 5% and 15% HC to low-fertility soils significantly increased rice yield [99]. Sun et al. found that the addition of cow manure HC resulted in an 89.37% increase in peanut seedling shoot weight, a 73.66% increase in seedling fresh weight, and a 206.00% increase in total root length. The addition of reed straw and HC resulted in a 31.52% increase in seedling fresh weight [66]. Wang et al. found that the addition of 3% (w/w) of the three HCs (sesbania straw, reed straw, and cow manure) increased the total aboveground and fruit dry biomass of tomato by 12.4–49.5% and 48.6–165%, respectively. The addition of cow manure HC gave the greatest increase in aboveground and fruit dry biomass [67]. Increased soil phosphorus availability due to labile phosphorus supply from cow manure HC dominantly accounted for elevated tomato growth and fruit production. Similarly, vinasse and sugarcane bagasse HC, when introduced to the soil, resulted in increased root growth and above-ground biomass in early maize [80].

The increase in N and P in the soil by the addition of manure HC was significant [45]. The short-term benefits of plant nutrition may be enhanced by the presence of available N and P in HC [52]. HC contains both inorganic nitrogen, such as NH₄⁺ and NO₃⁻, and organic nitrogen, such as amino acids, phospholipids, and amino saccharides. These nutrients are derived from feedstocks such as sewage sludge, animal manure, and plant residues, and can supply nitrogen directly to plants [100]. NH₄⁺-N could be released from the HC into the soil and made available for plant growth [69]. HC improves the adsorption of NH₄⁺ and NO₃⁻ in the soil by electrostatic adsorption and pore filling. This results in a gradual release of N in the soil for plant uptake and reduces N leaching from the soil [9].

P is an essential element for the growth, metabolism, and reproduction of plants [48]. HTC significantly retained the P in the feedstock [90]. HTC converts mobile P to insoluble inorganic P, which balances nutrient uptake by plants and the nutrient retention capacity of the soil. The binding of micronutrients to PO₄³⁻ in HC promotes the long-term release of P nutrients in agriculture [86]. HC can be directly used as a slow-release P fertilizer material, hence boosting the available P resources for plant uptake by increasing the available P and total P in the soil when added [67]. As a result, after the addition of HC, there was an increase in total organic carbon, total N, total DOC, WHC, and microbial biomass in the soil, which persisted even after one year [101].

Different HCs have been found to contain considerable amounts of micronutrients, such as S, Ca, and Mg [102]; these minerals are mainly involved in photosynthesis, redox reactions, and other metabolic processes, so the stability of these minerals in soils and plants is considered to be very important. Exogenous organic matter and nutrients accumulate primarily at the interface between the soil and the HC, promoting plant growth [103].

CEC is a factor in the effectiveness of nutrient dynamics and microbial activity [72]. Thus, the CEC of soil increases with the addition of HC, which is rich in surface cation exchange sites [67]. Greater SSA and pore volume of HC typically result in greater microporosity and CEC, which facilitates nutrient adsorption and transport and provides more space for microbial community development, all of which increase soil amendment capacity [104].

The addition of HC reduces the mineral N content of soil to varying degrees and promotes microbial immobilization, particularly when high C/N ratio additives such as activated sludge or primary sludge are used [105]. Application of HC with a C/N ratio > 16 will reduce the effective N supply to the plant [48]. The decline in plant growth may be attributed to the adverse effects of HC on soil properties. These effects include an increase in soil C/N ratio, which leads to enhanced N fixation by microorganisms and reduced N uptake by plants [106].

HC contains high levels of elements that provide nutrients for plant and microbial growth. HC addition improves the CEC of the soil, which improves the ability of plants to utilize the nutrients in the soil and improves seed germination, biomass, and yield. Excessive C/N in the soil negatively affects plant growth but can be controlled by HC addition content.

3.3. pH

Effective soil amendment requires careful management and regulation of pH levels. If the pH level is either excessively high or low, this can lead to toxicity and limited nutrient availability. In consequence, pH was the dominant driving factor affecting inorganic N in soil–water systems. Manure HC regulates pH in acidic soils, which reduces NH3 loss from the soil–water system [44]. The presence of abundant acidic functional groups in HC not only provides organic matter for plant growth but also reduces or buffers alkali stresses during plant growth [5]. Jager and Giani found that the addition of 5%, 10%, and 20% digestate HC to three different soils (Chernozem, Podzol, and Gleysol) improved the soil pH, supplied nutrients (PO4-P and K), and stimulated microbial activity in three soils of variable character, therefore increaseing the germination and biomass of Chinese cabbage seeds [73].

3.4. Harmful Compounds and Improvements

The addition of HC can have adverse effects on plants, depending on the feedstock and preparation parameters of the HC. Luutu et al. found HC additions resulted in a 38% reduction in seed germination and a 10% reduction in biomass [107]. Yin et al. found the growth of lettuce was significantly inhibited by the addition of reed straw HC [65]. Islam et al. found the untreated chicken feather waste HC addition reduced the germination and biomass of Acacia auriculiformis seeds, showing phytotoxic effects [108]. Al-Wabel et al. found that date palm leaflets HC containing PAHs reduced seed germination and the above-ground length of lettuce [55]. A thorough toxicity evaluation of HC is strongly recommended before its use in soil to improve plant productivity. Periodic observation of the side effects of HC on plants with varying doses is necessary [92].

Organic contaminants such as phenols, furfural, and organic acids, which are water soluble and volatile, cause inhibition of seed germination and plant growth [15]. At high temperatures of the HTC process (260 °C), the presence of toxic residual compounds in the HC is the most likely potential source of phytotoxicity. Residual organic compounds on HC may include O-functionalized breakdown products, some of which have been associated with phytotoxicity [109]. Feng et al. found higher application rates of wheat straw HC may lead to yield decreases and N use efficiency reductions due to excess toxic dissolved organic compounds (such as 2,6-dimethoxyphenol) and the presence of certain aromatic compounds in HC, which interfere with photosynthesis in plants and may be the cause of a decline in rice yields. However, nutrient-poor soils can mask the phytotoxic effects of HC [63].

The application of HC in plant production has shown inconsistent results between plants, mainly due to the feedstock type, HTC process parameters, plant species, soil conditions, and complex interactions. The relationship between HC characteristics and the response of different plants, as well as the roles and mechanisms of HC in promoting plant growth and productivity, need to be further investigated. It is important to use appropriate doses of HC for specific plants in specific soils. HTC may produce organic compounds that are toxic to plants, and most studies using washing, aging, modification methods, co-composting, or co-hydrothermal carbonization (co-HTC) could eliminate or reduce toxicity to plants [28].

Water washing of HC eliminates adverse effects on plant growth and ensures plant production [110]. Washing has been shown to effectively remove volatile fatty acids, furans, amines, amides, pyridines, pyrazines, and benzoic compounds in HC [27]. Chen et al. found that the application of washed poplar sawdust HC significantly increased the N uptake and N use efficiency of rice seeds, straw, and leaves, and the increased N uptake may affect soil N retention and N cycling and reduce N₂O emission from rice soils [111].

Modification of HC by bio-aging has low environmental and economic costs and is a viable modification method [20]. This process can increase the nutrient content of HC, resulting in improved plant growth and productivity [112].

Xia et al. prepared HC by mixing sawdust and lime at different ratios (0%, 10%, 20%, and 30%). This process enhanced the surface functional groups of HC [113]. P-rich HC with unique properties has been obtained by using H₃PO₄ as the liquid medium in the biomass HTC process. P-rich HC has high aromaticity and excellent thermal stability. The high chemical functionality and aromaticity of P-rich HC gives it the potential to adsorb and stabilize a wide range of pollutants in soil and water [114].

The utilization of HC and HC co-composting for agro-industrial applications has been demonstrated to enhance biostability and efficiently produce high-quality HC fertilizer [115]. Although high doses of HC induced phytotoxicity, when mixed with compost and fast aerobically stabilized, its performance as an amendment on Lactuca sativa L. was higher than compost applied alone [70]. Roehrdanz et al. found the addition of a higher percentage of HC (up to 50% in this study) to the co-composting mixture was found to be effective in improving plant growth [15]. HC accelerates compost maturation, shortens composting time, and reduces the phytotoxicity of compost products [59]. From an agri-environmental perspective, HC is very similar to unprocessed digestate, and the composting process is a viable solution to some of the environmental problems encountered when hydrocarbons are used as soil amendments. These problems include phytotoxic organic compounds, nutrient losses, and greenhouse gas emissions from the HTC reaction [116].

Sun et al. prepared co-hydrochar (co-HC) through reed straw and cow manure and adding co-HC to the soil improved its physicalchemical properties and nutrient status. This addition also increased the abundance of beneficial microorganisms, including bacteria and fungi, which improved soil quality and the growth of peanut seedlings [66]. In co-HTC, the characteristics of the co-HC are influenced by the different decomposition properties and reactivity of each feedstock [117]. Ebrahimi et al. found that using sewage sludge and biomass for co-HTC can reduce the heavy metal content in co-HC due to the dilution effect of biomass and the leaching of biomass-derived organic acids, which contribute to the stability of co-HC [118]. Co-HTC effectively reduces the ash content of co-HC, improves N recovery, and increases the porous structure of the surface, making co-HC suitable for application as a fertilizer. Additionally, co-HC has a rich loose porous structure and O-containing functional groups on the surface, which improve soil quality through adsorption during the application process [86].

Different correction methods result in different HC characteristics and should be used rationally to achieve the HC of the target characteristics.

4. Future

Practical applications necessitate careful consideration of specific factors:

In addition to temperature, residence time, and solid–liquid ratio, which are process parameters discussed in this text, pressure is a postulated parameter that can be explored in future research to investigate the effect of pressure change on hydrothermal carbon properties.

It is imperative to acknowledge the liquid phase by-products of hydrothermal carbonization. Further investigations are requisite to characterize the HTC products generated after specific recirculation cycles that have been previously established, and to evaluate the potential benefits for the environment and economic savings.

Consistency of feedstock composition is important for HC production, as there may be variations in the characteristics of different batches during large-scale production. The economics of feedstock transport and storage and the environmental and supply sustainability of the feedstock also need to be assessed. In order to improve the performance of HC, further research is needed to determine the optimum parameters for HTC.

Different types of HC have different effects on the soil and provide different levels of nutrients to plants. Further field experiments are necessary to fully comprehend the practical applications of HC. Additionally, the effectiveness of HC in the remediation of complex pollution situations needs to be considered. To provide a valuable theoretical basis for the development and application of HC as a soil amendment, it is necessary to have a database of its different properties in soil improvement.

Potential environmental risks must be taken into account when using HC. The presence of toxic substances has a negative impact on the application of HC in soil amendment. To ensure accurate application of HC, the relationship between the performance of HC and the effect of soil improvement needs to be established through targeted research.

Combining HC with other soil remediation technologies, such as advanced irrigation, phytoremediation, and microbial and plant-microbe remediation, is a promising area for future research and application. The efficacy, stability, risks, and costs of each technology must be taken into account to ensure the practicality and effectiveness of this coupled application.

5. Conclusions

The characteristics of HC are influenced by the feedstock and process parameters. Plant HC is low in nutrients and can be used as a slow-release material, manure HC has a significant effect on plant growth, and sludge HC is high in N and P but has a potential risk of heavy metals. In HTC, temperature plays a decisive role in the characteristics of HC. The effect of residence time is similar to that of temperature, but when HTC reaches a certain level, residence time does not affect the characteristics of HC. The solid–liquid ratio has a significant effect on the characteristics of HC, but relevant studies are lacking in this area. The addition of HC as a soil amendment has a corresponding effect on plant and soil properties depending on the feedstock and process parameters of HC, soil type, and application rate. The addition of HC to soil has a positive effect on plants due to its ability to enhance soil WHC, CEC, nutrients, and micronutrients, as well as its ability to neutralize soil pH and improve soil physicalchemical properties. Increased WHC by HC is significant in sandy soils. The negative effects of HC on plants are mainly due to the organic acids and some other chemical compounds formed during the HTC, some of which may inhibit plant germination and growth. Physical and chemical modifications, such as washing, aging, modification methods, co-composting, and co-HTC, can eliminate or reduce the negative effects of HC on plants. Despite the demonstrated beneficial effects of HC on soil properties, the effects of the interaction of soil organic carbon, minerals, microorganisms, and other components with HC on soil properties are unclear. Further investigation is needed to establish the relationship between HC properties and soil properties due to the diversity and complexity of soil environments.

Acknowledgments

The authors are grateful to the anonymous reviewers for their suggestive comments on the manuscript.

Conflicts of Interest

The authors declare no conflicts of interest.

Footnotes

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Figures and Table

Figure 1. Discovering trends in “hydrochar” and “hydrochar & soil” research. (a) flowchart of data collection for research on hydrochar. Hydrochar has been published in 3733 papers between 2013 and 2024, and hydrochar & soil has been published in 1014. (b) The number of published documents on hydrochar and hydrochar & soil each year.

Figure 2. Physicalchemical properties of HC influenced by HTC parameters.

Figure 3. The addition of HC alters the physical and chemical properties of the soil.

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