The introduction of hydrochar (HC) into the soil would have an immediate impact on soil physicochemical properties. Inherited pH from the hydrothermal conversion process would directly impact on the soil pH. In general, the pH of HTC slurry obtained from biomass is around 4.5 or below ^[1][95], while the ones from anaerobically digested sewage sludge generally have alkaline pH conditions ^[2][96]. Also, the proton-consuming activities of soil microorganisms might increase the pH by decreasing acidic metabolites ^[3][97]. Beneficial effects were shown mainly in degraded soils, for improving soil pH, cation exchange capacity (CEC, through high surface area and O-containing functional groups), electric conductivity (EC), and soil organic carbon (SOC) ^[4][5][71,98]. In general, it was reported that the hydrochar application improved soil porosity, decreased bulk density, and facilitated the formation of soil aggregates ^[6][99]. The porous structure of hydrochar and its hydrophilicity derived from oxygen-containing functional groups were beneficial in increasing the water retention capacity of the soil ^[7][100]. These beneficial effects are more evident in sandy soils than in clay and loamy soils ^[8][101]. In contrast, finer particles can block soil micropores and result in retarded water infiltration and retention ^[4][71]. In this context, the form of hydrochar application (e.g., post-processing methods: drying and pelletizing) and particle size may have a significant impact on the soil physical structure.

Due to easily degradable carbon, the addition of HC to soil usually leads to increased soil respiration, enzyme activity, and microbial abundance ^[9][102]. However, the presence of HC in the soil may lead to a significant change in the bacterial and archaeological community. While HC application generally increased the abundance and diversity of bacteria and fungi, in some cases, a decrease in bacterial abundance was found ^[6][99]. This effect could be attributed to the acidic nature of HC, favoring fungi rather than bacteria. Generally, hydrochar, with its rough and nutrient-rich surface, forms a suitable habitat for soil microorganisms and protects them from leaching and predators ^[6][99].

Both beneficial and detrimental effects on plant growth were observed from hydrochar application in soils. On the one hand, it could provide essential nutrients and improve soil properties. On the other hand, undesirable substances generated during hydrothermal conversion hampered seed germination and plant growth. In some cases, heavy metal contents in hydrochar inherited from its feedstock (e.g., sewage sludge and animal manure) need particular attention for its potential plant uptake and transfer through terrestrial food webs.

Nutrient contents in HC are mostly determined by its starting material and HTC conditions ^[10][103]. HC generally possesses plant nutrients, including N, P, K, Ca, and Mg. In some cases, hydrochar derived from manure, sewage sludge, and algal biomass is rich in phosphorus and/or nitrogen, and their potential application as slow fertilizer was reported ^{[6][11][12][13]}[99,104,105,106]. It was also suggested that hydrochar with low nutrient contents could still provide beneficial impacts when used with conventional fertilizer by reducing the amount of nutrient which is lost through surface run-off ^[14][15][107,108]. Also, nutrients available in subsoils can be adsorbed on the porous surfaces of hydrochar and be slowly released over time, which increases the efficiency of plant uptake ^[16][17][109,110]. Richness in surface functional groups of hydrochar would facilitate ammonium (NH₄⁺) and nitrate (NO₃⁻) retention in soils through electrostatic attraction and pore-filling ^[18][19][111,112]. In contrast, the supplement of hydrochar with a high C/N ratio could facilitate microbial N immobilization, which leads to inhibition in early plant growth ^[20][113].

The potential of hydrochar as the nutrient source would have more importance in recovering phosphorus, which is a finite resource which experiences depletion at an alarming rate ^[21][22][114,115]. Phosphorus in hydrochar derived from sewage sludge is predominantly associated with multivalent cations such as Fe and Al ^[23][24][116,117]. These compounds are considered as moderately labile pools in soil for plant uptake ^[17][110]. It was reported that the hydrochar can act as a direct P source and also as a reservoir through adsorption when excessive P is supplied (e.g., fertilizer application), which is readily available for plants ^[13][106].

Phytotoxic effects of hydrochar application are multi-variant phenomena consisting of the influence of soil properties, plant species, field settings, environmental conditions, and hydrochar characteristics ^[25][118]. Subsequently, researchers reported both positive and negative impacts of hydrochar application. Water-soluble phenols, furans, and organic acids could be the most representative by-products, resulting in acute phytotoxic effects ^[26][119]. The formation of these compounds can be attributed to the hydrothermal conversion of lignin, cellulose, and hemicellulose, which are the main constituents of plant-based feedstocks ^[27][120]. For the hydrochar materials derived from sewage sludge, the phytotoxic effects of other organic contaminants, including polychlorinated biphenyls, dioxins, and PAHs, as well as heavy metals, should be considered ^[26][119].

While some of the toxins in HC are organic substances built during the HTC process, heavy metals may be present in some input materials, like sewage sludge or other municipal organic waste ^[28][29][121,122]. Especially regarding the latter, caution is required; soil, as a finite and non-renewable resource, should not be additionally contaminated. To produce HC materials with more desired characteristics, the co-HTC of various feedstock blends could be an immediate solution ^[30][123]. Lang et al. (2018) ^[31][124] suggested that the co-HTC of swine manure and lignocellulosic biomass stabilized the heavy metal associated with hydrochar, decreasing the risk of heavy metal leaching. It was reported that the addition of corn cob into the hydrothermal treatment of swine manure (1-to-1 mass ratio) enhanced the nitrogen recovery and surface pore structure, which favors their application as fertilizers ^[32][125].

Washing: Water washing of hydrochar prior to soil application can remove the amount of labile C in hydrochar, resulting in less phytotoxicity. Busch et al. (2013) ^[33][126] reported that hot water (100 °C) effectively eliminated the detrimental effect of hydrochar and facilitated plant growth in a greenhouse setting. In the repetitive washing experiments, the number of washing cycles was shown to have significant improvement, while the duration of washing did not. Three-fold washing of 1 h at a mixing ratio of 1 to 30 (hydrochar to deionized water) was sufficient to remove labile carbon adsorbed on the hydrochar surface. However, washing with water was not effective at removing water-insoluble organics (e.g., high-molecular-weight PAH 98.8 mg/kg) and resulted in inhibited germination ^[34][127]. Also, it should be noted that the available essential plant nutrient was lost during the washing processes ^[35][128].

Aging: Several ageing techniques were investigated for their impacts on the changes in hydrochar properties and potential implications. Natural ageing of hydrochar could be the simplest solution for remediating the phytotoxic effects of hydrochar. In the pot tests performed on two subsequent barley cultivations, the detrimental effect of hydrochar application in the first round was not observed in the second cultivation round. The authors attributed this to the microbial degradation of phytotoxic agents which is supported by the high O/C and H/C ratios indicating the abundance of labile carbon compounds ^[11][104]. The idea corresponds with similar results obtained from another research on barley cultivation, which suggested that the harmful substances were degraded or water-leached during the ageing period ^[36][129]. Also, ageing in the air could be an appealing option. Puccini et al. (2018) ^[37][130] reported that storing pelletized hydrochar under a free air exchange chamber for four months was effective at remediating inhibited germination. The idea can be supported by the recent publication that reported rapid changes in the chemical properties of process water even in freezing temperatures ^[38][131].

Composting: Microbial degradation of phytotoxic chemicals can be accelerated by composting techniques. Co-composting of hydrochar, green waste and horse manure eliminated germination and plant growth inhibition within four weeks of a composting period without active aeration ^[33][126]. The beneficial effects of various co-composting blends were reported for green waste compost ^[39][132], organic fraction of municipal solid waste ^[40][133], and fresh compost (with high microbial activity—substrates not specified) ^[41][134]. Al-Naqeb et al. (2022) ^[42][135] tested the cytotoxicity of methanolic extracts of hydrochar made of municipal organic waste after anaerobic digestion. Untreated hydrochar was compared to composted hydrochar and compost. The results showed that untreated hydrochar has a higher cytotoxicity than the hydrochar co-compost, which is in the same range as standard compost cytotoxicity. The authors conclude that composting hydrochar is a good step to eliminate the cytotoxicity of hydrochar.

Similarly to other SOC components that are generally considered reactive (i.e., prone to degradation) ^[43][136], hydrochar introduced into soil can decompose through microbial activity, releasing CO₂ into the atmosphere. Detailed processes of degradation and stabilization remain incompletely understood ^[44][137]. The beneficial effects of hydrochar soil application in carbon sequestration would depend on its long-term persistence over decades to millennia. High carbon content, thermal stability, and recalcitrance could be favored properties for efficient carbon sequestration in the soil ^[45][46][138,139].

Several methodologies have been implemented to estimate the carbon sequestration potential of biochar materials: (i) ultimate analysis (focusing on H/C and O/C ratio), (ii) proximate analysis focusing on fixed carbon and volatile matter, (iii) thermal stability indices based on real-time analysis of gas and vapors during high-temperature pyrolysis (e.g., 900 °C) of test samples using the pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS) technique, and (iv) recalcitrance indices based on the fraction of carbon withstanding thermal oxidation measured by a combination of thermal gravimetric analysis and differential scanning gravimetry ^[46][139]. Among them, ultimate analysis becomes the most common approach, which provides an immediate estimate of the carbon sequestration potential. Higher contents of O and H in biochar (i.e., higher O/C and H/C ratio) indicate high aliphatic and less polyaromatic carbon content, which would lead to facilitated biotic degradation ^[47][48][140,141]. The majority of biochar materials (with exceptions for biochar from algal biomass and hydrochar with high H/C ratios) showed a close correlation between O/C and H/C ratios ^[49][50][142,143]. Based on the literature study, Spokas (2010) ^[51][144] coupled the O/C ratio with the expected half-life of biochar materials obtained through various laboratory studies on biochar degradation during its incubation in soil: an O/C ratio < 0.2 would indicate a half-life > 1000 years; an O/C ratio between 0.2 and 0.6 is for a half-life between 100 and 1000 years; a ratio greater than 0.6 is for a half-life < 100 years. Mostly, hydrochar showed intermediate to high O/C ratios ranging from 0.2 to > 1.7 ^[47][48][52][94,140,141].

However, this approach needs careful interpretation because most laboratory investigations are performed in idealized conditions, and other natural processes such as climate variability, infiltration, ozone and UV exposure, freeze–thaw cycling, or run-off are not considered ^[51][144]. It is advisable to perform field investigations over longer periods to obtain a more comprehensive understanding of hydrochar stability in a natural environment. To our knowledge, there is only limited research performed in field conditions. Lanza et al. (2018) ^[53][145] conducted a two-year field experiment to assess the degradability of biochar. Their findings estimated a half-life of 76–79 years for pyrochar and 49–61 years for hydrochar. In both cases, the degradation rate decreased over time, suggesting that more labile compounds are decomposed first, while the more stable fractions are more resistant to degradation. A similar result was reported by ^[54][146]. Around 1/3 of the initial hydrochar applied to a field lysimeter set-up was lost through leaching and decomposition within a year. However, the degradation rate of the remaining hydrochar slowed down significantly, having an estimated half-life of 19 years.

With respect to the priming effect of hydrochar application on the degradation of original soil organic matter (SOM), the results are contradictory. Short-term studies found positive priming effects indicating that hydrochar application leads to an increased degradation of SOM ^[55][56][57][27,147,148]. Malghani et al. (2015) ^[54][146] carried out a one-year field experiment and found a positive priming effect over the first three months, while the overall effect over the one-year period was negative. de Jager et al. (2022) ^[58][149] on the other hand, found a positive priming effect of hydrochar added to a podsol in a one-year experiment. The examination of the fate of hydrochar C showed interactions between particles of hydrochar with the original soil organic matter and indicated the possibility of incorporation into newly built aggregates.

Based on these results, it could be possible that priming effects may change over time. It has been shown that the degradation of hydrochar increases in the beginning when the less stable fractions decompose and slow down over time. Decomposition and soil respiration likely correlate with the abundance of soil microorganisms, which is affecting the degradation of SOM as well. Another factor is the effect of soil properties on the decomposition of hydrochar and SOM. The above-mentioned experiments were carried out on soils with different texture or pH. The meta-analysis of ^[59][150] for pyrochar showed that soil properties like the clay content affect the char decomposition as well as the priming effect.

Hydrochar has been suggested as a capable adsorbent in wastewater treatment processes based on its porous and reactive surface (i.e., rich in surface functional groups and polarity). Adsorptive removal of various contaminants was reported for heavy metals, dyes, pharmaceutical residues, endocrine-disrupting compounds, nitrates, phosphates, and sulphates ^[60][151]. It suggests the role of hydrochar in the soil as a contaminant barrier. Also, considering the fact that hydrochar is biodegradable and its recovery (i.e., separation) from the soil is unrealistic, more attention needs to be taken to the fate of non-biodegradable contaminants such as heavy metal and more recalcitrant organic contaminants retained in a soil–hydrochar matrix. They could be released into the soil and transported through plant uptake or infiltration into groundwater.

Isakovski et al. (2020) ^[61][152] investigated the immobilization and biodegradation of organophosphoric pesticides associated with hydrochar and pyrochar applied to river sediment. All carbonaceous materials slowed the migration of tested pesticides 4 to 18 times. Chlorpyrifos and chlorpyrifos-methyl were still being biodegraded. However, there was no visible degradation in the test for chlorfenvinphos. A similar result was observed from experiments targeting antibiotics (oxytetracycline). Hydrochar materials enhanced the microbial degradation of antibiotics and decreased their plant uptake ^[62][153]. The effect of biochar on organic contaminants varies and depends on the substance and its chemical structure. The selection of feedstock and HTC conditions affected the remediation performances of the soil–hydrochar mixture ^[61][152]. This would shed light on the application of hydrochar in the selective remediation or separation of organic contaminants. Also, it is of particular interest for the hydrochar derived from feedstock with potential heavy metal contamination (e.g., sewage sludge and animal manure). Yue et al. (2017) ^[63][154] observed an immediate and significant increase in heavy metal contents in soil amended by sewage sludge-derived hydrochar during 60 days of laboratory incubation. The authors suggested that the heavy metal contents embedded in the hydrochar were released as the hydrochar decomposed and were adsorbed by a soil matrix such as carbonates, iron oxides and clay minerals.

One of the most interesting and important phenomena in the soil–hydrochar matrix is the degradation of hydrochar. It is unavoidable, occurs over a long period spanning weeks to centuries at rates that change over time, and has various impacts, both in desirable and undesirable ways. As seen in the previous sections, degradation is accompanied by the release of nutrient contents embedded in hydrochar, which suggests its use as a slow-release fertilizer. Simultaneously, rapid decomposition limits its application for carbon sequestration and poses a potential risk of heavy metal contamination in the surrounding environment, particularly for the hydrochar derived from sewage-sludge, which generally has high contents of P and heavy metals.

In several research areas, it is stated that HC is composed of, at least, two different carbonaceous parts, which differ in their characteristics. Regarding HC degradation in soil, more labile carbon decomposes first at a significantly higher rate, and the recalcitrant carbon lasts for much longer periods. In a comparative investigation of conventional pyrochar and HC in field tests, a slowing down of the degradation rate was observed in both cases, but to a much smaller extent for pyrochar ^[53][145]. In water treatment research, the adsorption performance of hydrochar was improved by chemical activation, which removed the outer part. For instance, cold alkali washing of hydrochar (e.g., with 1 M KOH at room temperatures) was effective at removing the carbon layer deposited on the hydrochar surface and exposed the inner part, which provides improved surface areas with higher hydrophobicity ^[64][65][66][155,156,157].

More fundamental research on hydrothermal conversion processes has suggested the concept of primary and secondary char. Hydrochar is mainly composed of (i) primary char formed through solid-to-solid conversion of non-liquified remainders and (ii) secondary char generated through polymerization of dissolved organic substances through liquid-to-solid conversion which condenses on the surface of primary char ^[67][68][158,159]. The characterization of the secondary char is mainly based on its chemical extraction and subsequent analyses. Lucian et al. (2018) ^[68][159] provided important observations: (i) the secondary char is mainly comprised of organic acids, furfurals and phenols, which induce phytotoxic effects; (ii) the formation of the secondary char was most prominent at moderate reaction temperatures between 220 and 240 °C; at higher reaction temperatures, dissolved organics would be polymerized as the solid primary char; (iii) devolatilization rate of hydrochar was positively correlated with the higher secondary char contents, suggesting its responsibility for reactiveness. In contrast, Volpe and Fiori (2017) ^[69][160] reported higher thermal stability of secondary char based on its higher carbon content (lower atomic O/C ratio) than the primary char. Also, the methodological approach which identifies the secondary char as an extractable fraction would need careful consideration because, in harsh HTC conditions, it is likely that non-extractable primary char is generated through liquid–solid conversion processes. In-depth research on the hydrochar formation mechanisms would provide insights for developing more tailor-made hydrochar materials for its application in specific cases.

These results would have direct implications for soil–hydrochar interactions based on the strong correlation between thermal stability and biodegradability ^[70][161]. More comprehensive research under field-like conditions in the long term is essential. It is not only to obtain more information on hydrocar degradation in the soil, but also to examine the long-term effect of hydrochar on the soil under more realistic circumstances. Given that the soil–hydrochar interaction occurs in a much longer timespan than other reactions (e.g., combustion), the heterogeneous composition of hydrochar is of particular interest, resulting in responses in multi-stages as different char composites have different decomposition rates and characteristics. Because it is directly connected to climate change mitigation, a drastic surge in carbon permit prices could be a driving force for future research. The prices of EU carbon permits have risen from EUR 3.5 to 76.1 per ton of CO₂-eq ^[71][162].

Specific research topics would include the following:

• As primary and secondary char decompose at different rates, identifying spatial distribution (i.e., proportioning) of nutrients and heavy metals in hydrochar composite would provide fundamental knowledge that facilitates subsequent research streams in both hydrochar production and application. Finer tuning of hydrochar products could be a more tailor-made solution in a given context.
• The effects of hydrochar biodegradation on its adsorptive performance are not yet known. If the degradation of secondary char in the soil–hydrochar matrix occurs at a significantly higher rate than that of primary char, biodegradation would induce similar impacts as the chemical activation and provide a larger surface area. However, it has to be considered that the loss of rich surface functional groups (mainly O- and H-containing groups) would lead to an increase in hydrophobicity, resulting in a decrease in water holding capacity. Continuous monitoring of the hydrochar characteristics in the soil would provide crucial insights into long-term perspectives.
• The interaction between the soil–hydrochar matrix and other soil substances, such as chemical fertilizers and pesticides, would also change with the ageing of the soil–hydrochar matrix. This has to be examined in the long term. It might affect biogeochemical cycling and efficiency as well as the fate of pollutants.