*4.3. Soil Conditioning*

Char addition in soil has been widely recognized as an good practice to favor plant growth, enhancing soil properties in terms of available nutrients, soil porosity and water retention capacity [149–153]. In terms of plant growth, positive outcomes regarding roots nodulation and biomass production were reported in [154,155]. Moreover, different studies demonstrated also the positive effect of hydrochar on the retention of pesticides, allowing an optimized utilization and reducing the possible risk of leakage and contamination of ground waters [156–158]. The fast mineralization of the carbon contained in the char leads to a short-term release of nutrients in the soil, reducing the carbon content by 30–40% within the first 12–19 months [159–161].

The nature of the biomass used to produce the biochar, as well as the operating conditions, can significantly influence the final char composition and, therefore, the possible interaction with the soil [162,163]. A possible drawback of hydrochar addition can be represented by the release of organic compounds that can have toxic effects on soil. To minimize this effect, simple washing, aerobic or low temperature thermal treatment had been reported as further options to reduce the phytotoxic effect of pristine hydrochar [164,165]. Particular attention must be paid also when using HC from sewage sludges, due to the high content and release of heavy metal in the soil [166].

Tests to control ammonia volatilization and optimize nitrogen use for plant growth were proposed in [167–169]. Chu et al. tested the addition of algae-derived hydrochar to increase yield and mitigate N2O and NH3 emission [167]. Conversely to the main goal of the work, results showed a significant increase in grain yield up to approximately 25%, together with a rise in N2O emission and ammonia volatilization. The increased released of nitrogen compounds could be due to the low C/N ratio of the biomass and hydrochar, which could further prompt microbial activities, facilitating nitrogen gasification. Similar results related to emission of N-containing compounds were reported by Subedi et al. [169], as well as by Andert and Mumme [170] when adding pyro or hydro char into soil. On the same aspect, good results with high nitrogen retention, together with a significant decrease in ammonia volatilization was reported in [168] when using acid-modified hydrochar obtained from sewage sludge. Magnesium citrate or magnesium citrate with sulfuric acid were tested as potential additives in the reaction media for the HTC process. In the author's opinion, the acidity of the chars could reduce the ammonia volatilization, while the high porosity of the carbons can favor the adsorption of NH4 <sup>+</sup>. Moreover, the presence of surface functional groups rich in anion exchange sites can further reduce the nitrification rate, increasing N retention.

Base-modified hydrochars were also successfully tested on the removal of heavy metals like Lead (Pb) and Cadmium (Cd) [171,172]. In [171], authors evaluated the possible addition of 0, 10, 20 and 30% (*w*/*w*) of lime in the solution, before the HTC treatment. The obtained hydrochars were mixed with the contaminated soil in a proportion equal to 1, 2.5 and 5% by weight to evaluate the effect on leaching and contaminants immobilization. Results of the trials showed a remarkable increase in the removal of contaminants as long as the share of hydrochar was raised, achieving an improvement in the immobilization efficiency of 95.1% (Pb) and 64.4% (Cd), with respect to pristine hydrochar. Indeed, as long as hydrochar amount and lime content were raised, pH of soil shifted toward basic values, increasing electrostatic interaction between positively charged metals ions and the anions in hydrochar. Moreover, the formation of precipitates like metal (hydr)oxide or carbonate at high soil pH helped to further reduce the amount of Pb and Cd ions in the samples. Copper was also successfully removed from contaminated soil by Xia et al. [172], by using amino-functionalized hydrochar derived from pinewood sawdust. Authors reported an efficiency in Cu removal from the soil equal to 96.2%, together with a reduction of 98.1% in Cu amount in the leachate.
