Effects of selected process conditions on the stability of hydrochar in low-carbon sandy soil

Abstract

Biochars have been found to enhance soil properties and to reduce atmospheric greenhouse gases due to their stable carbon fractions. It is known that stable carbon fractions of pyrolysis-derived biochars usually exhibit mean residence times (MRT) of at least several hundred years. However, only a few studies exist on the stability of hydrochars, which are produced by hydrothermal carbonization (HTC). This study examined the influence of two feedstock materials, straw digestate and poplar, and several processing and treatment parameters (carbonization temperature, washing of hydrochars and recirculating of process water) on the stability of hydrochars in a carbon-poor sandy soil. The results show that HTC leads to a product of variable carbon fractions with different stabilities as reflected in the different rates of CO2-C release from soil incubations within the first weeks. The carbon pool of the hydrochars 2 could be classified into a readily available and a fast-cycling decade-scale fraction. No slow-cycling centennial-scale carbon fraction was observed in this 120-day study. Moreover, a high reaction temperature and enhanced recirculation rate of process liquor lead to higher stability of the hydrochars. Based on the two-pool model, the more stable carbon had an MRT of 4-15 y depending on the reaction temperature and an MRT of 11-14 y for the recirculation of process liquor. The main hypothesis, that this short-term study of 120 days allows a reliable description of the long-term degradability of hydrochars, could not be confirmed.

Biochar persistence, priming and microbial responses to pyrolysis temperature series

“…However, once this labile pool was exhausted, hydrochars decomposed at a slower rate than mild pyrolysis products. Schulze et al (2016) also found hydrochars to have both a readily available C pool and a slower mineralizable pool. While hydrothermal carbonization of our feedstocks was found to transform hemicellulose completely and cellulose partially (Calucci et al 2013), cellulose is not affected at temperatures of 200-230°C during mild pyrolysis treatment .…”

Evaluating climate change mitigation potential of hydrochars: compounding insights from three different indicators

“…Thus, data on the content of recalcitrant and labile carbon pools and the respective mineralization kinetic parameters were only included if derived from biexponential models. Furthermore, we combined data from studies which quantified amounts of CO 2 using one of the following methods: alkaline solutions used as CO 2 traps combined with titration (Gajić et al ., ; Qayyum et al ., ), methods using incubation vessels combined with gas chromatography (Dicke et al ., ; Schulze et al ., ), methods using an automated gas analysis systems (Lanza et al ., ), methods basing on measurements of isotope signature of CO 2 (d13C‐CO 2 ) (Naisse et al ., ; Budai et al ., ), or methods measuring evolution of 13 CO 2 from 13‐C labeled carbon (Bai et al ., ). It was assumed that the evolved CO 2 is solely a result of hydrochar mineralization.…”

Hydrochar from sugarcane industry by-products: assessment of its potential use as a soil conditioner by germination and growth of maize

“…In view of the results found in the literature, positive results from the hydrochar application may still be expected for subsequent crops, or even after a longer period of contact of hydrochar with the soil, since a longer incubation time of the material in the soil would help the mineralization process of both carbon and nutrients as present [80,81].…”

“…Apart from the factors linked to the structural characteristics of the material, according to the literature, the hydrophobic properties observed by the addition of hydrochar may also be due to certain fungi that can develop under the hydrochar surface, causing the water repellence effect observed [19]. Studies have shown that the lifetime of hydrochar in soils is shorter than that observed for biochar [80], explaining that the mineralization process for hydrochar is faster due to characteristics that facilitate the development and action of microorganisms [80,81]. So, better results may be expected from application of hydrochar after longer interaction of hydrochar and soil, for instance harvest or in a subsequently cultivation, once the hydrochar degradation by mineralization process can release more nutrients present in the carbon material and also may reduce the water repellence observed [82][83][84].…”