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

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 (MRTs) 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 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 years depending on the reaction temperature and an MRT of 11-14 years 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.