Microcosm study on the decomposability of hydrochars in a Cambisol

The process of hydrothermal carbonization (HTC) converts biomass into a carbonaceous product named hydrochar. It is hypothesized that due to a high recalcitrance against microbial decomposition in soil, hydrochar may contribute to carbon (C) sequestration, thereby sustaining its function as a soil conditioner. The objective of this microcosm study was to identify process parameters of hydrochar production affecting the stability of hydrochar-C against decomposition, and thus its C sequestration potential. A variety of hydrochars differing in processing temperature (180-250 degrees C) and time (4 -12 h), and feedstock material (sugarbeet pulp, draft) as well as reference materials (wheat straw (WS), mature compost (MC), white peat (WP), sugarbeet pulp biochar (SB)) were applied to soil in a concentration equivalent to 30 t ha(-1) incorporated into 15 cm soil depth. After 248 days of incubation, C mineralized from the hydrochars ranged from 12 to 32%; it decreased considerably with increasing processing temperature from 200 to 250 degrees C, and less pronounced with increasing processing time from 4 to 12 h, whereas feedstock had no distinct effect. Higher processing temperature reduced oxygen content in hydrochar thus decreasing its reactivity, which resulted in both a higher amount and mean residence time of the stable hydrochar-C fraction. The mean residence times of tested organic materials followed the order: WS << hydrochars < WP <<< SB, MC. Thus, the application of hydrochar as a soil conditioner under field conditions may offer a moderate potential for C sequestration. A comprehensive evaluation of the complete HTC process chain including C and energy balances is prospectively required. (C) 2012 Elsevier Ltd. All rights reserved.