Degradation of Miscanthus x gigantheus biochar, hydrochar and feedstock under the influence of disturbance events
Little is known about the degradation and environmental impacts of carbon (C) amendments such as hydrochar and biochar in soil under the influence of disturbance events such as wetting, freeze-thaw cycles, manual stirring, and glucose additions. Thus, we assessed the degradation and greenhouse gas (GHG) emissions of Miscanthus x giganteus biochar (from pyrolysis), hydrochar (from steam and water hydrothermal carbonization, HTCs and HTCw), the uncarbonized feedstock material in a sandy and a loamy soil, compared to a control, with four replicates per treatment. The C amendments were mixed with soil at a rate of ~620 t/ha wt% and incubated at 30° C over the period of 441 days. Over the whole incubation period, the soil mixtures were exposed consecutively to different disturbance events, with the intention to simulate a worst-case scenario for C degradation and GHG emissions. The degradation kinetics were quantified by source partitioning of the headspace 13C-CO2 and the application of an isotope two-component mixing model. Additionally, microbial biomass and composition were quantified and characterized at the end of the experimental period by chloroform fumigation extraction and phospholipid fatty acid analysis. The molecular composition and structural properties of the C amendments obtained by elemental analysis and NMR spectroscopy proved to be suitable indicators for the CO2 emissions which mostly followed the sequence feedstock > HTCs > HTCw > biochar over the experimental duration. The addition of glucose triggered a short-lived, emporary co-mineralization of the otherwise recalcitrant materials HTCw and biochar. Among all C amendments, biochar proved most recalcitrant against decomposition and disturbance in both soils with a calculated recovery rate of 95– 99% of the initially added biochar-C. Additionally, biochar amendment led to a decreased decomposition of soil organic C, especially in sandy soil.
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