Degradation kinetics of biochar from pyrolysis and hydrothermal carbonization in temperate soils

Estimates of biochar residence times in soils range over three orders of magnitude. We present the first direct comparison between the biodegradation of a char from hydrothermal carbonization (htcBC) and pyrolysis (pyrBC) with high temporal resolution. Mineralization of the biochars and their shared Miscanthus feedstock in three soils was determined directly by the (CO2)-C-13 efflux using a novel method incorporating wavelength scanned cavity ring-down spectroscopy. Biochar half-life (t(1/2)) was estimated with three empirical models. (1) The htcBC was readily biodegradable, whereas pyrBC was more recalcitrant. (2) Cumulative degradation of both biochars increased with soil organic carbon and nitrogen content. (3) The corrected Akaike information criterion (AIC(C)) showed an overall preference for the double exponential model (DEM) reflecting a labile and a recalcitrant C-pool, over the first-order degradation model (FODM) and a logarithmic model. (4) The DEM resulted in t(1/2) ranging from 19.7-44.5, 0.7-2.1 and 0.8-1.3 years for pyrBC, htcBC and feedstock, respectively. The degradation was rather similar between feedstock and htcBC but one order of magnitude slower for pyrBC. The AIC(C) preferred FODM in two cases, where the DEM parameters indicated no distinction between a labile and recalcitrant carbon pool.