Stability of Black Carbon/Biochar.

Both black carbon as an important pool of the global carbon cycles and biochar as a management tool to reduce emissions of greenhouse gases are relatively recalcitrant to decomposition and mineralization in the environment. Several key features of what I collectively call here BC have led to uncertainties about its mean resident time: (i) the high stability of BC poses obstacles to the quantification, and (ii) the variability of BC properties leads to variation in decomposition and mineralization rates. Quantification of mean residence times that for most BC types exceed 100 years, require a sufficiently long period of observation. Curve fitting to hypothetical BC decay data required at least 50 years of measurements to estimate the order of magnitude correctly. Since such time frames are typically beyond the capacity of most projects, and existing long-term experiments are still at early stages, other approaches to estimate the BC decay have to be explored. In addition to a limited number of opportunities where BC input or initial stocks can be estimated such as with archived soils or chronosequences or by modeling of long-term dynamics, new field or laboratory experiments are required. A better understanding of the processes of BC mineralization and calibrating short-term decomposition experiments with a limited number of long-term decay data may provide an approach to predicting BC stability over decadal and centennial time periods. Combining short-term experiments with long-term data is especially appropriate in the context of the variety of different BC that need to be studied. The proportion of aromatic carbon is very different for BC produced from different feedstocks and at different temperature, significantly affecting decomposition rates of new BC.