Biochar carbon stability in four contrasting soils

There is a limited understanding of the effects of soil properties on biochar carbon (C) stability. This knowledge is essential to evaluate the capacity of biochar for long-term soil C sequestration fully. In this study two biochars, produced by slow pyrolysis at 450 or 550 degrees C from a C-13-depleted (-36.4 parts per thousand) Eucalyptus saligna Sm. woody material, were incubated in four soils (Inceptisol, Entisol, Oxisol and Vertisol) of contrasting chemical and mineralogical properties. The total biochar-C mineralized over 12months was 0.30-1.14 and 0.97-2.71% from the soil-biochar mixtures incubated at 20 and 40 degrees C, respectively. The total biochar-C mineralized (mg CO2-C per unit of native soil organic C (SOC) basis) from soils incubated with the 450 degrees C biochar was approximately twice the corresponding amount mineralized from the 550 degrees C biochar systems. The influence of soil properties on biochar-C mineralization was greater for the 450 degrees C biochar than the 550 degrees C biochar. The smallest proportion of C mineralized from the 450 degrees C biochar occurred in the Inceptisol incubated at 20 degrees C and in the Oxisol at 40 degrees C. However, when expressed on a per unit of native SOC basis, the C mineralization of the 450 and 550 degrees C biochars was least in the Oxisol and greatest in the Inceptisol at both incubation temperatures. Mean residence times (MRTs) of the biochars estimated using the two-pool exponential model varied between 44 and 610years. The estimated MRT of the biochars may vary under field conditions depending upon the environmental conditions and addition of labile C from plants. Our results indicate that biochar-C was stabilized by variable charge minerals in the Oxisol and that the stabilization occurred rapidly at high temperatures.