Biochar carbon stability in four contrasting soils

Abstract

SummaryThere 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°C from a δ13C‐depleted (−36.4‰) 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 12 months was 0.30–1.14 and 0.97–2.71% from the soil‐biochar mixtures incubated at 20 and 40°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°C biochar was approximately twice the corresponding amount mineralized from the 550°C biochar systems. The influence of soil properties on biochar‐C mineralization was greater for the 450°C biochar than the 550°C biochar. The smallest proportion of C mineralized from the 450°C biochar occurred in the Inceptisol incubated at 20°C and in the Oxisol at 40°C. However, when expressed on a per unit of native SOC basis, the C mineralization of the 450 and 550°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 610 years. 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.