Biochar properties affecting carbon stability in soils contrasting in texture and mineralogy

A pot experiment involving growing three consecutive corn crops in two contrasting tropical soils-a coarse-textured, Al-rich Ultisol and a fine-textured, Mn-rich Oxisol-treated with two eucalyptus wood biochars-at low (350 °C) and high (800 °C) pyrolysis temperatures-at weight per weight rates of 0%, 1%, 2% and 4%, was conducted to assess their effects on the soil organic carbon (SOC) stability (soil C remaining relative to initial soil C) 144 d after biochar application (after the third crop harvest). The low temperature biochar had higher volatile matter but lower ash and fixed C contents than its high temperature counterpart. In the Ultisol, the SOC stability significantly increased with both biochars at up to the 2% rate but beyond which it did not further increase, whereas, in the Oxisol, the SOC stability significantly decreased at all rates of high temperature biochar and at the 2% and 4% rates of the low temperature biochar. Proposed mechanisms underlying these contrasting responses of the two soils involve their different buffering capacities and their mineralogy in relation to the Al in the Ultisol and the Mn in the Oxisol interacting with the different contents of the three main biochar constituents (volatile matter, ash and fixed C) of the two biochars.