The soil carbon (C) saturation concept suggests an upper limit to the storage of soil organic carbon (SOC). Biochar has the capacity to protect new C, including rhizodeposits and microbial necromass. This study shows that the soil C storage ceiling of a Ferralsol under subtropical pasture was raised by a second application of *Eucalyptus saligna* biochar 8.2 years after the first application. High spatial heterogeneity of C functional groups resulted in the retention of rhizodeposits and microbial necromass in microaggregates and the mineral fraction. Microbial C-use efficiency was increased by lowering specific enzyme activities, decreasing native SOC mineralization by 18%. The study suggests that the SOC ceiling can be lifted using biochar in (sub)tropical grasslands.

Zhe (Han) Weng, Lukas Van Zwieten, Ehsan Tavakkoli, Michael T. Rose, Bhupinder Pal Singh, Stephen Joseph, Lynne M. Macdonald, Stephen Kimber, Stephen Morris, Terry J. Rose, Braulio S. Archanjo, Caixian Tang, Ashley E. Franks, Hui Diao, Steffen Schweizer, Mark J. Tobin, Annaleise R. Klein, Jitraporn Vongsvivut, Shery L. Y. Chang, Peter M. Kopittke, Annette Cowie