Quantifying and correcting for pre-assay CO2 loss in short-term carbon mineralization assays

Abstract. The active fraction of soil organic carbon is an important component of soil health and often is
quickly assessed as the pulse of CO2 released by re-wetting dried soils in short-term
(24–72 h) assays. However, soils can lose carbon (C) as they dry and, if soil samples vary in moisture content at sampling, differential C loss during the pre-assay dry-down period may
complicate the assay's interpretations. We examined the impact of pre-assay CO2 loss in
a long-cultivated agricultural soil at initial moisture contents of 30 %, 50 %, and 70 %
water-filled pore space (WFPS). We found that 50 % and 70 % WFPS treatments lost more C during
drying than did those in the 30 % WFPS treatment and that dry-down losses led to a 26 %–32 % underestimate of their CO2 pulses. We developed a soil-specific
correction factor to account for these initial soil moisture effects. Future C mineralization
studies may benefit from similar corrections.