Artificial humic substances activate a rapid anoxic microbial carbon pump

Organic carbon accumulation in anoxic environments is typically a slow process. Here, we test whether this rate is set by the organic input rather than by anoxia, using Artificial Humic Substances (AHS) — engineered substrates with labile and complex fractions. Amending anoxic sediment mesocosms with AHS raised surface organic carbon to 18.5 g L-1 within six weeks, against 0.4 g L-1 in unamended controls, retaining 73% of the added carbon, and 80 ± 5% in an independent three-column experiment. Less bioavailable amendments retained more, so AHS was the most extensively processed rather than the best preserved input. A low-diversity community dominated by Firmicutes and sulfate-reducing bacteria developed, the two chemically distinct amendments converged on nearly identical fulvic-acid spectra, and composition shifted toward more reduced formulae. Anoxic carbon accumulation therefore depends on the bioavailability of the organic input rather than on its recalcitrance, and substrate design can exploit this in controlled systems.