Abstract
Decades of conflicting results have fueled a debate about how O 2 affects organic matter (OM) degradation and carbon cycling. In a laboratory study, using both OM taken directly from a humic lake and chemically isolated fulvic acid, we monitored the mineralization of dissolved OM in freshwater under purely oxic and anoxic conditions, under oxic then anoxic conditions, and under anoxic then oxic conditions, for 426 d. Between 5% and 24% of the initial OM was mineralized, with most extensive mineralization occurring under purely oxic and anoxic—oxic conditions. A sequential change in the O 2 regime did not result in greater overall degradation, but initially anoxic conditions favored subsequent oxic mineralization. A substantially greater fraction of the OM was degraded than in previous shorter studies, with as much as 50% of the total OM degradation occurring after 147 d into the experiment. Three fractions of the degradable OM were identified: OM degraded only under oxic conditions (68–78%), OM degraded more rapidly under anoxic conditions than under oxic conditions (16–18%), and OM degraded at equal rates under both oxic and anoxic conditions (6–14%). The degradation patterns of natural dissolved OM from a humic lake and chemically isolated fulvic acid were very similar, which indicates a similar level of bioavailability. The difference between anoxic and oxic degradation was greater in our long‐term studies than in previous short‐term experiments, which indicates that the oxic and anoxic degradation potentials vary with increasing overall OM recalcitrance and that similar oxic and anoxic degradation rates can be expected in short‐term experiments in which