Oxygen (O2) limitation contributes to persistence of large carbon (C) stocks in saturated soils. However, many soils experience spatiotemporal O2 fluctuations impacted by climate and land-use change, and O2-mediated climate feedbacks from soil greenhouse gas emissions remain poorly constrained. Current theory and models posit that anoxia suppresses C decomposition irrespective of temporal scale, yet microbial community acclimation to periodic O2 deprivation could potentially mitigate the suppressive effects observed previously. We tested the impacts of short-term fluctuations between oxic and anoxic conditions on decomposition in two contrasting soils (a tropical forest Oxisol and temperate cropland Mollisol) that experience anoxia under field conditions. In the lab, soils were exposed to repeated cycles of 0, 2, 4, 8, or 12 days of anoxic conditions followed by 4 days of oxic conditions for more than one year. DNA was sampled after 0, 48, and 384 days for 16S sequencing. We used the Microbial-ENzyme Decomposition (MEND) model to predict C losses.
Results/Conclusions
Microbial communities subjected to periodic anoxia largely sustained decomposition relative to static oxic conditions, counter to model predictions. Even after 384 days, soils that were anoxic for 75% of the time had similar decomposition as the oxic control. The MEND model closely reproduced oxic control results but systematically underestimated C losses under O2 fluctuation. Acclimation of the extant microbial communities to O2 variability may have contributed to the recovery of decomposition rates after several weeks of suppression observed early in the experiment. This hypothesis is supported by significant separation of communities among treatments as a function of O2 exposure and significant log2-fold increases of 886 bacterial OTUs in fluctuating O2 treatments vs. the control after 48 days, by which time steady-state decomposition dynamics had established. Community composition (Bray-Curtis distance) was significantly related to C losses. The Mollisol showed greater change in 16S composition among treatments than the Oxisol and also had smaller variation in cumulative decomposition among treatments. Incorporating microbial acclimation to repeated perturbations may be critical for modeling the responses of biogeochemical processes to environmental variability.