Separate Effects of Flooding and Anoxia on Soil Biogeochemistry and Greenhouse Gas Emissions

Soils are large sources of atmospheric greenhouse gases, and their magnitude and composition is strongly controlled by soil redox conditions. Though the effects of redox dynamics on greenhouse gas emissions have been well studied for flooded soils, oxygen can also be depleted without soil inundation. This can occur when rates of oxygen consumption exceed the rate of atmospheric replenishment. We hypothesized that greenhouse gas emissions may differ under these distinct anaerobic scenarios. We investigated the effects of soil anaerobiosis, generated with and without flooding, on greenhouse gas emissions and a range of redox-sensitive soil characteristics. We collected soil from just above the water table in a partially drained peatland pasture and from a clay-rich humid tropical soil in the Luquillo Long-Term Ecological Research forest in Puerto Rico that never experiences flooding. We produced anaerobic conditions with crossed treatments of flooding and nitrogen headspace, applied to homogenized soil microcosms. Gas fluxes of carbon dioxide, nitrous oxide, and methane were repeatedly measured over the course of a 2 or 3-week incubation and soil was extracted for mineral nitrogen and iron, as indicators of soil redox, post-treatment.

We found that anaerobic conditions under a nitrogen headspace, with or without flooding, suppressed carbon dioxide emissions by 50%. Flooding further suppressed emissions temporarily, but the largest effect of flooding on soil carbon dioxide emission was equivalent to the effects of a nitrogen headspace. Longer-term impacts of flooding on carbon dioxide emission differed by soil type. Under anaerobic conditions flooding led to greater carbon dioxide emissions than unflooded soils from the tropical forest. The magnitude and temporal trends in nitrous oxide emissions differed greatly across the treatments. Emissions of nitrous oxide were likely sensitive to changes in pore-space diffusivity associated with flooding in addition to the soil redox conditions. Interestingly, only the tropical soil, and not the peatland histosol, produced significant methane effluxes in the anaerobic incubation and this was significantly greater with flooding. Our results indicate that oxygen depletion suppressed carbon dioxide emissions as expected, but that this effect was at least partially offset by the stimulation of anaerobic respiration. Flooding led to greater soil respiration relative to unflooded soils under anaerobic conditions. Flooding also stimulated methanogenesis more than oxygen depletion alone in the tropical soil. We conclude that soil biogeochemistry and greenhouse gas emissions are sensitive to the redox effects of oxygen depletion as a driver of anaerobiosis, but that flooding can have additional effects independent of oxygen depletion.