NEAR SURFACE SOIL OXYGEN DYNAMICS: PATTERNS FROM SIX YEARS OF HIGH FREQUENCY MONITORING

Soil oxygen (O₂) is a fundamental control on terrestrial biogeochemical cycles including processes producing and consuming greenhouse gases (GHG), yet it is rarely measured. Instead, soil O₂ is assumed to be proportional to soil moisture and physical soil properties. For example, soil O₂ is often inferred from a 25-year old steady-state diffusion model; however, few data exist to test this model in stochastic systems. The variability of soil O₂ may be particularly important to GHG emissions from aquatic-terrestrial interface zones because of the convergence of variable hydrology and rapid biogeochemical processing. Our objective is to gain a better understanding of soil O₂ variation and its role in controlling GHG emissions across aquatic-terrestrial interface zones. Specifically, we hypothesize that in aquatic-terrestrial interface ecosystems, soil moisture predicts O₂ concentration under stable conditions, but under dynamic conditions (e.g., water table fluctuations or precipitation) heterogeneous distributions of water-filled soil pore space complicate this prediction. Furthermore, we hypothesize that GHG emissions will correspond to variation in soil O₂.

Twenty-four near-continuous (30-minute frequency) soil O₂ and moisture sensors were monitored for more than six years. The sensors were installed at 10 cm of depth across an aquatic-terrestrial interface of a constructed wetland in April 2012 and removed in July 2018. Diurnal, precipitation and drainage events, seasonal, and longer-term patterns were in soil O₂ observed. Drought conditions (2012) resulted in minimal soil O₂ variation; however, a diurnal pattern of lower soil O₂ during the day was observed. When precipitation increases within and among sensor soil O₂ variation increases. The relationship between soil moisture and soil O₂ was non-linear during periods of soil drainage and precipitation. Commonly, a rapid (change of 10% over <24 hours) increase in soil O₂ occurred during soil drainage near a common threshold. As soil moisture increased due to precipitation, soil O₂ decreased slower than predicted by simple diffusion models. Soil O₂ was an important predictor of weekly methane and nitrous oxide emissions correspond to variation in soil O₂. These soil O₂ data will be useful for understanding multiple soil biogeochemical functions.