Soil Oxygen Dynamics: Patterns and Lessons from Six Years of High Frequency Monitoring

Abstract

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.