Variation in soil oxygen across an aquatic-terrestrial interface

Background/Question/Methods

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 where diffusion dominates, but fails under dynamic conditions (e.g., water table fluctuations or precipitation) due to advective flux during soil drainage and lags in biological O₂ consumption.  Furthermore, we hypothesize that GHG emissions will correspond to variation in soil O₂. 

Results/Conclusions

Twenty-four near-continuous (hourly readings) soil O₂ and moisture sensors were installed across an aquatic-terrestrial interface zone of a constructed wetland in April 2012.  Drought conditions (2012) resulted in minimal soil O₂ variation.  Despite dry conditions a diurnal pattern of lower soil O₂ during the day was observed.  When precipitation increased in September (due to the remnants of Hurricane Isaac), soil O₂ variation increased substantially with 20 of the 24 sensors recording soil O₂ concentrations below 5%.  The relationship between soil moisture and soil O₂ was non-linear during periods of soil drainage and precipitation.  A rapid (change of x% over <24 hours) increase in soil O₂ occurred at ~ 40% water filled soil volume during soil drainage.  This rapid increased appears related to drainage of soil macropores ahead of changes in bulk soil moisture.  As soil moisture increased due to precipitation, soil O₂ decreased slower than predicted by simple diffusion models.  A lag in O₂ consumption is hypothesized to explain this divergence from the diffusion model.  Weekly methane and nitrous oxide emissions corresponded to variation in soil O₂.  Future research will explore the importance of temporal soil O₂ and moisture variation in driving GHG emissions.