On the Coupled Dynamics of Soil Moisture, Temperature, and Oxygen in an Urban Agricultural Soil

microbial metabolism in mesic to humid environments. For example, models of root and microbial respiration include a multiplier function that represents the roles of soil moisture in generating physiological stress under dry conditions and in limiting substrate diffusion under wet conditions. With respect to wet, saturated conditions, soil moisture is often treated as a surrogate for oxygen supply in modeling metabolic limitations and anaerobic processes such as denitrification and methanogenesis. Modeling of these processes accurately is limited by an understanding of the coupled dynamics of soil moisture and oxygen. We addressed this gap by sampling the near surface oxygen concentration, volumetric water content, and temperature of an urban agricultural soil at five-minute frequency throughout the 2018 growing season. The data reveal relationships between the three variables from the diurnal to daily and seasonal time-scales. A temporal analysis of oxygen and soil moisture revealed clear diurnal variability, indicating evapotranspiration and respiration are stimulated during the day. Memory in the soil oxygen signal dissipates within a few hours and soil moisture and oxygen show strong coherence at time-scales longer than daily. At the daily scale, a strong negative and non-linear relationship between soil moisture and oxygen emerged in a cross-spectrum analysis of the two variables; at significant frequencies the two variables were shown to be nearly completely out of phase. This relationship was approximated with a reduced-order approximation of a one-dimensional, vertical model coupling soil moisture and oxygen dynamics. Given the strong relationship between soil moisture and oxygen at the daily scale, we estimated the crossing properties of the soil oxygen dynamics using a transformed stochastic water balance nsity of anaerobic conditions given soil, climate, and vegetation parameters. This modeling approach can be used to estimate oxygen stress on soil metabolism in humid environments and the production of greenhouse gases, such as Nitrous Oxide and Methane, under anaerobic conditions.