Soil aeration: the relation between air‐filled pore volume and redox potential

Abstract

SummaryThe soil water content affects rates of oxygen diffusion and redox potentials (EH). When water‐saturated soils become aerated, a switch from reducing to oxidizing conditions occurs. However, limited information is available on the air‐filled pore volume (ϵ) at which this shift happens. To obtain values of ϵ, undisturbed soil cores were taken from a Fluvisol and a Gleysol that differed in structure and clay content. Experiments on submergence and drying following a new experimental design were performed in the laboratory. After submergence, the cores were sealed with a glass hood to exclude oxygen and to achieve reducing conditions (EH < −100 mV). We then aerated the sample by removal of glass plugs in the hood and measured EH consecutively by platinum (Pt) electrodes and ϵ by matric potential readings on an hourly basis. From the drying curve we determined two characteristic values: (i) ϵPt reaction indicates the air‐filled pore volume at which a response of the Pt electrode to contact with oxygen occurs (i.e. EH increase > 5 mV hour−1) and (ii) ϵPt aeration indicates when oxidizing soil conditions are present (i.e. EH > 300 mV at pH 7). The Fluvisol was characterized by an ϵPt reaction value of 0.036 ± 0.006 cm3 cm−3 and an ϵPt aeration value of 0.047 ± 0.005, whereas for the Gleysol these values were 0.048 ± 0.008 and 0.085 ± 0.007 cm3 cm−3, respectively. We aimed to obtain such characteristic values for different soils to estimate the aeration status of a soil when ϵ is known, but EH measurements were unavailable.Highlights We studied the relation between EH and air‐filled pore volume (ϵ) for two soils varying in texture. Two characteristic ϵ values are proposed: ϵPt reaction (EH increases > 5 mV hour−1) and ϵPt aeration (EH > 300 mV at pH7). ϵPt aeration was larger for a clayey Gleysol than a sandy Fluvisol, but ϵPt reaction was similar. Small‐scale heterogeneity in soil structure had no effect on the EH–ϵ relation.