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

Abstract

Summary The soil water content affects rates of oxygen diffusion and redox potentials (E H ). 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 (E H < −100 mV). We then aerated the sample by removal of glass plugs in the hood and measured E H 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) ϵ Pt reaction indicates the air‐filled pore volume at which a response of the Pt electrode to contact with oxygen occurs (i.e. E H increase > 5 mV hour −1 ) and (ii) ϵ Pt aeration indicates when oxidizing soil conditions are present (i.e. E H > 300 mV at pH 7). The Fluvisol was characterized by an ϵ Pt reaction value of 0.036 ± 0.006 cm 3 cm −3 and an ϵ Pt aeration value of 0.047 ± 0.005, whereas for the Gleysol these values were 0.048 ± 0.008 and 0.085 ± 0.007 cm 3 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 E H measurements were unavailable. Highlights We studied the relation between E H and air‐filled pore volume ( ϵ ) for two soils varying in texture. Two characteristic ϵ values are proposed: ϵ Pt reaction (E H increases > 5 mV hour −1 ) and ϵ Pt aeration (E H > 300 mV at pH7). ϵ Pt aeration was larger for a clayey Gleysol than a sandy Fluvisol, but ϵ Pt reaction was similar. Small‐scale heterogeneity in soil structure had no effect on the E H – ϵ relation.