Redox potential of bulk soil and soil solution concentration of nitrate, manganese, iron, and sulfate in two Gleysols

While the reduction of nitrate-N, Mn(III,IV), Fe(III), and sulfate-S in soil has been studied intensively in the laboratory, field research has received only limited attention, This study investigated the relationship between redox potential (E-H) measured in bulk soil and concentrations of nitrate, Mn2+, Fe2+, and sulfate in the soil solution of two Gleysols differing in drainage status from the Marsh area of Schleswig-Holstein, Northern Germany. The soils are silty-sandy and developed from calcareous marine sediments. Redox potentials were monitored weekly with permanently installed Pt electrodes, and soil solution was obtained biweekly by ceramic suction cups from 10, 30, 60, and 150 cm depth over one year. Median E-H at 10, 30, 60, and 150 cm depths was 470, 410, 410, and 20 mV in the drained soil and 500, 480, 30, and -170 mV in the undrained soil, respectively. A decrease in E-H below critical values was accompanied in the soil solutions (pH 7.4 to 7.8) by disappearance of nitrate below 0 to 200 mV, appearance of Mn2+ below 350 mV, and Fe2+ below 0 to 50 mV. Both metals disappeared from soil solution after aeration. In the sulfide-bearing environment of the 150 cm depth of the undrained soil, however, the sulfate concentrations were highest at such E-H values at which sulfate should be unstable. This discrepancy was reflected in the fact that at this depth bulk soil E-H was about 400 mV lower than soil solution E-H (250 mV). When investigating the dynamics of nitrate, Mn, and Fe in soils, bulk soil E-H provides semi-quantitative information in terms of critical E-H ranges. However, in sulfidic soil environments the interpretation of E-H measured in bulk soil is uncertain.