In situ long-term redox potential measurements in a dyked marsh soil

The long-term measurement of soil redox potential (E-H) by permanently installed Pt electrodes may be restricted by electrode breakdown (electrode rupture and resin leakage) and contamination, especially under wet and strongly reducing soil conditions. The E-H of a slightly alkaline (pH 7.1 to 7.3) Calcaric Gleysol developed from marine sediment in the dyked marsh of Schleswig-Holstein, Northern Germany, was monitored weekly during a 4-year period using permanently installed Pt electrodes. Measurements were performed in fivefold at 10, 30, 60, 100, and 150 cm. Furthermore, water table level was recorded. Sulfide occurred in 150 cm as a heritage of the previous marine environment. Mean water table level was 84 cm below the soil surface but was characterized by both short-term and seasonally strong fluctuations. Levels of water table ranged from 33 to >200 cm below soil surface. In consistence with water table level, the E-H continually decreased with soil depth. Mean redox conditions were oxidizing at 10 (550 mV) and 30 cm (430 mV), weakly reducing at 60 cm (230 mV), and moderately reducing at 100 (120 mV) and 150 cm depth (-80 mV). Soil hydrology differed markedly during the study as expressed by periods of water saturation for each depth. This was reflected by Pt electrodes response, since period of water saturation and E-H were significantly negatively correlated as calculated for each year and depth (r(s) = -0.971; n = 20; P < 0.01). The 60-cm depth was most frequently influenced by water table fluctuations, showed the largest E-H range (920 mV) and the most distinct seasonal pattern in E-H. Good function of the electrodes in this depth can be concluded even after long time of installation in soil. Although established in a sulfide-bearing environment, three of five electrodes at 150 cm showed a substantial increase (+500 mV) in E-H during summer of the third and fourth years of investigation, which had low water tables. It is not clear whether the non-response of two electrodes was due to electrode contamination or spatial variation in E-H. When operating in reducing systems, this problem can be circumvented by installing a large number of electrodes or by a regular replacement of electrodes. Using properly constructed and permanently installed Pt electrodes, soil E-H can be monitored for extended periods under wet and reducing soil conditions.