Inferring oxygen status in soils with iron rods (Texas).

摘要

The determination of oxygen in soils is a fundamental parameter that is needed for understanding and application of biogeochemical processes regarding many predictions and uses of soils. The objectives of this research were: (1) to develop a simple, rapid, cost effective and non-intrusive field technique to infer or estimate oxygen status in soils using a zero-valent polished iron rod, and (2) to determine the seasonal changes in oxygen status, reduction-oxidation potentials, soil moisture contents, ferrous iron contents and the iron oxide/oxyhydroxides coatings that form on iron metal rods placed in nine soil environments of the Texas Coast Prairie. Three toposequences representing nine soils (Alfisols and Vertisols) of variable texture and pH were used for this study. In each of the nine sites, there were polished iron rods, piezometers, tensiometers, Eh electrodes, soil water/gas collection chambers installed to quantify the hydrology, redox processes and oxygen concentration in the soils. The commonly used field methods of Eh, soil tension were examined and compared to polished iron rods as indicators of O₂ status. All methods were useful at predicting anoxic/paraoxic and oxic conditions but Eh and soil tension had limitations at near saturated conditions. The anoxic conditions (0–2% O₂) occurred at Eh's below 200 mV and at soil tensions of 0 which yielded no bright coatings on the iron rods. At near saturated conditions (2–5% O₂), Eh's were highly variable and ranged from 200 to 400 mV, soil tension had considerable variability and ranged from 1 to 80 Kpa soil tension, and the iron rods yielded variegated and pore shaped bright coatings, which was the only method that indicated microsite variability. The oxic conditions were indicated by Eh's >400 mV, soil tensions from 1 to 100 KPa, and iron rods were almost completely covered with bright coatings. The results of this field study indicate that zero valent low carbon iron metal rods are a promising field technique for determining ranges in O₂ content of soils. This research has applications in agronomic/horticultural sciences, bioremediation, on-site wastewater design, soil classification, and wetland determination.