Laboratory comparison of soil redox conditions between red soils and brown soils in Minnesota, USA

Certain soil morphological characteristics create significant problems for hydric soil identification due to lack of commonly observable Fe-based redoximorphic features that indicate seasonally saturated and reduced conditions. Examples include soils with high levels of organic C and red soils (7.5YR or redder) containing high amounts of hematite iron. The objectives of this research were to examine the effects of organic C content and Fe mineralogy on (i) soil redox response and (ii) the development of Fe-based redoximorphic features. Redox reactions related to the fundamental biochemical processes that occur in saturated soils were studied in a controlled laboratory experiment. The experimental design included six columns of red soils (5YR) from northeastern Minnesota and six columns of brown soils (10YR), possessing varying organic carbon contents, from southeastern Minnesota. Redox measurements taken throughout the experiment suggest different soil redox environments between the brown and red soils. The brown soils had redox potentials that decreased gradually and then remained at a constant decreased state around 0.0 mV. Redox potentials for the red soils decreased rapidly and equilibrated in the range of 100 to 300 mV. Variations between these soil responses may be due to different Fe-oxide mineralogies and/or the amount of bio-available organic carbon.