Effects of redox on the solution chemistry and aggregate stability of Midwest upland soils.

摘要

Many upland soils in the Midwest of the US are temporally flooded during the spring and can remain saturated for days. The effects of prolonged saturation on the biogeochemical processes that occur in these soils are not fully understood and are the subject of this study. To evaluate the biogeochemical changes that occur in these soils during prolonged saturation the redox-induced changes in the soil solution and its effects on nutrients, with emphasis on phosphorus and nitrogen; redox sensitive constituents; and soil erodibility were investigated for three cultivated and three uncultivated soils with different organic carbon concentrations. Two types of experiments were conducted in this study (1) an open-air experiment and (2) controlled experiments under anoxic conditions. The open-air experiment was conducted in a representative soil using rainfall-runoff simulation under controlled saturation conditions up to 28 days. Results showed that prolonged saturation and reducing conditions near surface decreased nitrate concentration, increased manganese concentration and soil loss, and had no effect in the concentration of soluble phosphorus in runoff. The experiments under anoxic conditions were performed in a biogeochemical reactor designed to maintain a constant flow of oxygen-free gas. Two experiments were carried out: a soil solution and a soil aggregate stability studies. In both studies, cultivated and uncultivated soils were saturated up to 14 days. The chemical transformations in the uncultivated soils were faster than those of cultivated soils. In the uncultivated soils, there was a faster lowering of the redox potential, higher increase in pH, and quicker increase in manganese, iron, dissolved organic carbon, ammonium, and phosphorus concentrations. In both soils groups, nitrate was undetectable in the soil solution after 1 day of anoxic incubation. The CO2 production was higher in the uncultivated soils and was correlated with increases in the concentration of reduced iron in the soil solution. An overall decrease in the stability of soil aggregates was observed under reducing conditions, which may cause increased soil loss under prolonged saturation. There was a general agreement between the changes in nutrients and soil erodibility observed in the open-air and anoxic experiments.