The Biogeochemistry of Boreal Beaver Ponds

The flooding of soils by beaver dams causes the depletion of oxygen and development of anaerobic conditions that alter biogeochemical processes. Field studies showed that, as anaerobic conditions increased, porewater concentrations of Fe2+ increased and SO42− decreased, consistent with the predictions of redox reactions. Porewater PO43− concentrations also increased under anaerobic conditions, presumably due to dissolution of iron phosphates following iron reduction. Ammonium concentrations increased with increasing saturation; under aerobic conditions ammonium would normally be converted to nitrate. Porewater Ca2+ and Mg2+ concentrations also increased with increasing saturation, but the mechanism was probably carbonate dissolution due to increased CO2 pressure in the soil during waterlogging, rather than changes in redox potential. Soil type influenced beaver meadow chemistry: glaciolacustrine soils had higher total N, oxalate-extractable Fe, cation exchange capacity, and base cations (K, Ca, Mg) than did glaciofluvial soils. A split moving window was used along a beaver meadow transect to identify biogeochemical ecotones, the locations where sharp breaks occurred in anaerobiosis and nitrate concentrations. A distributed parameter biogeochemical model was applied to the Finlander watershed of the Kabetogama Peninsula, and indicated that beaver impoundments there would decrease nitrogen concentration, chemical oxygen demand, and sediment yield measured at the watershed outlet cell.