Peatland Humification and Subsidence Related to Ditching.

A 23,000 acre drained peatland in Northeastern Minnesota was investigated for its potential conversion to a wetland mitigation bank. Approximately 68% of the peatland soils are classified as Typic Haplohemists and consists of very deep, very poorly drained soils formed from sphagnum moss on a glacial lake plain. The peatland was extensively ditched and drained prior to the 1920’s to create a mixture of pasture, peat mining, sod farming, agrarian and/or silviculture land, however, the majority of the site currently remains dominated by a bog habitat. The drainage effects on the peatland soils were measured on transects perpendicular to the ditches in 2014. Soils were sampled and characterized and the organic materials were assessed using the von Post Degree of Humification method. Soil data were complemented by topographical, well monitoring, floristic quality assessment, and tree core data. The results show the peatland has subsided adjacent to drainage ditches and that the peat adjacent to the drainage ditch has a higher degree of humification and has been transformed from a nutrient poor ombrotrophic system to a more nutrient rich minerotrophic system. In sections (640 acres) that only have perimeter ditches the peatland soils have lower degrees of humification and the effects of ditching does not seem to extend beyond 500 feet whereas sections that are bisected by several lateral ditches show much higher degrees of humification that extend beyond 500 feet. The results of the soil, hydrological, vegetation, and topographic assessments suggest that ditching has depressed the water table in the peatland adjacent to the drainage ditches which has accelerated decomposition and subsidence of the peatland soils.  It is likely that continued transformations occur as an “oxidation front” where unsaturated conditions and humification of the soils travels away from the ditch with time.