Environmental controls on bioavailable manganese concentrations in soils of the Boulder Creek watershed, Colorado
Patton, Annette Irene
May 8, 2013
Manganese (Mn) is an essential micronutrient for biological communities and is present as a trace or minor element in soils, water, and solid minerals. However, Mn can become toxic to both plants and animals at high concentrations. In order to characterize the effects of wildfire and other environmental factors on soil Mn bioavailability in watersheds of the Front Range, Colorado, DTPA-extractable Mn tests and percent loss-on-ignition (%LOI) tests were performed on soil samples collected in the Boulder Creek watershed. Bulk Mn concentrations of dry-ashed vegetation samples collected near the soil sampling sites were also measured. The Boulder Creek watershed is the primary source of drinking water for the city of Boulder and other small communities; an understanding of Mn cycling in response to environmental factors thus provides insight into water quality issues and water treatment plans for these communities.
Soils throughout the Boulder Creek watershed display addition profiles of soil Mn, indicating a primary input from surface processes such as atmospheric dust. Litter-fall and decomposition of organic matter also return soluble Mn to the soil surface. A positive correlation between %LOI and extractable Mn concentration indicates an association between bioavailable Mn and organic matter in the soil, although other pedogenic processes likely influence Mn cycling as well, particularly in watersheds where the correlation between extractable Mn concentration and %LOI is weak.
This study also demonstrates that wildfire releases Mn from combusted vegetation and supplies large inputs of readily bioavailable Mn to the soil surface, although Mn concentrations at depth remain relatively constant. Comparison between soil treatments further indicate that Mn in mineral-rich ash is highly mobile and demonstrates a relatively short-term residence time (less than two years) in post-fire soils. Elevation and climate also demonstrate a significant effect on bioavailable soil Mn; high-elevation, high-precipitation soils contained lower concentrations of soluble Mn than soils collected from lower elevations. Other factors that demonstrate significant impacts on bioavailable Mn concentration and distribution include location on hill slope transects. Within the Boulder Creek watershed, soil pH and slope aspect do not appear to have a profound impact on bioavailable Mn concentrations. It is also unlikely that industrial pollution adds significant quantities of Mn to the soils in this watershed.
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