Anomalous calcareous hydromorphic soils in western WyomingMatthew King and Karen VaughanEcosystem Science and Management Department, University of Wyoming

ABSTRACTSoils comprised of greater than 50% calcium carbonate possess unique problems related to wetland delineation in western Wyoming.These light-colored soils lack traditional field indicators of hydric soil (redoximorphic features and organic C accumulation) indicativeof anaerobic conditions despite the presence of hydrophytic vegetation and wetland hydrology. To determine the limiting factor forthe lack of redoximorphic feature development, a mesocosm study was conducted using 15 intact soil cores from a representative site.These soil cores inundated with water for 16 weeks and subject to one of three treatments, including the addition of FeCl3, organic Cas dextrose, and a combination of both FeCl3 and organic C, as well as a control. Once per week, oxidation-reduction potential and pHwere measured within each mesocosm. After the 16-week study duration, the mesocosms were drained and soil cores weredisassembled. The abundance, color, distinctness, and type of redoximorphic features were described in 5-cm increments throughoutthe cores. The FeCl3 and FeCl3 plus organic C treated mesocosms formed a mean of 21% and 22% distinct or prominent ironconcentrations respectively. In contrast the organic C treated and control mesocosms averaged 0.2% and 0.1% distinct or prominentiron concentrations respectively. Based on these results, Fe appears to be the limiting factor for redoximorphic feature development inthese calcareous hydromorphic soils. Future work will explore how these hydric soils can be successfully identified in the field.

OBJECTIVE• To determine the limiting factor/s responsible for the absence of visible redoximorphic

features in these soils.

• To develop an alterative field indicator of hydric soil for these systems.

Microbially mediated reduction

left- cutting open a mesocosm and right – describing the soil with special attention to percent, color, and distinctness of redoximorphic features.

above – General location of research sites in western Wyoming; upper right - Soil profile excavated at Duck Creek west of Pinedale, WY and landscape; lower right –parameters assessed at each of the four study sites.

To form redoximorphic features indicative of anaerobic conditions, microorganisms require a C source and terminal electron acceptor (TEA) such as Fe, Mn, or S.

Mesocosm extraction

A – cores were installed in large containers and secured with cam straps. B – cores were inundated with water for 16 weeks and monitored for soil temperature (continuous), redox potential (weekly), and pH (weekly). C – each block of 4 was amended with either 2% Fe by weight (FeCl3), 36 mg l-1 organic C (C6H12O6, dextrose), or Fe+OC(FeCl3 and dextrose).

Environmental SettingSituated west of Pinedale, Wyoming at ~7,000 ft elevation, the site receives ~30 cm of annual precipitation and has a mean annual temperature of 1 C. The soil temperature regime is frigid and soil moisture regime is aridic (outside these wet area, where it’s aquic). Mean values for select properties shown below:

United States Department of Agriculture, Natural Resources Conservation Service. 2017. Field Indicators of Hydric Soils in the United States, Version 8.0. L.M. Vasilas, G.W. Hurt, and J.F. Berkowitz (eds.). USDA, NRCS, in cooperation with the National Technical Committee for Hydric Soils.

REDOXIMORPHIC FEATURE FORMATIONMESOCOSM DESCRIPTION RESEARCH FINDINGS

Potential field indicatorWith no typical soil morphological features to indicate anaerobic conditions, an alternate field indicator must be employed to identify these areas as hydric soils. In addition to the use of IRIS tubes and/or the application of alpha alpha dipyridyldye, we propose the following as a working concept specific to MRLA 34A (Cool Desertic Basins and Plateaus):• Dominant matrix colors with a value ≥ 5 and a chroma ≤ 2 from soil surface to

a depth of ≥ 30 cm (12 in);• strong or greater effervescence after the application of 1 M HCl within 30 cm

(12 in) of the soil surface; and• sites observed under this indicator require either the presence of hydrophytic

vegetation and wetland hydrology indicators or a positive reaction to alpha alpha dipyridyl dye.

User Notes: This indicator could also be limited to use in riparian floodplains; unlike indicator F3 which excludes calcic horizons without the presence of redoximorphic features this indicator requires the presence of CaCO3.

4 sites

wetland hydrology

via automated ground water monitoring

hydricsoils via

IRIS tubes

hydrophyticvegetation via

USACOE requirements

monitoring à

capable microbial community

terminal electron acceptor (TEA)

Fe3+

C source

A – 4-inch diameter PVC cores were driven into the ground at the Randall site in close proximity to reduce soil variability. B - cores were extracted by capping then digging them out manually. The Cores were transported to the lab.

Treatments to facilitate redox feature formation

C

OXIDATION-REDUCTION POTENTIAL

control organic carbon

Fe Fe + OC

Representative cores from each treatment group showing:

FeS

Fe oxide

Fe concentrations

FeS concentrations

Percentage iron redoximorphic features formed by treatment within mesocosms. Box plots show median values (solid horizontal line), 50th percentile values (box outline), 90th percentile values (whiskers), and outlier values (circles, data falling outside 1.58 times the interquartile range above or below the upper or lower quartile). *significance indicated at p-value= 0.05.

Percentage iron monosulfide (FeS) redoximorphic features formed by treatment within mesocosms. Box plots show median values (solid horizontal line), 50th percentile values (box outline), 90th percentile values (whiskers), and outlier values (circles, data falling outside 1.58 times the interquartile range above or below the upper or lower quartile). *significance indicated at p-value= 0.05.

Fe oxide

FeS

After 16 weeks of inundation the mesocosms showed significant differences in redoximorphic feature formation between mesocosms treated with Fe (Fe and Fe+OC) and those that did not (OC and control). • Iron is the limiting factor in the formation of iron redoximorphic features

in situ.• Field observation of hydric soils in these environmental settings remains

complicated due to the lack of observable redox features. • Despite the presence of wetland hydrology and hydrophytic vegetation,

these soils do not meet current Field Indicators of Hydric Soils (USDA-NRCS, 2017).

• Inundated, highly calcareous soils (>75%) show no observable evidence of anaerobic conditions.

• Four sites meet wetland hydrology and vegetation requirements.• Field wetland delineation is challenging.• Something is limiting redoximorphic feature formation.

THE ISSUE:

Fe

soil organic C CaCO3 total Fe soil pH0.02% 75% 0.5% 8.5

Zone of predicted Fe reduction

Zone of predicted SO42- reduction

ß Redox potential decreased in all mesocosms below the NTCHS technical standard for anaerobic conditions after the first week. All cores were anaerobic and susceptible to redoximorphic feature formation.

Acknowledgements: The authors acknowledge USDA-NRCS scientists Dan Mattke, Dan Perkins, Karen Clause, Gabe Fancher, and Kimberly Cumella for their help with identifying the problematic calcareous hydromorphic soils and field sampling soils and vegetation; Jill Randall for the use of her property and vegetation characterization; and Taylor Bush for his field and lab assistance.

Fe2+ Fe3+