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DIVISION S-10 - WETLAND SOILS

Soils and Hydrology of a Wet-Sandy Catena in East-Central Minnesota

^a Dep. of Rangeland Resources and Wildland Soils–NR 200, Humboldt State Univ., Arcata, CA 95521
^b Dep. of Soil, Water, and Climate, Univ. of Minnesota, 439 Borlaug Hall, 1991 Upper Buford Circle, St. Paul, MN 55108

* Corresponding author (rjr11{at}humboldt.edu)

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Soil properties are strongly related to the retention and movement of water within the soil system. The purposes of this study were to document the near-surface hydrology of a wetland–upland hillslope on a sandy glacial outwash plain in east-central Minnesota and to describe the patterns of soil morphology with respect to observed hydrology. Water levels, soil temperature, soil-water tension, and redox potential were monitored at seven points along a 41-m hillslope transect composed of Psamments, Aquents, Aquods, and Saprists. In addition to standard field descriptions, particle-size distribution, percentage of organic C, and citrate-dithionate and ammonium-oxalate extractable Fe (Fe_d and Fe_o, respecitively) were determined for profiles at each transect point. During the study period, the 30-yr mean annual precipitation (MAP) was exceeded in 4 of 5 yr. Mean water levels were highest in spring and water levels typically rose from September through May. The depth to redoximorphic features increases with elevation above the peatland and the upper extent of redoximorphic features is 15 to 60 cm above the measured mean water table level. In the upper landscape positions, the redoximorphic features were located 12 cm above the maximum recorded water level. The distribution characteristics of Fe throughout the soil system indicate that Fe has been removed from the upslope soils and reconcentrated in organic-rich horizons in the lower landscape positions. The combination of Fe distribution and the location of redoximorphic features well above the mean water table suggest that the regional water table has been lowered.

Abbreviations: CCHNA, Cedar Creek Natural History Area • ET, evapotranspiration • Fe_d, citrate-dithionate extractable Fe • Fe_o, oxalate extractable Fe • LTER, Long-Term Ecological Research • MAP, mean annual precipitation • OC, organic C • WSMP, Wet Soil Monitoring Project

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
WATER SERVES as one of the primary energy sources for landscape processes, such as sequestration of organic C (OC), erosion, colonization of vegetation, and distribution of soluble and mobile compounds.

The correlation between water status, landscape, and soil properties has been well-documented in pedologic studies. A brief review indicates that the correlation persists across land types and geographic locations. Simonson and Boersma (1972) documented strong correlation between depth to water table and soil color patterns in a drainage sequence in western Oregon. In Illinois, Kreznor et al. (1989) found that landscape position and terrain attributes affect distribution of soil properties such as A horizon thickness and clay and OC content. Evans and Franzmeier (1986) discussed the relationships between duration and season of occurrence of soil saturation with soil color patterns in north-central Indiana. Pickering and Veneman (1984) also found evidence of relationships between duration and time of soil saturation and distribution of soil color in a hydrosequence in Massachusetts.

The Wet Soil Monitoring Project (WSMP) is a cooperative project between the NRCS-National Soil Survey Center, under the Global Change Initiative, the U.S. Army Corps of Engineers Wetlands Research Program, and several universities in the USA. The overarching objective of the project is to collect baseline data for hydrology and soil properties in different climatic regimes and landscapes so that these data can be used in the future to evaluate the effects of climate change on landscape hydrology and ecosystems. Eight states—Alaska, Indiana, Louisiana, North Dakota, New Hampshire, Texas, Oregon, and Minnesota—participate in the WSMP (Lynn et al., 1996). One of the specific areas that WSMP researchers are focusing on is morphological indicators of hydric soils.

This study documents the observations and results of the Minnesota WSMP Site at Cedar Creek on the Anoka Sandplain (Fig. 1) . Review of the literature for the area indicates that the hydrology of the Anoka Sandplain prior to the 1970s lacks documentation, especially on a landscape scale. The Cedar Creek WSMP site provides a platform to document the current hydrology and its relationship to soil morphology from a catena perspective. Objectives for the study were to (i) document the near-surface hydrology of an upland–wetland transect representative of undisturbed conditions within the Anoka Sandplain and (ii) describe the spatial distribution of soil morphological properties with respect to hillslope hydrology. Our hypothesis is that the spatial distribution of redoximorphic features in soils is a function of the location of the near-surface water table relative to the soil surface.

View larger version (111K): [in this window] [in a new window]   Fig. 1. Location of the Cedar Creek site in Minnesota and section of the Isanti digital orthoquad showing the location of the study transect.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

As a LTER site, the ecology of CCNHA has been well documented by hundreds of studies. Several studies have documented aspects of the hydrology and soils. Buell and Buell (1941) recorded the seasonal variability of the hydrology of Cedar Creek Bog, noting that, in 1934, the elevation of the water table in the peatland fluctuated as much as 70 cm between May and October. Reiners (1968) measured CO₂ evolution along upland–peatland transects at Cedar Creek Bog, finding a negative correlation between precipitation events and CO₂ emissions and a positive correlation between soil temperature and CO₂. Grigal et al. (1974) characterized and mapped the soils for CCNHA. Nitrogen and C cycle dynamics and availability, which have been related to successional vegetative stages in CCNHA (Tilman, 1986; Zak et al., 1994), have been shown to be closely related with soil water availability and topographic position (Zak and Grigal, 1991; Hairston and Grigal, 1991; Homann and Grigal, 1996). In addition to published research, multiple theses have been written on the ecology and physical characteristics of CCNHA, including that of Basiletti (1994), which documents the topography of the water table within CCNHA.

The Anoka Sandplain straddles the humid–subhumid border and has MAP of 76 cm. Mean July temperature is 22°C and mean January temperature is -10°C (Minnesota Climatology Working Group, 1999). Vegetative communities of the sandplain include tall-grass prairie, deciduous hardwood, and coniferous forest (Grigal et al., 1974). The immediate area of the study site is oak (Quercus L.)-dominated forest surrounded by oak savanna. We theorize that drainage ditches in the sandplain have lowered the historical water table; however, preditching well records are not available for verification. Aerial photos of the area indicate extensive ditching as well as channelization of existing streams. Cedar Creek is the major drainage for the area and has been straightened in reaches outside of CCNHA.

Field Methods
A 41-m transect, representative of the regional topography and extending from forested upland into a peatland, was chosen for the study (Fig. 1). The peatland covers 10 ha and is located 1 km east of Cedar Creek. The transect is located on the eastern edge of the peatland and has a west aspect. Seven monitoring stations were installed along the transect in the summer of 1993 and surveyed for elevation. The stations were assigned names based on the hillslope positions of Ruhe (1975) (Fig. 2) . The instrumentation array for each station is summarized in Table 1. Depth to the water table was recorded in screened polyvinyl Cl (PVC) wells installed to 300 cm. Soil temperature (2 repitions) was recorded at 10-, 25-, 50-, and 100-cm depths in the profile. Soil water tension was recorded at 25-, 50-, and 100-cm depths with in situ tensiometers and a digital handheld tensimeter. Redox potential (2 repitions) was measured at 25-, 50-, and 100-cm depths with in situ Pt electrodes and a using an Ag–AgCl reference electrode. Field measurements were taken using the method described by Patrick et al. (1996) and corrected to the standard H halfcell by adding 199 mV to the measurement (Patrick et al., 1996). Given the coarse-textured parent materials for the site, monitoring wells, instead of piezometers, were used to record the depth to the near-surface aquifer. Data were collected every 2 wk during spring, fall, summer, and monthly during the winter since fall of 1993. There are occasions of missing data because of various environmental problems, including frozen tensiometers, frozen or broken wells, and fouled or broken Pt electrodes.

View larger version (33K): [in this window] [in a new window]   Fig. 2. Hillslope profile of the study transect indicating location and names of stations.

In conjunction with site instrumentation, soil pits were excavated adjacent to the monitoring stations (outside of the fall line) and were described using standard National Cooperative Soil Survey procedures (Soil Survey Division Staff, 1993). Samples were taken from described horizons in each profile and sent to the National Soil Survey Laboratory for physical and chemical analyses. Measured parameters included particle-size distribution (pipette method), organic C (acid dichromate digestion), and pH (1:1 water dilution). Iron percentages were determined in soil samples using ammonium oxalate (Fe_o) and citrate-dithionate (Fe_d) extraction techniques (Soil Survey Laboratory Staff, 1996). Taxonomic classifications of the soils were based on field descriptions and lab analyses (Soil Survey Staff, 1998) (Table 1).

Lastly, we are interested in the variation in the seasonal water table response. Using the soil temperature (T) brackets of T < 5°C (winter), 5°C < T < 10°C (fall and spring), and T > 10°C (summer), we separate hydrologic seasons as follows: (i) 1 December to 31 March (winter), (ii) 1 April to 31 May (spring), (iii) 1 June to 30 September (summer), and (iv) 1 October to 30 November (fall).

	RESULTS AND DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Climate
Annual precipitation exceeded the MAP in 4 of the 5 yr (Fig. 3a) ; precipitation in both 1995 and 1996 exceeded the MAP by 20 cm and precipitation in 1994 and 1998 was 6 cm greater. Precipitation for 1997 was 5 cm below the MAP. Soil temperatures at 50-cm depth typically ranged between 18 and 2°C for the study period (Fig. 3b). The upper landscape positions, which are drier, experience both the highest and lowest temperatures while the lower parts of the landscape are thermally buffered because of nearly continual saturation and had a slightly smaller range of temperature.

View larger version (63K): [in this window] [in a new window]   Fig. 3. Climate data for the study site: (a) cumulative monthly precipitation and 30-yr mean from Cedar, MN (Minnesota Climatology Group, 1998), and (b) soil temperature at 50 cm for the monitoring stations (PS = peatland; TS = toeslope; BSL = lower backslope; SS = shoulder).

View larger version (41K): [in this window] [in a new window]   Fig. 4. (a) Average, minimum, and maximum locations of the well observations for the study period and the zone of redoximorphic features. The shoulder station does not have a mean or minimum because the water level was frequently below the 300 cm, the deepest extent of the well. The vertical relief is exaggerated to show the trends in the water table. (b) Well observations for the study period (corrected for elevation: z = 0 at the edge of the peatland).

View larger version (37K): [in this window] [in a new window]   Fig. 5. Mean seasonal levels of the water table along the catena. The error bars represent the standard deviation of the sample population. Values are reported with respect to the elevation of the peatland (dashed line). The shoulder station is not included because water levels were frequently below the depth of the observation well.

In this landscape, peatlands are the areas where the aquifer is expressed at the surface. Visual inspection of the recorded water table levels indicates that the timing of fluctuation is similar between stations across the catena. However, the magnitude of the fluctuations is variable with hillslope position. The largest fluctuations occur in the lower and upper footslope positions. For the 5 yr of the study, these positions recorded the deepest water table (with respect to z = 0) (Fig. 4). Water levels at the peatland and toeslope stations remain more stable and fluctuate less when compared with the upslope soils (Fig. 5). This is consistent with the observations of Grigal and Homann (1994) who found less groundwater fluctuations in wetlands versus uplands at CCHNA.

Verry (1997) indicated that evapotranspiration (ET) rates for northern peatlands and forests are similar, so that ET is not the likely reason for the discrepancy between water levels across the landscape. The most likely cause of the differences in the water table may be stratification of the peatland.

The sandplain is underlain by dense gray till which acts as an aquaclude and allows the formation of the Anoka Sandplain Aquifer (Helgesen and Lindholm, 1977). Basiletti's (1994) topographic map of the local water table indicates that the water table in CCNHA has a lateral gradient towards Cedar Creek. Lindeman (1941) found a deposit of marl underlain by the basal sand in Cedar Creek Bog. The marl forms a hydraulic barrier between the peat and sand. The marl would have a much lower hydraulic conductivity when compared with the sand and peat. In effect, the marl layer cups the water in the peatland, protecting it from lateral losses that occur in the uplands, resulting in the discrepancy seen in water table both in this study and that of Homann and Grigal (1996). Since we have not extracted deep cores for this specific peatland, we cannot verify this theory, and present it only as a testable hypothesis.

Saturation, Redox Potential, and Redoximorphic Features
Peatland
The peatland soil was saturated at 15 cm for 86% of the observations during the study period. At 50 cm, the soil was saturated for 100% of the observations. The continuous saturation at this station is reflected in the redox potential (E_H) readings (Fig. 6a) . Redox potentials are centered around 0 mV. Redoximorphic features because of Fe oxidation and reduction were not visible or expected in this organic soil.

View larger version (29K): [in this window] [in a new window]   Fig. 6. The redox potential (EH) at 50-cm for the (a) peatland, (b) toeslope, and (c) lower backslope stations. Values from two Pt electrodes are depicted for each station.

Toeslope
The toeslope position has a similar hydrology to the peatland, with a slightly larger standard deviation (Fig. 5). The near-surface aquifer was within 15 cm of the soil surface for 80% of the observations and within 50 cm for 99% of the observations. On average, the water level was within 3 cm of the surface. The E_H values at 50 cm for the toeslope station are also concentrated around 0 mV (Fig. 6b). While there is an obvious relationship between saturation and E_H (McBride, 1994; Patrick et al., 1996), given the dynamic nature of redox chemistry, it is not surprising that there is little correlation between the measured water table elevation and E_H (r² < 0.01).

The depressed E_H values at the toeslope, combined with a pH between 5.2 and 6.1 (Table 2) suggests that Fe should be in reduced form. Black colors (N 2/0) dominate the upper 34 cm of the profile and gley colors indicative of reduced Fe are not evident. Below 34 cm, the 10YR 4/1 matrix suggests depletion of Fe. Based on the extraction data, Fe is present in the upper 34 cm but is indeed depleted in underlying horizon (Fig. 7) .

View larger version (33K): [in this window] [in a new window]   Fig. 7. Iron extraction data for the sampled profiles of the transect. Feo = oxalate extractable Fe and Fed = citrate-dithionate extractable Fe.

The mixed colors (7.5YR 3/2 and N 2/0) between 4 and 11 cm for lower footslope soil are probably the result of bioturbation, either by ant (Formica sanguinea) or tree throw. Thatch ants (Formica obscuripes) are abundant in the Cedar Creek area (Haarstad, 1985) and mix the upper surfaces of the soil to various depths and strongly influence profile morphology. Several anthills (0.5–1 m diam.) are in close proximity to the transect. The next horizon is black (N 2/0) followed by depleted (10YR 3/1) matrix parent material at 18 cm.

Fluctuating water levels in the upper footslope soil affect the morphology in the upper reaches of the profile. Redoximorphic features in this profile start at 12 cm and below 23 cm, the soil matrix is depleted with Fe concentrations (Table 2). Iron data for the upper and lower footslope profiles supports the depletion of Fe below 20 cm (Fig. 7). Like the toeslope profile, the Fe maximum occurs in the darker horizons that are richer in OC (Fig. 6). For both the upper and lower footslope soils, the mean well readings are between 15 and 25 cm deeper than the upper limits of the depleted horizons while the maximum well observation is 5 cm above the depleted zone (Fig. 4a).

Backslope
The water table in the lower backslope soil was within 50 cm of the soil surface only once during the 5-yr period (Fig. 4b). However, redoximorphic features are found within 20 cm of the soil surface and the matrix is depleted below 20 cm. Thus, the zone of redoximorphic features extends 12 cm above the maximum well reading and 60 cm above the mean location of the water table (Fig. 4a). The monitoring data cover a period of time where precipitation values exceeded the 30-yr mean, not a dry period. However, the tensiometer data along with the depressed E_H and pH values suggest that reduction of Fe could be occurring above the apparent water level. At times, soil water tension is at or near zero (Fig. 8) suggesting nearly saturated conditions and the possibility of anaerobic conditions in smaller soil pores resulting in an alternating redox environment. Redox potentials at 50 cm fluctuate frequently between 700 and 200 mV and the pH ranges between 4.2 and 5.6 within the profile. Based on the E_H–pH relationship (McBride, 1994) there is a strong likelihood that Fe can be in reduced form. If these conditions have been present over the pedogenic history of the soil, much of the secondary Fe could have been moved out of the profile.

View larger version (29K): [in this window] [in a new window]   Fig. 8. The 50-cm soil tensiometer data for the (a) lower backslope and (b) summit stations.

Iron data for the lower and upper backslope positions depart from the observations of the previous landscape positions. In both profiles, Fe appears to be evenly distributed rather than decreasing to correspond with redox features. In the lower hillslope positions, Fe concentration dropped noticeably at the depth of the low chroma colors. Also, the disparity between Fe_d and Fe_o extractable Fe is higher in the backslope soils than in the soils on the lower landscape position. In the backslope soils, Fe_o is 50% of Fe_d, suggesting that less of the Fe is in amorphous forms, which are more easily reduced (Loeppert and Inskeep, 1996).

Shoulder
At the highest point of the transect, the maximum observed water table was 120 cm below the surface. We did not calculate a mean for this position because the water table was frequently located below the maximum depth of the well (300 cm). The tensiometer data indicate soil water tension near zero for a few observations at 25, 50, and 100 cm, however, the subsequent observations are negative (Fig. 8). These events are likely periods of snowmelt or rain events that have temporarily increased the water content of the surface horizons. Additionally, lamellae were found below 100 cm (Table 2) and may result in temporary perching of water.

The soil was described to 155 cm and no redoximorphic features were observed. The Fe profile is similar to the backslope profiles; there is no peak in Fe and Fe_d is more than twice Fe_o.

In the soils adjacent to the peatland, redoximorphic features are found 15 to 60 cm above the observed mean elevation of water table but the observed features are within the maximum elevation recorded during the study. However, the higher landscape positions had redoximorphic features well above the mean and maximum observed water tables. These observations nullify our hypothesis that the spatial distribution of redoximorphic features is a function of the mean elevation of the water table relative to the soil surface, at least with respect to the observed water table during the study.

Tensiometer data suggest nearly saturated conditions. The E_H and pH data suggest conditions suitable for reducing Fe. These three variables may be responsible for the observed redoximorphic features; however, this is not consistent along the transect as no features were observed in the upper parts of the backslope and shoulder profiles. Another confounding issue is the change in character of the Fe distribution moving from wetland to upland. The wetter soils had sharp peaks of both Fe_d and Fe_o in the OC rich near-surface horizons, below which Fe decreased. This was not the case for the upland soils where Fe was distributed evenly throughout the profile and Fe_d was the dominant type.

Because of its association with organic rich horizons in the lower landscape positions, it appears that Fe is tied up in organic complexes. However, Fe is considerable less in the upper landscape positions. This suggests that the Fe has been redistributed in the landscape, accumulating in the organic rich horizons of the soils bordering the peatland.

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Our results indicate that the hydrology of this landscape, while subtle, has resulted in a complex morphology that cannot be readily explained by the current observations of the water table. The current water table in this landscape was not well associated with the depths to redoximorphic features. These features may be remnants of an earlier, higher water table, although we cannot document that at this site. Alternatively, zones of accumulation and depletion may be suitable indicators of finer-scale processes that are not directly controlled by the hydrology observed at the scale of this study.

	ACKNOWLEDGMENTS

 
The authors acknowledge the NRCS for their funding and support for this project and the University of Minnesota Cedar Creek Natural History Area for use of the site for research and educational purposes. Special thanks to Warren Lynn for help in site characterization and his technical expertise. Several individuals assisted in site setup and data collection, including Charlie Butler, Jim Thompson, and Dan Wheeler.

Received for publication January 4, 2000.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 

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This Article Abstract Figures Only Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in ISI Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via ISI Web of Science (1) Citing Articles via Google Scholar Google Scholar Articles by Reuter, R. J. Articles by Bell, J. C. Search for Related Content PubMed Articles by Reuter, R. J. Articles by Bell, J. C. GeoRef GeoRef Citation Agricola Articles by Reuter, R. J. Articles by Bell, J. C. Related Collections Wetland Soils Soil Classification and Mapping Pedology