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DIVISION S-6—SOIL & WATER MANAGEMENT & CONSERVATION

Manganese Distribution and Patterns of Soil Wetting and Depletion in a Piedmont Hillslope

^a Dep. of Soil Science, North Carolina State Univ., Raleigh, NC 27695-7619
^b Dep. of Soil Science, Isfahan Univ. Of Technology, Isfahan, Iran

* Corresponding author (keith_cassel{at}ncsu.edu)

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION SUMMARY REFERENCES

 
The distribution of Mn in soils across the landscape is a function of mineralogy, topography, vegetation, and soil properties that control soil water movement and solute transport. We hypothesized (i) that current landscape properties and processes would explain the observed distribution of acid-extractable soil Mn in a cropped hillslope in the Carolina Slate Belt of the southern Piedmont, and (ii) that the current spatial patterns of soil water movement would be related to the observed Mn distribution. Soil samples were collected in 10-cm increments to the 1-m depth at 5-m intervals along 110-m-long Transect AB and at 10-m intervals along 80-m-long Transect CD. The air-dried samples were analyzed for acid-extractable (1 M HCl) Mn. Soil water content () along the transects was periodically monitored by neutron attenuation to the 120-cm depth. Duplicate banks of tensiometers were installed at depths of 30, 45, and 60 cm at these locations. The concentration of acid-extractable soil Mn was greatest in the footslope (FS), exceeding values >500 and 600 mg kg^-1 soil for Transects AB and CD, respectively. For a wet period in July 1989, along Transect AB varied from 0.40 m³ m^-3 for the FS to 0.50 m³ m^-3 at the summit and was significantly correlated with clay content. Increases in water content of a dry soil after rainfall of 80 mm in July 1989 were similar at all landscape positions, but further increases following additional rainfall were less for the FS, indicating that soil at the FS was already near saturation. The slightly coarser-textured FS consistently had the lowest water contents. Tensiometric and lateral Br transport data for this site, coupled with the water content measurements, indicate that subsurface flow of water and dissolved Mn from higher to lower elevations on the hillslopes is occurring.

Abbreviations: BS, backslope • FS, footslope • K_sat, saturated hydraulic conductivity • RB, recharge basin • SH, shoulder • SU, summit • , soil water content

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION SUMMARY REFERENCES

 
THEORETICAL AND FIELD STUDIES suggest that lateral subsurface water flow contributes to the accumulation of water at the lower positions on landscapes and hillslopes (Hewlett and Hibbert, 1963; Zaslavsky and Sinai, 1981a,b; Hanna et al., 1983). In the southern Piedmont region, it is common in late winter and early spring to observe seeps at the backslope (BS) and FS positions. Indeed, higher corn (Zea mays L.) yields at the FS position in Piedmont soils have been attributed to the occurrence of lateral subsurface water flow from the higher to lower landscape positions (Daniels et al., 1987).

Topographic position and soil physical characteristics have major impact on short- and long-term water balance. Soil water storage at any given time is influenced both by present and previous processes of water infiltration, runoff, evapotranspiration, and internal drainage. Hanna et al. (1982)(1983) reported that the FS and BS positions of a Mollisol in Nebraska retained an average of 4 cm (equivalent depth) more (total) water than soils on the SU and shoulder (SH) positions. Sinai et al. (1981) found strong correlation between and soil surface curvature for Israeli soils; water contents at concave landscape positions were about two times greater than water contents at other landscape positions.

Lateral transport of Br^- has been reported in Piedmont soils (Bruce et al., 1985; Bathke et al., 1992). Manganese is a mobile component of soil systems and is often found in higher concentrations at the lower positions on the landscape (Yaalon et al., 1972; Gilkes and McKenzie, 1988). For a Piedmont landscape in North Carolina, McDaniel and Buol (1991) found greater Mn concentrations in the surface horizons at the FS and concave positions. The FS position (see Schoeneberger et al., 1998) is concave in shape and often receives water, solutes and solids from the SH and BS positions (see diagrams in Fig. 1) . In some cases a level to nearly level toeslope adjoins the FS at a lower elevation. The BS position occurs at the steepest position on the landscape, is erosional, and is the most unstable position. The SU is relatively flat, occurs at the highest elevation in the landscape, and is considered to be the most stable position. For the transect shown in Fig. 1A, the SH is so small that it is not shown, but is included with the SU. The closed depression near the SU in Fig. 1A is a depression focused recharge basin (RB).

View larger version (26K): [in this window] [in a new window]   Fig. 1. (A) Elevation and landscape positions along Transect AB, (B) elevation and landscape position along Transect CD, and (C) saturated hydraulic conductivity, Ksat (m s-1), along Transect AB. (FS = footslope, BS =backslope, SU = summit, RB = recharge basin, and SH = shoulder).

	MATERIALS AND METHODS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION SUMMARY REFERENCES

 
The study was conducted from May 1989 through February 1991 in a farmer-managed field in the Carolina Slate Belt located in Orange County, North Carolina (36° N lat., 79° W long.). On this region the landscape is rolling and interfluves are irregular (Daniels et al., 1999). Valley sides are short and topographic breaks such as knolls and saddles are common. The water tables can be as deep as 10 m or more below the surface, but perched water tables often occur. The entire experimental area was mapped as Georgeville soil (fine, kaolinitic, thermic Typic Kanhapludults). The soil is variable and small inclusions of Tetotum (fine-loamy, mixed, semi-active, thermic Aquic Hapludults) often occur in the FS positions, but are not delineated. These soils developed from residuum and saprolite that underlay the field at depths exceeding 2 m. The field had been intensively farmed and eroded phases were common throughout the field. Continuous dryland corn silage with a wheat (Triticum aestivum L.)–clover (Trifolium pratense L.)–oat (Avena sativa L.) mixture of silage winter-cover crop was the cropping sequence during the study and for many years preceding it. The field was disked 1 d prior to seeding corn (cv. ‘NK-PX89’) on 29 May 1989 and 1 June 1990 (cv. ‘Pioneer 3154’). Corn rows were spaced 90 cm apart and were oriented in the up and down slope direction.

Two transects normal to the contour were selected. Transect AB (Fig. 1A and Table 1) was 110 m long and was divided into 22 contiguous plots, each being 5 m long and 2.7 m (four corn rows) wide. Transect CD (Fig. 1B and Table 1) was 80 m long and was divided into eight contiguous plots, each being 10 m long and 2.7 m wide. The two transects occurred in a watershed that occupied <2 ha. For Transect AB a minimum of three contiguous plots fell within each hillslope position and the FS terminated in a grassed waterway. Transect CD was dominated by the BS and terminated in a nongrassed drainage way. No drain tile was installed at either FS.

Soil water content was measured by neutron attenuation at 20-cm depth intervals to 120 cm from June 1989 through February 1991, but we concentrated our effort during the summer when corn was actively growing. To facilitate these measurements, we visited the site at least three times weekly during the corn-growing seasons, but less often during fall, winter, and early spring. This effort provided 16 data sets that were taken within 14 h after major rainfalls during the 1989 cropping season, and between February 1990 and February 1991. If was determined 1 or 2 d before a rainfall, and we obtained a set of neutron readings within 14 h after rainfall stopped, then we included the data in the analysis. Dates for these measurement intervals are listed in Table 2 and in all cases was measured on the last date of each time interval. Changes in soil water content ( in m³ m^-3) as a function of soil depth and hillslope position were calculated using

[1]

where i, j, and k refer to soil depth interval, location on the transect, and measurement time, respectively. Net water-depletion periods selected were those with the longer continuous periods without rainfall to minimize errors in computing the changes in using Eq. [1].

Conventional analysis of variance methods are of limited use in the analysis of transect data (Marcia Gumpertz, personal communications, 1995). This is because landscape positions on landscapes cannot be randomized—we have to accept them as they occur in nature. Moreover, individual soil samples taken within each landscape position cannot be treated as independent replications. For these reasons no attempt was made to statistically analyze increases in and depletion as functions of soil depth and hillslope position. Rather, the water content differences for selected time intervals, coupled with previously published tensiometric and Br^- transport data for these two transects were plotted and interpreted with respect to hillslope position and soil depth. Data in the figures were plotted using the contour option of Coplot and Surfer (Golden Software, Inc., Golden, CO).

	RESULTS AND DISCUSSION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION SUMMARY REFERENCES

 
We begin by presenting soil physical property data for the different hillslope positions along the transects, followed by information on the measured acid-extractable Mn distribution as functions of depth and hillslope position. Changes in for several wetting and depletion events are presented. Finally, reference is made to previously published Br^- transport and tensiometric data for these two transects (Afyuni et al., 1993, 1994).

Soil Physical Properties
Transect AB has a total change in elevation of 4 m and the Ap horizon is thickest at the RB and at the FS position (Table 1), where deposition of soil material eroded from the upslope positions has occurred. In general, the texture is silt loam in the FS and gradually grades to clay at the RB (Table 1). The silt content throughout the transect is high, ranging from a maximum of 54% in the Ap of the FS to a minimum of 28% in the Ap of the RB. Except for the SU, clay content increases with depth. In general, saturated hydraulic conductivity (K_sat) in Transect AB decreases with depth at all positions (Fig. 1C). The low K_sat associated with the Bt horizons would encourage lateral water transport when this sloping soil (8% slope for the BS) approaches saturation. Bulk density is generally highest for the Ap horizon in Transect AB, but not CD (Table 1). The chroma values reported in Table 1 indicate that the soil at the FS in Transect AB is aquic and classified at Tetotum.

Transect CD (Fig. 1B) is shorter, has an elevation change of 5 m (Table 1), and has a slope of 8.1% for the BS. Thickness of the Ap horizon ranges from 37 cm at the FS to 10 cm at the SH. Soil texture is coarsest at the FS and becomes finer with increase in elevation. Silt content at the FS of Transect CD is similar to that for Transect AB, but the clay content decreases with depth. In general, the clay content and bulk density of Transect CD are higher at the FS compared with Transect AB. The K_sat values decrease with depth at all three landscape positions (Afyuni et al., 1993).

Manganese Distribution
Large differences in the concentrations of acid-extractable soil Mn, hereafter referred to as soil Mn, occurred with depth and landscape position for each transect (Fig. 2A,B) . For Transect AB, the soil Mn content exceeded 400 mg kg^-1 soil in the upper 20 cm in the FS and exceeded 500 mg kg^-1 soil in the soil surface at the lowest elevation of the FS. A second region where soil Mn content exceeded 400 mg kg^-1 was at the RB. Based on elevation changes along Transect AB, both surface and subsurface water flow would be expected to converge on these two positions. Thus transport of soluble soil Mn to lower elevations during periods of greatest soil water pressures (presence of free water) would be expected. The greater soil Mn content observed at the FS in Transect AB occurs where low chroma was also measured (Table 1) and indicates that the soil remains wet for long periods. Other investigators have found that the largest Mn contents on landscapes typically occur at the wetter FS positions in response to lateral transport of soluble Mn (McDaniel and Buol, 1991).

View larger version (34K): [in this window] [in a new window]   Fig. 2. Acid-extractable soil Mn distribution (mg kg-1) as a function of soil depth and hillslope position for (A) Transect AB and (B) Transect CD.

The large concentrations of soil Mn at the FS of both transects, and at the RB for Transect AB, are attributed to the lateral transport of water over an extended time period. The measured differences in K_sat with depth along each transect (Table 1) also support the process of lateral transport. The concentration of soluble Mn would be highest at the FS or recharge position where flowing water converges. When water from the FS was utilized by vegetation, the secondary Mn precipitated thus increasing the acid-extractable Mn content. The concentration patterns with respect to soil depth and position along the transect are similar to those reported by Biswas and Gawande (1964), Yaalon et al. (1972), and McDaniel and Buol (1991). McDaniel et al. (1992) observed Mn nodules at the FS position where Mn accumulated and precipitated when the soil dried. The small K_sat of the Bt1 and Bt2 horizons, combined with the slope gradient at higher hillslope positions would favor lateral flow of water and soluble Mn from the higher to lower elevations. The slightly coarser texture of the soil at the FS position, combined with accumulation of water from the upslope positions, would promote percolation of water and result in accumulation of Mn deeper in the profile. A similar pattern, but on a smaller scale, was observed at the RB, suggesting short distance lateral transport of water into this position as well.

Soil Water Content
Having established that soil Mn concentrations were larger at the lower hillslope positions, we evaluated the measured patterns of soil water changes. The extremes in (m³ m^-3) for each transect as a function of soil depth and hillslope position occurred on 19 July and 15 Sept. 1989 (Fig. 3) . The wettest conditions occurred on 19 July 1989 after 130 mm of rainfall over a 9-d period (Table 2). Because at 60 sites on the two transects could not be measured instantaneously at the cessation of rainfall, it is possible that at some depths reached higher values than those measured. Evaporative water loss during the humid 12-h period of water content measurement on 19 July is estimated to be <2 mm.

View larger version (33K): [in this window] [in a new window]   Fig. 3. Soil water content, (m3 m-3), as a function of depth and hillslope position after a 130-mm-rainfall event in mid July 1989 and in mid September following an 18-d-long rainless period with high evaporative demand for Transect AB (A, B) and Transect CD (C, D).

Measured soil water contents for both transects were driest (Fig. 3B,D) on 15 Sept. 1989 following 18 d without rainfall (Table 2). The relative differences in with respect to hillslope position and soil depth on this date were similar to those reported for equivalent depth of water (cm cm^-1) for each transect on 19 July (Afyuni et al., 1993). They found a significant correlation (P = 0.01) between equivalent depth of soil water retained in the upper meter of the soil and the mean clay percentage in the upper meter of soil; correlation coefficients were 0.92 and 0.79 for Transects AB and CD, respectively. The least water contents occurred at the FS where clay contents were small, in contrast to the large water contents at the SU and SH where clay contents were greater.

Soil Wetting After Rainfall
Increases in along Transect AB for three significant rainfall events are shown in Fig. 4 . The patterns observed for changes in were a function of landscape position and antecedent water content of the soil profile. Using the relatively dry soil condition on 7 July 1989 as a reference, the mean change in following an 80-mm rainfall from 7 July to 14 July was 0.072 m³ m^-3 (equivalent to 75 mm of precipitation) and ranged from 0.05 to 0.11 m³ m^-3 (Fig. 4A). The greatest increase in occurred at the nearly level SU and RB. If runoff at the SU occurred, some of it would flow into the RB. The smaller increase in at the FS position, however, might result from more rapid drainage out of the coarser-textured and more permeable soil at this position. The FS terminated in a grassed waterway normal to the transect which could have transported some subsurface water away from this position. An additional 50 mm of rainfall between 14 July and 19 July resulted in little change in except at the FS (Fig. 4B). The soil was essentially saturated at all positions except the FS on 14 July when rainfall began. The slope gradient and the low K_sat of the Bt horizon are sufficient to promote lateral flow toward the FS.

View larger version (33K): [in this window] [in a new window]   Fig. 4. Net soil water increase (m3 m-3) for Transect AB as a function of soil depth and hillslope position for (A) 7 July to 14 July 1989 and (B) 12 Mar. to 4 Apr. 1990.

The wetting pattern for Transect CD was similar to that for Transect AB. The increase in between 7 July and 14 July was smallest at the FS position (Fig. 5A) . Texture of the Bt1 was only slightly coarser at the FS than at the BS position, and by itself, cannot account for the large difference in water stored during the 7-d period. The increases in between 14 July and 19 July were 0.02 m³ m^-3 or less and occurred primarily at the FS and SH positions (Fig. 5B). Little change in occurred between 12 March and 4 April.

View larger version (32K): [in this window] [in a new window]   Fig. 5. Net soil water increase (m3 m-3) for Transect CD as a function of soil depth and hillslope position for (A) 7 July to 14 July 1989 and (B) 12 Mar. to 4 Apr. 1990.

View larger version (37K): [in this window] [in a new window]   Fig. 6. Net soil water depletion (m3 m-3) for Transect AB as a function of soil depth and hillslope position for (A) 19 to 21 July 1989, (B) 18 Aug. to 15 Sept. 1989, and (C) 15 Jan. to 22 Feb. 1991.

During the 18 August to 15 September period of moderate evapotranspiration by the corn crop (Table 2), soil water was depleted primarily from the upper 40 cm of the profile (Fig. 6B). Reductions in ranging from 0.05 to 0.09 m³ m^-3 occurred at this depth, with the maximum losses occurring at the FS-BS boundary (Fig. 6B). For this time period no definite pattern of water depletion with respect to hillslope position or soil texture was observed.

The pattern of net water depletion for Transect AB during the mid-winter period from 15 Jan. to 22 Feb. 1991 with low evapotranspiration by the cover crop (Fig. 6) is similar to that shown in Fig. 6B. Again, no definite conclusion with respect to lateral movement of water can be made for this period. Water depletion results for Transect CD are similar to those for Transect AB and are not shown.

	SUMMARY

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION SUMMARY REFERENCES

 
Water transport in soils in the southern Piedmont is complex and often gives rise to intermittent droughty or excessively wet areas, depending on landscape position. Crop response is highly variable on these landscapes and is highly correlated with the soil water regime (Stone et al., 1985). The observed acid-extractable Mn content distribution along two transects indicates that the net transport of water over time has carried soluble Mn to lower elevations on the landscape where it has precipitated out of solution. Using data collected both spatially and temporally, we present evidence that the changes in as a result of rainfall and depletion vary with antecedent and hillslope position. Given the constraint in data collection, that is, the inability to measure the maximum values at 60 locations for the two transects at the moment rainfall ceased, we still were able to document a wide range in spatial and temporal soil water contents throughout the year. The measurements, and K_sat, Br^- transport, and tensiometric data lend solid support to the hypothesis that the present soil water regime is consistent with long term transport of soluble soil Mn from the higher elevations to the FS position where it accumulated over time.

The slightly coarser-textured soil material at the FS generally had a higher net increase in water content following rainfall. Data also suggest that the FS had the lowest net water depletion during periods when crops were actively transpiring. Corn grain yields measured along the transects support this idea. Mean corn grain yields reported by Afyuni et al. (1993) for the 2-yr study for Transect AB were 11.5, 10.5, 10.5, and 9.2 Mg ha^-1 for the FS, BS, SU, and RB, respectively, and 12.5, 11.2, and 6.5 Mg ha^-1 for the FS, BS, and SH, respectively, for Transect CD. The BS position had the least recharge during rains and was subject to earlier depletion of soil water when crops were transpiring. Lateral subsurface water flow, which is very difficult to quantify, plays a major, but as yet unquantifiable, role in the complex water and solute transport processes and their interactions with crop growth in these soils.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION SUMMARY REFERENCES

 
Contribution from the Dep. of Soil Science, North Carolina State Univ., Raleigh, NC.

Received for publication March 30, 2001.

	REFERENCES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION SUMMARY 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 Cassel, D. K. Articles by Robarge, W. P. Search for Related Content PubMed Articles by Cassel, D. K. Articles by Robarge, W. P. GeoRef GeoRef Citation Agricola Articles by Cassel, D. K. Articles by Robarge, W. P. Related Collections Watershed and Landscape Processes Soil Physics Vadose Zone Processes and Chemical Transport