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Published online 27 February 2006
Published in Soil Sci Soc Am J 70:631-642 (2006)
DOI: 10.2136/sssaj2005.0185
© 2006 Soil Science Society of America
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Soil & Water Management & Conservation

Soil Physical and Biological Properties as Affected by Land Leveling in a Clayey Aquert

Kristofor R. Brye*, Nathan A. Slaton and Richard J. Norman

Dep. of Crop, Soil, and Environmental Sciences, Univ. of Arkansas, Fayetteville, AR 72701

* Corresponding author (kbrye{at}uark.edu)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Land leveling is a government-subsidized, water-conserving agricultural practice common in the Mississippi River Delta region of the midsouthern United States. Though practiced to more uniformly deliver irrigation water, land leveling is a severe soil disturbance that alters the quasi-equilibrium among near-surface soil biogeochemical properties. The objectives of this study were to characterize the short-term impacts of land leveling on the magnitude, variance, and spatial variability and distributions of soil physical and biological properties and to evaluate the impact of land leveling on the relationships among soil properties in a Sharkey clay (very-fine, smectitic, thermic Chromic Epiaquert) used for irrigated soybean [Glycine max (L.) Merr.] and rice (Oryza sativa L.) production in the Mississippi Delta region of northeast Arkansas. Bulk density and clay increased (P < 0.001), while sand, silt, fungal biomass, and fungal/bacterial biomass ratio decreased (P < 0.001) as a result of land leveling. Despite the effect on the fungal/bacterial biomass ratio, hence a significant alteration of the soil microbial community structure, soil bacterial biomass was unaffected by land leveling. The variance associated with silt increased (P < 0.01), while the variance for fungal biomass and the fungal/bacterial biomass ratio increased (P < 0.001) as a result of land leveling. Spatial variability and distributions of soil physical and biological properties and relationships among them were noticeably altered by land leveling. Increased variations in soil physical and biological properties as a result of any soil disturbance will make uniform field management difficult. Compared to a similar study, the magnitudes of near-surface soil physical and biological property change as a result of land leveling were lower on a clayey Vertisol than on a silt-loam Alfisol.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
IN THE 2002 FARM BILL, the United States Congress reauthorized substantial financial assistance in the form of government subsidies to producers willing to adopt water-conserving agricultural practices (USDA, 2002). Land leveling (i.e., precision leveling or precision grading) is considered a water conservation practice and is relatively common in the south-central United States. Land leveling creates a slight, but uniform, slope gradient to facilitate more even distribution of irrigation water and is routinely performed in fields where crops such as rice and soybean are grown. In eastern Arkansas, where more than 45% of all rice in the United States is produced (Agricultural Statistics Board, 2005), it is estimated that between 28 000 and 33 000 ha of cropland are land leveled annually. In some Arkansas rice-producing counties, as little as 5%, while in other counties as much as 92% of the land area has already been leveled (Young et al., 2004). The potential agronomic benefits of land leveling, including the ability to flood-irrigate previously nonlevel soils, increased irrigatable acreage, improved distribution of irrigation waters, soil and water conservation, and improved uniformity of crop growth and yield, have been recognized for more than half a century (Whitney et al., 1950), but land leveling also has disadvantages.

Though recognized and subsidized as a water conservation practice, land leveling is a severe soil disturbance (Brye et al., 2005). Nearly the entire equilibrium among soil physical, chemical, and biological properties is disrupted or altered during the process of land leveling (Brye et al., 2003; 2004c). Land leveling can result in major changes in surface soil pH, decreased organic C, and exposure of highly acidic subsoil (Miller, 1990). Subsoil exposure following land leveling has also been shown to result in numerous nutrient deficiencies, particularly N and P (Whitney et al., 1950; Eck, 1987; Robbins et al., 1997, 1999). Land leveling has been shown to significantly alter magnitudes and variability of numerous soil physical and chemical properties in a silt-loam soil (Brye et al., 2003; 2004c) and result in hard-pan migration toward the soil surface that can decrease the volume of soil a subsequent crop is grown in by as much as 25% (Brye et al., 2005). Eck (1987) reported that the physical characteristics of exposed subsoil remained relatively unchanged 16 yr after topsoil removal and suggested that restoration of productivity would be difficult. Unger et al. (1990) reported that the clay content of a clay loam was higher in areas within the field that were cut than in areas that were filled and that the textural class changed from a clay loam in cut areas to a silt loam in filled areas.

Aside from affecting soil property magnitudes, Miller (1990) speculated that the near-surface spatial variability of certain soil properties in eastern Arkansas soils cropped to rice was affected by land leveling and that this variability was related to postleveling variability in crop growth. Similarly, spatial variability of soil properties has been implicated in nonuniform growth of tropical lowland rice in the Philippines (Dobermann et al., 1995, 1997). In addition to affecting crop production, changes in soil physical and chemical properties as a result of land leveling also impacts soil biological properties (Brye et al., 2003).

Soil biological properties are intimately related to the soil physiochemical environment and are as important in contributing to soil tilth as soil physical and chemical properties. Parkin (1993) and Wardle (1995) suggested that spatial distributions of soil microbial biomass are still not completely understood in terrestrial ecosystems, especially in agroecosystems under various tillage practices. In addition, the role that biological properties play in assessing soil quality is also poorly understood and, therefore, has not been addressed sufficiently (Parr et al., 1992; Turco et al., 1994). In response to shallow-cut land leveling activities in a silt loam, Brye et al. (2003, 2004c) demonstrated significant alteration of the magnitudes and spatial distributions of fungal and bacterial biomass. Soil biological properties may be as important as physical and chemical properties in restoring leveled fields to preleveling conditions and expected crop productivity levels (Brye et al., 2004b). Walker et al. (2003) reported significant rice yield declines in cut compared to filled areas on recently leveled clay soils in Mississippi.

Rice production in the United States is somewhat unique compared to other cereal grains in that rice is typically grown for an extended period in flooded-soil conditions. Consequently, clay soils with low surface and subsurface hydraulic conductivities are desirable because slowly permeable soils have lower vertical water fluxes, thus facilitating ponded water and maintenance of flooded-soil conditions. However, much of the previous research evaluating the effects of land leveling has been conducted on somewhat coarser clay- and silt-loam soils with little attention given to clay-soil response to land leveling in areas of concentrated rice production. In Arkansas alone, approximately 41% of the total rice production occurs on soils with a clay or clay-loam surface texture (Wilson and Branson, 2004). The frequency of land leveling of clay soils has substantially increased in the past few years; a trend that will likely continue into the near future given the recent movement of rice to alluvial soils near plentiful surface water and the increased availability of government-sponsored subsidies for water conservation practices. Therefore, evaluating soil property change and how a subsequent crop will respond to altered and potentially more variable soil properties will improve management capabilities to ensure maximum or near-maximum production from graded fields.

The objectives of this study were to characterize the short-term effects of land leveling on (i) the magnitudes, variance, and spatial variability and distributions of selected soil physical and biological properties, and (ii) the relationships between soil physical and biological properties in a Vertisol of the Mississippi Delta region in eastern Arkansas that is commonly used for rice production. Similar to previous results of shallow-cut land leveling on a silt-loam soil (Brye et al., 2003), it was hypothesized that land leveling significantly alters the magnitude, variance, and spatial variability and distribution of soil bulk density, particle-size fractions, and fungal and bacterial biomass and that land leveling significantly alters the relationships between soil physical and biological properties of a clay-textured Vertisol.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Site Description and Experimental Design
In fall 2003, a 4.9-ha field, previously cropped to soybean, on Sharkey clay soil (Ferguson and Gray, 1971; Soil Survey Staff, 2005) at the Northeast Research and Extension Center in Keiser, AR (35°40' N, 90°4' W), was chosen as the study site. The Sharkey clay is a very deep, poorly and very poorly drained, very slowly permeable soil that formed in clayey alluvium. It is typically located on flood plains and low terraces of the Mississippi River and is a component of Major Land Resource Area 131A (Soil Survey Staff, 2005). The Sharkey clay is commonly used for rice production in northeast Arkansas, Mississippi, and Louisiana. The field was slightly sloped (<2%), such that irrigation water flowed from East to West, and contained <15-cm-tall, north-south–oriented raised beds spaced every 12 m throughout the field. Before land leveling, a 40-m-wide by 90-m-long study area containing a 50-point grid was established with sampling points spaced evenly 10 m apart. The grid-sampling approach employed in this study is similar to that used in several previous studies on a silt-loam soil to ascertain the effects of land leveling on soil properties and crop response (Brye et al., 2003, 2004a, 2004b, 2004c, 2005).

Study Site Manipulations
Land leveling activities occurred over a 3-d period in mid-April 2004. Following initial land leveling activities, the entire field was disked and land planed numerous times to reduce soil clod size to an approximate diameter of <2 cm. Land leveling altered the surface drainage from the original east-to-west orientation to a south-to-north orientation.

Soil Sampling and Measurements
Immediately before and following land leveling, relative elevation was measured and soil samples were collected from the top 10 cm at each of the 50 grid points in the study area to characterize elevational and soil physical and biological property changes as a result of land leveling. Elevational changes were measured using a laser level and stadia rod. A single 4.8-cm-diameter soil core was collected from the 0- to 10-cm depth within a 20-cm radius surrounding each grid point, oven dried at 70°C for 48 h, and weighed for bulk-density determination. The soil-core sampling chamber was beveled to the outside to minimize compaction on sampling. Measured bulk density values were qualitatively compared to predicted values based on regression relationships including sand and clay percentages (Saxton et al., 1986; Juma, 2002). Oven-dry soil was subsequently crushed and sieved to pass a 2-mm mesh screen for particle-size analysis using a 2-h hydrometer method (Arshad et al., 1996). A second set of samples consisting of ten 2-cm diameter soil cores were collected and composited from the 0- to 10-cm depth from within the same 20-cm radius surrounding each grid point. Samples were kept cool and within 1 d of collection were sent to the Soil Foodweb (Soil Foodweb, Inc., Corvallis, OR) for total fungal and bacterial biomass determinations.

Total fungal biomass concentration was determined using methods described by Ingham and Klein (1984). For each sample, a suspension was prepared from fresh soil mixed with 90 mL of 60 mM phosphate buffer (pH 7.6). One-milliliter aliquots were removed, stained for 3 min with 1 mL of 20 µg mL–1 fluorescein diacetate (FDA) solution in 60 mM phosphate buffer, and filtered. One milliliter of 1.5% agar in distilled water was added to a 1-mL FDA suspension in phosphate buffer and mixed. An aliquot was placed on a microscope slide of known dimensions. Total fungal biomass was estimated from stained hyphal lengths and diameters determined by direct microscopic examination of the soil–agar film in three transects of 20 fields per slide using phase-contrast microscopy.

Total bacterial biomass concentration was determined using methods described by Babiuk and Paul (1970). Twenty grams of fresh soil were mixed with 190 mL of sterile distilled water and dispersed for 2 min. Aliquots were placed within a 1-cm2 area of a microscope slide, allowed to air dry, and slightly fixed with heat. Soil smears were stained with fluorescein isothiocyanate for 2 min and washed. Stained soil smears were immediately mounted in glycerol and observed using phase-contrast microscopy. Total bacterial biomass was estimated from the number of bacteria and their mean diameter and length per field. Soil fungal and bacterial biomass concentrations, expressed on a mass-per-mass basis [µg (g dry soil)–1], and measured bulk densities were used to calculate microbial biomass contents and expressed on a mass-per-area basis (g m–2).

Statistical Analyses
Paired t tests were performed to determine the overall effect of land leveling on soil bulk density, particle-size fractions, bacterial and fungal biomass concentration and content, and fungal/bacterial biomass ratios (Minitab Version 13.31, Minitab Inc., State College, PA). Pearson linear correlations were performed to ascertain relationships among soil physical and biological properties, elevational changes, and estimated amounts of manipulated soil, which were calculated using preleveling bulk densities and measured elevational changes. Pre- and postleveling soil properties were pooled and linear regression was performed to ascertain general relationships among soil physical and biological properties in a clay-textured Aquert.

Land leveling effects on the variability and spatial distributions of soil physical and biological properties studied were determined by several methods. Pre- and postleveling homogeneity of variance was evaluated using Levene's test (Levene, 1960). Geostatistical analyses were also conducted (GS+ Version 5.1, Gamma Design Software, Plainwell, MI). Only isotropic semivariograms were considered and semivariance parameters of the best-fitting (i.e., smallest residual sum of squares) spherical, exponential, or linear models were reported for pre- and postleveling soil properties. In addition, multiple regression analyses were performed using relative spatial coordinates (x, y, xy, x2, and y2) as simultaneously fit model parameters to evaluate the effect of land leveling on spatial variability ignoring potential spatial correlations (Minitab).

Changes in the spatial distributions of soil physical and biological properties were determined by mapping pre- and postleveling soil properties using Surfer (Version 7, Golden Software, Inc., Golden, CO). Point kriging with no search radius was used as an unbiased, weighted linear interpolation method that minimizes total parameter variance by incorporating semivariogram functions to create contour maps (Isaaks and Srivastava, 1989). Only the linear semivariogram function for all parameters was used to facilitate uniform mapping and comparisons among all soil properties.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Land leveling, whether shallow or deep, represents a significant and severe form of soil disturbance (Brye et al., 2004c, 2005). The immediate disruption of a previous quasi-equilibrium that land leveling activities impart on the soil can have lasting negative effects on soil properties (Brye et al., 2003, 2004c, 2005) and crop production, particularly for rice and soybean (Brye et al., 2004a, 2004b), that are not easily reversed solely with additional inorganic fertilizers.

In this study, land leveling resulted in an average surface elevational change of –0.13 m (i.e., an overall cut), ranging from +0.03 m (i.e., a fill) to –0.29 m, across the 0.36-ha study area (Fig. 1 ). A maximum cut of nearly 0.3 m in this study represents a fairly common and moderate soil manipulation for the land leveling activities in the region. More severe cuts of ≥1 m occur in northeast Arkansas on clay soils, but are less common. Based on the elevational change, land leveling resulted in an average of –1511 Mg ha–1 of soil manipulated throughout the study area (Fig. 1). Consequently, land leveling significantly affected the magnitude, variability, and spatial distribution of selected soil physical and biological properties.


Figure 1
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  		Fig. 1. Surface elevation changes and estimated amounts of soil manipulated throughout a 0.36-ha study area as a result of land leveling a clay soil in northeast Arkansas. Positive (+) values indicate increased elevation or deposited soil. Negative (–) values indicate decreased elevation or removed soil.

 
Soil Physical Properties
Before land leveling, soil bulk density ranged from 0.93 to 1.24 g cm–3 and averaged 1.12 (SE = 0.01) g cm–3 over the 0.36-ha sampling area (Table 1). Soil particle size distributions ranged from 0.16 to 0.29 kg kg–1 and averaged 0.21 (SE = 0.01) kg kg–1 for sand (0.02- to 2-mm diameter), from 0.22 to 0.25 kg kg–1 and averaged 0.24 (SE < 0.01) kg kg–1 for silt (0.002 to 0.02-mm diameter), and from 0.48 to 0.61 kg kg–1 and averaged 0.55 (SE < 0.01) kg kg–1 for clay (<0.002-mm diameter; Table 1). All grid points throughout the study area had a clay texture in the top 10 cm.


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  		Table 1. The effect of land leveling on soil physical and biological properties from the 0- to 10-cm depth of a clay soil. Mean values (± SE) are reported (n = 50).

 
Avoiding compaction from soil sticking to the inside of the sampling chamber while employing the soil-core method for bulk density determination in clay soils can be quite challenging. However, 68% of preleveling measured bulk density values fell within 10% of the predicted bulk density and 28% of measured values fell within 5% of the predicted value based on regression relationships with sand and clay percentages (Saxton et al., 1986; Juma, 2002).

Preleveling range parameters from best-fit (i.e., smallest residual sum of squares) semivariogram models were >165 m for sand and clay, indicating spatial autocorrelation among sampling points at the uniform 10-m spacing and that these properties were not truly independent within the sampling area (Table 2). The range parameter was 47 m for bulk density, indicating some degree of spatial independence within the study area (Table 2). In contrast, with a linear best-fit semivariogram, the uniform 10-m spacing was too large to ascertain a spatial dependency for silt within the sampling area. Multiple regression analyses with relative spatial coordinates (x, y, xy, x2, and y2) support this geostatistical result with no spatial factor being significant in the model for silt (Table 3). The spatial component [i.e., the C/(C0 + C) column in Table 2] explained <50% of the variation in silt, 80% of the variation in bulk density, and >90% of the variation in sand and clay. Multiple regression analyses also support these geostatistical results with at least two spatial factors being significant in each model (Table 3).


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  		Table 2. Summary of geostatistical parameters for soil physical and biological properties measured before and after land leveling a clay soil.

 

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  		Table 3. Multiple regression summary of the significance of relative spatial coordinates at explaining pre- and postleveling soil property variability. All model parameters were fit simultaneously and in the following order: x, y, xy, x2, y2.

 
Following land leveling, mean bulk density increased 12% (P < 0.001) to 1.25 (SE = 0.01) g cm–3 (Table 1). Mean clay increased 6% (P < 0.001) to 0.61 (SE = 0.01) kg kg–1, while mean sand and silt decreased 3% each (P <0.001) to 0.18 (SE = 0.01) and 0.21 (SE = 0.01) kg kg–1, respectively (Table 1). Similar to preleveling values, 88% of postleveling measured bulk density values fell within 10%, and 68% of measured values fell within 5% of the predicted bulk density value (Saxton et al., 1986; Juma, 2002). Although land leveling significantly altered individual soil particle size fractions, all grid points retained a clay texture in the newly exposed or deposited top 10 cm due to a similarly clayey, deep, and uniform subsoil of alluvial parent material. Although the coefficient of variation (CV) decreased 16% following land leveling for bulk density and increased between 7% for clay and 102% for silt (data not shown), only the silt variance ({sigma}) was significantly affected (P < 0.01), which decreased from 0.012 before leveling to 0.007 after leveling (Table 4). In addition to the significant effect on overall soil physical property means, land leveling also significantly affected their spatial variability.


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  		Table 4. Effect of land leveling on the variance ({sigma}) of soil physical and biological properties from the 0- to 10-cm depth of a clay soil. Asterisks next to postleveling values represent significant differences in the variance of pre- and postleveling soil properties.

 
For bulk density and clay, the same preleveling semivariogram model was the best-fit model after land leveling (Table 2). However, the range parameter increased 350% for bulk density, indicating that after leveling bulk density values were autocorrelated and no longer somewhat independent within the study area, while the range parameter for clay did not change (Table 2). Silt, which was spatially autocorrelated before leveling, achieved some degree of independence with a range parameter of 52 m following land leveling. Though the best-fit semivariogram model changed, sand remained spatially autocorrelated within the study area following land leveling. The amount of variation explained by the spatial component following land leveling increased 12% for bulk density and 15% for silt, decreased 19% for sand, and did not change for clay. Multiple regression analyses generally support geostatistical results with at least three spatial factors being significant in each model (Table 3). Though the uniform 10-m sample spacing may not have been adequate enough to consistently capture spatial dependency related to the variability of soil physical properties within the study area, the spatial distribution of similar bulk densities and particle-size fractions changed as a result of land leveling.

Numerous locations within the study area with low bulk densities before leveling had noticeably higher bulk densities after leveling as a result of either compaction due to machinery weight or the exposure of subsoil that had inherently higher bulk density than the original surface layer (Fig. 2 ). Similar changes in the spatial distributions occurred for sand, silt, and clay (Fig. 3 ). Clay most noticeable increased throughout most of the study area upon exposure of and mixing subsoil with slightly higher clay contents (Fig. 3).


Figure 2
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  		Fig. 2. Pre- and postleveling spatial distributions of soil bulk density from the 0- to 10-cm depth. The x-direction is east and the y-direction is north on the land surface.

 

Figure 3
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  		Fig. 3. Pre- and postleveling spatial distributions of soil particle-size fractions from the 0- to 10-cm depth. The x-direction is east and the y-direction is north on the land surface.

 
Significant alterations of soil surface physical properties due to land leveling can create potential agronomic problems that cannot be remedied solely by increased inorganic fertilizer applications. Elevated bulk densities following land leveling indicates that some degree of compaction occurred during the leveling process most likely from vehicular traffic, which may in turn negatively impact soil water-holding capacity and plant growth (Brye et al., 2003). Compacted soils have a lower total porosity, consequently they cannot hold as much water as less- or noncompacted soils. In this study, postleveling bulk density values were equal to or exceeded the 1.40 g cm–3 minimum root-restricting bulk density suggested by Arshad et al. (1996) for clay soils at only two of the 50 grid points. However, for rice production, which is the target crop following most land leveling activities, soil compaction, reduced water-holding capacity, and the possible promotion of soil crusting (Miller and Radcliffe, 1992; Valentin and Bresson, 1998) are lesser issues because of the way water is manipulated for rice production and in the flooded-soil environment. Alternatively, the non–flood-irrigated crops grown in rotation with rice, primarily soybean, may be negatively impacted by land leveling's significant effects on soil surface properties. However, the relatively high shrink-swell potential of the smectitic clays commonly present in Vertisols may help to alleviate potential postleveling production problems caused by soil compaction.

Soil Biological Properties
In contrast to soil physical properties and similar to the results of Brye et al. (2003) on a silt loam, soil biological properties in the top 10 cm were quite variable within the study area before land leveling (Table 1). Soil bacterial biomass concentrations ranged from 219 to 1678 µg g–1 and averaged 827 (SE = 51) µg g–1, and soil fungal biomass concentrations ranged from 14.4 to 123 µg g–1 and averaged 53.1 (SE = 3.4) µg g–1 before land leveling (Table 1). The mean fungal/bacterial biomass ratio, which provides an indication of the vegetation-associated microbial community structure was 0.09 (SE = 0.01). A ratio of this magnitude is substantially lower than the desired range reported for typical row-crop agricultural soils (e.g., 0.6–1.2; E.R. Ingham, personal communication, 2005). Microbial biomass contents were also quite variable, where bacterial biomass contents ranged from 23 to 180 g m–2 and averaged 92.9 (SE = 5.9) g m–2, and fungal biomass contents ranged from 1.5 to 13.9 g m–2 and averaged 5.9 (SE = 0.4) g m–2.

All measured or calculated microbial biomass parameters were spatially autocorrelated within the study area with range parameters ≥ 170 m before land leveling (Table 2). Consequently, the spatial component explained only 50% of the variation in soil biological properties. Multiple regression analyses support geostatistical results with only between 23 and 51% of the variation in soil biological properties being explained by the five spatial factors evaluated (Table 3).

Mean fungal biomass concentration and content and the fungal/bacterial biomass ratio decreased (P ≤ 0.001) by more than 50% following land leveling (Table 1). Since it is well known that organic matter concentration and microorganism numbers decrease with soil depth, decreases in soil biological properties following land leveling were likely simply due to the exposure of previous subsoil as a result of removing some thickness of the original topsoil during the leveling process. Bacterial biomass concentration and content did not change as a result of land leveling. The significant affect on the fungal/bacterial biomass ratio, despite the statistical no change with bacterial biomass, indicated that land leveling significantly altered the structure of the microbial community. These results support those of Brye et al. (2003) indicating that biological properties, especially fungal biomass, are sensitive to such extensive soil disturbance as results from land leveling and that restoring complete preleveling biological functioning may be challenging.

Soil biological property variability was also significantly affected by land leveling. Coefficients of variation increased between 25 and 56% for fungal and bacterial biomass concentrations and contents, but decreased 13% for fungal/bacterial biomass ratios (data not shown). Fungal biomass concentration and content and fungal/bacterial biomass ratio variance increased significantly (P ≤ 0.001) as a result of land leveling, while the bacterial biomass concentration and content variance did not change (Table 4). Similar to soil physical properties, land leveling significantly affected the spatial variability and distributions of soil biological properties.

Land leveling caused little change in the spatial structure of fungal biomass concentrations and contents as range parameters following land leveling remained ≥170 m and the spatial component explained ≤56% of their variation (Table 2). Multiple regression analyses support geostatistical results with only one spatial factor being significant in either parameter's model and the models only explained between 18 and 22% of the variation in fungal biomass (Table 3).

The range parameter for bacterial biomass concentrations and contents decreased by 78% to achieve some degree of spatial independence within the study area as a result of land leveling, and nearly all of the variability of these parameters was explained by the spatial component. This result was also supported by multiple regression analyses with the x and x2 spatial factors highly significant (P < 0.001) in each model and the models explained 91% of the variability in bacterial biomass (Table 3). Similarly, the fungal/bacterial biomass ratio achieved some degree of spatial independence as its range parameter decreased by 90% following land leveling, but the proportion of variability explained by the spatial component only increased from 50% before leveling to 58% following leveling; a result also supported by only an 6% increase from 40 to 46% in the amount of variability explained by multiple regression analyses (Table 3).

Similar to soil physical properties, the spatial distributions of soil biological properties were, based on visual comparison, significantly altered as a result of land leveling. However, spatial patterns of physical properties (Fig. 2 and 3) did not visually correlate well with spatial patterns of biological properties (Fig. 4 and 5) . Brye et al. (2003) reported that even slight variations in soil texture do not account for changes in soil structure as a result of land leveling, in which the disruption of soil structure by land leveling activities in turn impacts microorganism habitat.


Figure 4
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  		Fig. 4. Pre- and postleveling spatial distributions of soil microbial biomass contents from the 0- to 10-cm depth. The x-direction is east and the y-direction is north on the land surface.

 

Figure 5
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  		Fig. 5. Pre- and postleveling spatial distributions of soil fungal/bacterial biomass concentration ratios from the 0- to 10-cm depth. The x-direction is east and the y-direction is north on the land surface.

 
The effects of land leveling on fungal and bacterial biomass concentration and content were similar, a result also reported by Brye et al. (2003) on a silt loam. Despite no change in the overall mean within the study area (Table 1), spatial distributions of bacterial biomass concentrations (data not shown) and contents (Fig. 4) were most noticeably affected by land leveling activities. The streaked appearance to the spatial distribution of bacterial biomass (Fig. 4) was likely the residual effect of the predominant travel direction of earth movers during the land leveling process itself. Numerous spots within the sampling area with relatively high fungal biomass concentrations or contents before leveling were removed creating the appearance of a more uniform distribution after leveling despite a significant increase in sample variance within the study area (Table 4). Some degree of spatial-distribution alteration as a result of land leveling was also observed for fungal/bacterial biomass ratios (Fig. 5). These results are consistent with those of a similar study on the effects of relatively shallow-cut land leveling on a silt-loam soil (Brye et al., 2003) and with that of previous studies that have reported similar microbial responses to other soil disturbances, such as tillage (Doran, 1980; Karlen et al., 1994).

Visually, postleveling spatial distributions of soil biological properties did not correspond with the patterns of soil mass manipulation (i.e., either cut or fill) during the land leveling process (Fig. 1). This is likely the result of the common practice of surface smoothing following initial cutting and filling with a variety of different implements to reduce soil clod size to something more manageable for seedbed preparation before planting.

Relationships Among Soil Physical and Biological Properties
Based on results of previous studies (Brye et al., 2003, 2004b), land leveling was expected to influence correlations among soil physical and biological properties. However, contrary to that hypothesized, there were no significant correlations among soil physical and biological properties before or after land leveling in the Sharkey clay of this study. Sand was positively (0.38 < r < 0.67; P < 0.01) and clay was negatively correlated with bulk density (–0.39 < r < –0.63; P <0.01) both before and after land leveling. Postleveling sand (r = 0.42; P < 0.01), silt (r = 0.31; P < 0.05), clay (r = 0.40; P < 0.01), and the fungal/bacterial biomass ratio (r = 0.46; P < 0.001) were weakly correlated to their preleveling values, while postleveling bulk density and bacterial and fungal biomass concentrations and contents were not. Interestingly, of the soil physical and biological properties measured, only postleveling bacterial biomass concentrations (0.45 < r < 0.48; P ≤ 0.001) and contents (0.47 < r < 0.50; P ≤ 0.001) were weakly correlated with soil surface elevation change and the estimated amount of soil manipulated as a result of land leveling. Walker et al. (2003) indicated that rice yield loss was positively correlated with the volume per hectare of soil moved in cut areas across seven sites on common rice soils in Mississippi including three sites on a Sharkey clay. Walker et al. (2003) also reported rice yields were between 6 and 45% lower in cut compared to filled areas on recently leveling clay soils in Mississippi and attributed some of the yield decline to decreased soil P. However, in contrast to this study, Walker et al. (2003) did not soil sample immediately before land leveling but only afterward in areas known to have been cut and filled.

Despite the lack of pre- or postleveling correlations among soil physical and biological properties, pre- and postleveling data were combined to evaluate general relationships between soil physical and biological properties in a clay-textured Aquert. In contrast to that of Brye et al. (2003) on a silt-loam soil, bacterial biomass concentration (data not shown) and content (Fig. 6 ) were unrelated to bulk density, sand, silt, or clay, while fungal biomass concentration, but not fungal biomass content, were negatively related to bulk density (r2 = 0.19; P < 0.001). Both fungal biomass concentration (data not shown) and content (Fig. 6) were positively related to sand (0.07 < r2 < 0.09; P < 0.01). Similar to that of Brye et al. (2003), fungal biomass concentration (data not shown) and content (Fig. 6) were positively related to silt (0.23 < r2 < 0.26; P < 0.001) and negatively related to clay (0.15 < r2 < 0.16; P < 0.001). However, all significant relationships were poor for predictive purposes with rather low r2 values (r2 < 0.26). Frey et al. (1999) also reported no strong relationship between fungal biomass and soil texture.


Figure 6
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  		Fig. 6. Relationships between soil bacterial and fungal biomass contents and bulk density, sand, silt, and clay from the 0- to 10-cm depth for combined pre- and postleveling data.

 
Previous results, along with the results of this study, indicate that relationships between soil physical and biological properties are neither simple nor consistent. Though not directly evaluated in this study, soil structure likely plays a significant role in the magnitude and distributions of biological properties, particularly fungal biomass, in a clay-textured Aquert.

Similarly, time of year when land leveling is performed likely affects the resulting magnitude of soil property change. In this study, land leveling was conducted in the spring a few weeks before planting. Had land leveling been conducted later in the year when the field could be left fallow for several months or less intensively managed with a winter cover crop, the magnitude of soil property change would likely have been less due to extra time for microorganisms to get acclimated to a new surface soil physical and chemical environment and the addition of organic matter from weed or crop roots.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Land leveling significantly and immediately altered the magnitude, variance, and spatial variability and distributions of near-surface soil physical and biological properties in deep, relatively uniform clayey alluvium. Increased bulk density and clay contents as a result of land leveling in a clay soil by themselves will not necessarily negatively impact rice production. The concomitant decrease in surface hydraulic conductivity will actually contribute to the maintenance of the flooded-soil environment. However, the more subtle effects of altered soil physical properties on soil microbial biomass will likely have immediate effects on postleveling crop productivity.

With the alteration of soil surface biogeochemical properties and their spatial distributions by any means of soil disturbance, including land leveling, comes the potential for negatively affecting crop growth and ultimately yield, in which crop uniformity and high yields are constant goals for all producers. Increased variations in soil biogeochemical properties as a result of any soil disturbance may make the practicality and appropriateness of uniform field management (i.e., tillage, soil fertility, pest and disease control) difficult. Consequently, more advanced and expensive management practices, such as precision agriculture technologies (i.e., variable-rate fertilization and/or site-specific organic soil amendments), may be required to restore graded-soil productivity to predisturbance levels. Longer-term field studies will be necessary on clay soils to evaluate whether the potential water conservation benefit outweighs the restoration time required after initial productivity declines. Furthermore, it will be necessary to evaluate certain management practices on clay soils, such as deep tillage and the use of poultry litter, for their ability to alleviate potential problems like compaction and reduced soil fertility and tilth.

Received for publication June 14, 2005.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 




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K. R. Brye*
Soil Biochemical Properties as Affected by Land Leveling in a Clayey Aquert
Soil Sci. Soc. Am. J., May 23, 2006; 70(4): 1129 - 1139.
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