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a Dep. of Crop, Soil, and Environmental Sciences, Univ. of Arkansas, 115 Plant Sciences Building, Fayetteville, AR 72701
b Dep. of Crop, Soil, and Environmental Sciences, Univ. of Arkansas, P.O. Drawer 767, Marianna, AR 72360
* Corresponding author (kbrye{at}uark.edu).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: EC, electrical conductivity • FDA, fluorescein diacetate • OM, organic matter
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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The potential agronomic benefits of land leveling have been recognized for more than half a century and include improved distribution of irrigation waters, soil and water conservation, and improved uniformity of crop growth and yield within a field (Whitney et al., 1950). However, the positive effects of land leveling on crop production are equally as numerous as the negative effects of growing crops in exposed subsoil. Deficiencies in essential plant nutrients (e.g., N and P) can limit crop growth following land leveling (Whitney et al., 1950; Eck, 1987; Robbins et al., 1997, 1999). Exposing subsoil can result in major changes in soil surface pH, decreased organic C, and Na toxicity (Miller, 1990). Miller (1990) also 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 eastern Arkansas, where approximately 40% of the total rice production in the USA occurs (National Agricultural Statistics Service [NASS], 2001), rice yields have been shown to increase on land-leveled fields when fertilized with inorganic N, P, and Zn and/or amended with composted and uncomposted poultry litter (Miller et al., 1990, 1991). In contrast, fertilization with inorganic K and/or S did not significantly affect rice yields on land-leveled soil (Miller et al., 1990). These studies indicated that land leveling caused decreased contents of soil organic C, N, P, and Zn, but not K or S. In addition to affecting yield, changes in soil chemical properties as a result of land leveling may also impact soil biology.
Spatial distributions of soil microbial biomass, especially fungi and bacteria, are still not completely understood in terrestrial ecosystems (Parkin, 1993), especially in agroecosystems using various tillage practices (Wardle, 1995). Soil microorganisms are important for maintaining soil quality due to their role in decomposition of OM and nutrient cycling and storage, and potentially represent a very sensitive biological marker (Turco et al., 1994). To our knowledge, few studies have characterized the effects of land leveling on soil biological properties. Soil biological properties are intimately related to the chemical environment in the soil and are as important in controlling soil tilth as soil chemical and physical properties. In general, the microbial community in agroecosystems is poorly understood and not well characterized (Parkin, 1993; Wardle, 1995). Since the combined populations of fungi and bacteria represent a large fraction of the total soil microbial biomass, changes in land management or disturbance of the soil profile from land leveling would likely affect the populations or population ratio of the soil fungal and bacterial communities (Bardgett et al., 1996; Brye et al., 2003) and the chemical properties that control their existence.
The objectives of this study were to (i) characterize the short-term impacts of land leveling on the magnitudes, spatial variability, and spatial distributions of soil chemical properties, and ii) evaluate the impact of land leveling on the relationships among selected soil chemical properties and between soil chemical and microbial biomass in a soil of the Mississippi Delta region in eastern Arkansas commonly used for irrigated rice and soybean production. We hypothesized that land leveling significantly alters the magnitude, spatial variability, and distribution of soil macro- and micronutrients. We also hypothesized that land leveling significantly affects the relationships between microbial biomass, soil pH, and OM and between extractable soil nutrient contents, soil pH, EC, and OM content.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Experimental Design
Before land leveling, a 40 by 90 m sampling grid (0.36 ha) was established as the study area within approximately 5 ha that was to be land leveled (i.e., manipulated). Grid points (i.e., sampling points) were spaced evenly at 10-m apart, for a total of 50 grid points, to facilitate statistical evaluation of the effects of land leveling on the magnitude, variability, and spatial distribution associated with soil chemical and microbial biomass. The grid was positioned in the field so that roughly one-half of the sampling area was cut (i.e., topsoil was scrapped and removed from an area of relatively high elevation) and the other half was filled (i.e., deposition into an area of relatively low elevation of soil previously scrapped and removed from another area within the same field).
Study Site Manipulations
Land leveling occurred at the site in April 2002 resulting in a uniform slope throughout the study area. After initial cutting and filling occurred, the entire manipulated area was regraded to eliminate minor topographic variations. During the regrading process, some material originally removed from the cut area was pushed from the filled area back onto the cut area. The maximum depth of cut was roughly 15 cm. Within 2 wk following land leveling, semi-solid composted poultry litter was broadcast at approximately 2.2 Mg ha–1 throughout the entire study area using a tractor-drawn manure spreader as recommended by the University of Arkansas Cooperative Extension Service (Slaton, 2001). The poultry litter application so soon after completion of the land-leveling activity was the result of miscommunication with the cooperating landowner and was not originally intended to be part of the experimental design of this study. However, in general, composted poultry litter from northwest Arkansas has a pH of 8.0 to 8.7, and contains 257 to 302 g total C kg–1, 35 to 39 g total N kg–1 (fresh-weight basis), 1.6 to 3.0 g soluble reactive P kg–1, 31 to 35 g total P kg–1, and approximately 36 g total N kg–1 (dry-weight basis) (DeLaune, 1999). These concentrations and contents are similar to the chemical composition of other sources of composted poultry litter in the southeastern USA (Tyson and Cabrera, 1993; Warren and Fonteno, 1993; Freeman and Cawthon, 1999). In addition, Warren and Fonteno (1993) indicate that composted poultry litter has a total porosity of approximately 78% and a bulk density of approximately 0.5 Mg m–3.
Sampling Scheme and Measurements
On 11 Jan. (i.e., preleveling) and 9 May 2002 (i.e., postleveling), soil samples were collected for chemical (i.e., extractable nutrients, pH, EC, and OM) and biological (i.e., fungal and bacterial biomass concentrations) property determination. No vegetation existed in the study area between sampling dates. Postleveling samples were collected only 1 wk after poultry litter application, with no significant precipitation events between litter application and postleveling soil sampling, and the litter was physically scrapped aside from each sample location so that it was not included in the actual sample.
A single 4.8-cm diameter soil core (the soil-core sampling chamber was beveled to the outside to minimize compaction upon sampling) 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. Ten 2-cm diameter soil cores were collected from the 0- to 10-cm depth within a 20-cm radius surrounding each grid point and composited, oven dried at 70°C for 48 h, crushed, and sieved to pass a 2-mm mesh screen for soil chemical analyses. Dried and sieved soil was extracted with Mehlich-3 extractant solution (Tucker, 1992) in a 1:10 soil/extractant solution ratio and analyzed for extractable nutrients (i.e., P, K, Ca, Mg, Na, S, Fe, Mn, Zn, Cu, and B) using an inductively coupled argon-plasma spectrophotometer (CIROS CCD model, Spectro Analytical Instruments, MA). Soil pH and EC were determined with an electrode on a 1:2 soil/water solution. Organic matter was determined on sieved soil by weight-loss-on-ignition after 2 h at 360°C (Schulte and Hopkins, 1996). A second set of ten 2-cm diameter soil cores were also collected from the 0- to 10-cm depth within a 20-cm radius surrounding each grid point and composited for biological assays. These samples were immediately iced in the field then refrigerated <3 d before fungal and bacterial biomass determinations were conducted.
Total fungal biomass 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% (v/v) 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 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 shaken for 2 min. Aliquots of the suspension 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.
Data Manipulation and Statistical Analyses
Extractable soil nutrient and microbial biomass concentrations, expressed on a mass per mass basis, and bulk density were used to calculate extractable nutrient and microbial biomass contents, expressed on a mass per area basis. Soil chemical properties are reported as mean values (±standard error [SE]). Coefficients of variation (CV) were also calculated for soil chemical properties.
Although the sampling grid was positioned such that approximately one-half of the area was cut and one-half of the area was filled, the regrading process that occurred following initial site manipulations caused the entire study area to have some degree of fill material. Therefore, the roughly cut and filled areas were not treated as experimental treatments and were not separated during statistical analyses.
Without true replication, but rather pseudo-replication in time, paired t tests were performed, retaining the identity of each sampling point, to determine the short-term effects of land leveling on extractable soil nutrient contents, pH, EC, and OM (Minitab 13.31, Minitab Inc., State College, PA). Soil moisture was above field capacity and near saturation on both sample dates; therefore water content was not considered a variable that affected pre- and postleveling comparisons. Pearson linear correlations were performed to ascertain correlations among selected soil chemical properties, between microbial biomass and soil pH, EC, and OM, and whether significant correlations determined before land leveling changed due to land leveling. Based on significant correlations, linear and multiple regression analyses were used to determine the relationships among selected soil chemical properties and between soil chemical properties and fungal and bacterial biomass contents. Analysis of covariance was used to determine if the slope and/or intercept differed between pre- and postleveling linear relationships (SAS Version 8.1, SAS Institute, Inc., Cary, NC).
The effects of land leveling on the spatial variability of soil chemical properties were determined by several methods. Homogeneity of variance was evaluated using Levene's test (Levene, 1960). Geostatistical analyses were also conducted using GS+ (version 5.1, Gamma Design Software, Plainwell, MI). Only isotropic semivariograms were considered and semivariance parameters for spherical, exponential, or linear models are reported for pre- and postleveling pH, EC, OM, and extractable soil nutrient contents.
The effects of land leveling on the spatial distributions of soil chemical properties were determined by mapping pre- and postleveling pH, EC, OM, and extractable soil nutrient contents using Surfer 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 mapping and comparisons among soil chemical properties.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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In contrast to soil pH, decreased soil OM is a particularly negative effect of land leveling. Since soil nutrient content is related to soil OM content, natural nutrient availability and cycling is affected by decreased levels of soil OM (Rochette et al., 1999). The decline in soil OM will also adversely impact soil structure (Caravaca et al., 2001; Watts et al., 2001; Franzluebbers, 2002), aggregate stability (Chenu et al., 2000; Caravaca et al., 2001), infiltration (Franzluebbers, 2002), and water-holding capacity (Haynes and Naidu, 1998; Diaz-Zorita et al., 1999), and increase the potential for soil erosion.
As indicated by the somewhat large positive and negative changes in the coefficients of variation for pre- and postleveling mean values (Table 1), land leveling affected the overall variability associated with soil chemical properties. Results from Levene's test for equal variances support this observation (Table 2). The variances associated with soil pH and S, Mn, and Cu contents significantly increased (P < 0.01) due to land leveling.
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Preleveling range parameters from exponential and spherical semivariogram models for most soil chemical properties were large, >80 m, indicating spatial autocorrelation among sampling points at the 10-m spacing and that data were not truly independent within the sampling area (Table 3). Some degree of spatial independence (i.e., the range parameter <17 m) existed within the sampling area for the contents of Mn and B. A spherical model best characterized the structure of the preleveling semivariograms for soil pH, EC, and the contents of K, Ca, Na, and Cu, but the model fit was very poor for Ca and Cu (r2 < 0.02). An exponential model best characterized the structure of the preleveling semivariograms for the contents of soil OM, P, S, Fe, Mn, Zn, and B, but the model fit was also poor for P and Zn (r2 < 0.36). A linear model best characterized the preleveling semivariogram for soil Mg content. The spatial component [i.e., the C/(C0 + C) column in Table 3] explained between 50 and 99% of the variation in preleveling soil chemical properties. However, land leveling affected spatial relationships for a number of soil chemical properties.
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The spatial distributions of all soil chemical properties measured in the top 10 cm were altered by land leveling. Portions of the sampling area with an alkaline preleveling soil pH had an acidic pH following land leveling as a result of exposing and mixing alkaline surface soil with the typically acidic subsoil (Fig. 1) . Similar spatial distribution changes occurred with soil-extractable macro- (Fig. 2) and micronutrient contents (Fig. 3, 4) . The most noticeable changes occurred with the spatial distributions of soil OM (Fig. 1), K (Fig. 2), and S (Fig. 3) contents. Land leveling resulted in an average decrease of 18% in soil OM in the top 10 cm. The decline in OM represents a substantial loss of potential native soil fertility for subsequent crops. In contrast, land leveling caused the content of extractable K in the top 10 cm to increase by an average of 63%. As a cation susceptible to leaching, K+ would have the tendency to leach from the surface and accumulate in the subsoil of flood-irrigated soils. Similarly, K is known to be associated with certain clay minerals like vermiculite and illite (Rich, 1968), which are found in higher amounts in clayey subsoil, particularly in subsoil with argillic horizons like the soil in this study. The exposure of K-rich subsoil by land leveling would potentially benefit the K nutrition of subsequent crops grown in shallowly cut fields, such as rice and soybean (Norman et al., 2003). Although, Miller et al. (1990) indicated that K fertilization did not increase rice grain yields on some land-leveled soils in eastern Arkansas.
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One might expect that the areas that were initially cut and the areas that were initially filled are observable in the maps of the spatial distributions of soil chemical properties (Fig. 1– 4). However, it is difficult to associate changes in soil properties with cut and fill areas, except in cases of extreme or deep-cut land leveling, because land-leveled areas are often graded several times after the initial cutting and filling to smooth out topographic variations that resulted during the leveling process.
The effects of land leveling on soil chemical properties reported here are similar to those reported by Brye et al. (2003) for soil physical properties and microbial biomass. It is evident that the agricultural practice of land leveling to improve the distribution of irrigation water has several significant drawbacks, namely, the decline in potential long-term fertility with the decrease in OM and the significant disruption of the spatial variability and distributions of soil physical, chemical, and biological properties.
Despite the numerous significant pre- to postleveling differences observed in this field study, collecting postleveling soil samples approximately 1 wk after application of poultry litter to the study site may have also introduced additional variability into the postleveling results. The mode of litter application may have resulted in a nonuniform distribution of litter to the soil surface within the study area. Despite removing surface-applied litter from the 20-cm radius around each grid point, relatively moist soil conditions could have facilitated the leaching of readily solublized constituents contained in the litter into the soil.
The application of poultry litter could have accounted for the entire increase in extractable soil P and could have contributed to increased extractable K (Table 1). However, litter application could not have accounted for the increased magnitude of most other extractable nutrients because the content of most other nutrients in poultry litter is not that high. The increases in extractable Ca and Mg were likely due to exposure of subsoil following land leveling with increased concentrations of Ca and Mg due to a long history of using irrigation water high in these nutrients. In addition, if litter application had a significant affect on postleveling soil properties, one would have expected the soil pH and OM to have increased somewhat, but pH and OM actually decreased. In all likelihood, the poultry litter masked more significant decreases in soil pH throughout the study area. Furthermore, the long history of irrigation in eastern Arkansas has resulted in ground water that has an elevated pH. Consequently, the ground water, which is the most common source of irrigation water in eastern Arkansas, has contributed to the near-surface subsoil, which is the portion of the soil manipulated in shallow-cut land leveling, having a similarly high pH at least to the depth of the hardpan before acidic soil conditions are frequently encountered. Therefore, since relatively little time elapsed between litter application and postleveling sample collection, we feel that the poultry litter application did not compromise the results obtained to a significant degree. Furthermore, as discussed in detail in Brye et al. (2003), the addition of poultry litter following land leveling likely resulted in an underestimation of the actual effect of land leveling on soil microbial biomass (Table 1).
Similar to the potential effects of the 1-wk between litter application and postleveling soil sampling, the 4-mo period between pre- and postleveling soil sampling may have affected some variables, such as pH, EC, and several extractable soil nutrients, in ways that were unrelated to land leveling. However, periodic observations indicated the soil moisture status remained uniformly wet between pre- and postleveling soil samplings, in which the prolonged wetness prevented use of heavy land-leveling equipment in the field until the soil dried out sufficiently, and the study area did not experience wetting and drying cycles. Furthermore, natural mineralization of OM would have been too slow under the relatively cool and wet soil environment to have been responsible for the significant increases in numerous extractable soil nutrients (Table 1). Therefore, we feel that any potential time effect between samplings on soil chemical properties was insignificant compared with the effects of the land leveling itself.
Relationships among Soil Chemical and Microbial Biomass
Numerous significant (P < 0.05) linear correlations existed among soil chemical and microbial biomass before, after, and both before and after land leveling (Table 4). Only the preleveling relationship between Zn and OM content resulted in a relatively strong correlation (r = 0.76, P 
0.001). All other significant preleveling correlations among soil chemical and microbial biomass had correlation coefficients 0.60. Following land leveling, the relationship between the Zn and OM content remained relatively strong (r = 0.71, P 0.001). In addition, postleveling Ca content and pH had a relatively high positive correlation (r = 0.81, P 0.001), while S content and pH had a relatively high negative correlation (r = –0.80, P 0.001). Bacterial biomass content did not correlate with any soil chemical properties before or after land leveling indicating that the bacterial population that exists in the top 10 cm of this silt-loam soil under a flood-irrigated, rice–soybean rotation is relatively insensitive to the chemical status in the bulk soil of its surrounding environment. Microscale soil chemical properties most likely had a greater influence on bacterial biomass. In contrast, the fungal biomass appeared to be much more sensitive to the chemical status of its surrounding environment as fungal biomass content was significantly (P < 0.05), but generally not highly (–0.33 < r < 0.50) correlated with numerous soil chemical properties before and/or after land leveling. The numerous pre- and/or postleveling correlations among soil chemical and microbial biomass also suggests that land leveling may have resulted in significant changes in the linear relationships among soil chemical and microbial biomass.
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0.012) and between the fungal/bacterial-biomass ratio and extractable K and Cu contents (P 0.05) were significantly affected by land leveling. Only the intercept parameter for the linear relationships between bacterial biomass content and EC and extractable K, Fe, Zn, and B contents were significantly (P < 0.04) affected by land leveling. The slope and intercept parameters for the linear relationships between the fungal/bacterial biomass ratio and extractable Na and S contents were significantly (P < 0.02) affected by land leveling.
Management Implications
Variable growth and yield of rice and soybean on recently leveled fields in the mid-South are frequently attributed to the disruption of soil fertility and plant nutritional status, which is typically manifested as plant nutrient deficiencies and/or toxicities. Preleveling soil-test results are ineffective at predicting postleveling soil conditions and crop growth. The application of inorganic fertilizers often fails to improve or restore soil productivity compared with the application of organic amendments like poultry litter, which, in the absence of plant-nutrition-related disorders, suggests a significant contribution from soil biological properties in restoring productivity to leveled soils. Though the addition of readily available plant nutrients and organic substrates from commercial fertilizers and organic amendments offer a quick fix for the subsequent cropping cycle, restoration of chemical and biological equilibrium and proper soil functioning will take time before newly exposed subsoil becomes acclimated again for high-yielding crop production. Numerous observations have been made that suggest that the greater extent or depth of land leveling, the greater the grain yield decline, the longer the time required for grain yields to be restored to acceptable levels, and the more nutrient inputs required (Miller et al., 1990, 1991; Norman et al., 2003).
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Soil with sufficient moisture alone will not be highly productive. The benefit of improved water distribution must be weighed against the relatively severe and immediate alteration of soil properties and natural processes and the potential for future plant-growth problems brought on by land leveling. Short- and long-term studies that evaluate the inter-relationships between soil physical, chemical, and biological properties will be essential for defining and/or refining management practices that expedite the restoration of productivity to precision-leveled soils.
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ACKNOWLEDGMENTS |
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Received for publication April 29, 2004.
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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This article has been cited by other articles:
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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. [Abstract] [Full Text] [PDF]
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K. R. Brye, N. A. Slaton, and R. J. Norman Soil Physical and Biological Properties as Affected by Land Leveling in a Clayey Aquert Soil Sci. Soc. Am. J., February 27, 2006; 70(2): 631 - 642. [Abstract] [Full Text] [PDF]
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CV) of pre- to postleveling soil properties [(CVpost – CVpre)/CVpre x 100]. Asterisks next to postleveling means represent significant pre- and postleveling differences.
