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Dep. of Crop, Soil, and Environmental Sciences, Univ. of Arkansas, 115 Plant Sciences Building, Fayetteville, AR 72701
* 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: CV, coefficient of variation • SE, standard error
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Many have reported a decline in soil fertility coupled with reduced crop productivity as a result of land leveling. 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 the subsoil can result in major changes in surface soil pH, decreased organic C, and exposure of sodic horizons (Miller, 1990). Miller (1990) also speculated that the near-surface spatial variability of certain soil properties in eastern Arkansas soils used for rice production was affected by land leveling and that this variability was related to post-leveling variability in crop growth. Similarly, spatial variability of soil properties has been implicated as being responsible for nonuniform growth of tropical lowland rice in the Philippines (Dobermann et al., 1995; 1997). In general, previous research has focused on the impact of land leveling on the magnitude and spatial variability of soil chemical characteristics and restoring full productivity of the exposed subsoils.
Few studies have focused on the effects of land leveling on soil quality by investigating several soil physical properties and their impacts on soil microorganisms. Soil quality is a concept that generally refers to the soil's ability to sustain productivity, environmental quality, human and animal health (Doran and Parkin, 1994). Papendick et al. (1995) suggested that an analysis of soil quality should consist of a minimum data set that includes measures of soil physical, chemical, and biological properties. However, 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). 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. Eck (1987) also examined the effect of depth of topsoil removal on particle-size distribution in a Pullman clay loam (fine, mixed, superactive, thermic Torrertic Paleustoll) in Texas, and determined that particle-size fractions 23 yr after subsoil exposure did not differ significantly from those in reference plots representing soil conditions at the time of exposure. Unger et al. (1990) reported that the clay content of a Pullman 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.
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 organic matter and nutrient cycling and storage, and potentially represent a very sensitive biological marker (Turco et al., 1994). To our knowledge, no studies have characterized the impact of land leveling on soil biological properties. 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 be reflected in the population ratio of soil fungi and bacteria (Bardgett et al., 1996). Morris and Boerner (1999) suggested that the spatial distributions of soil microorganisms and the factors affecting them should be further investigated since both soil chemistry and vegetation are affected by soil microbial communities.
The objectives of this study were to (i) characterize the short-term impacts of land leveling on the magnitudes, overall variance, and spatial variability and distributions of soil physical (i.e., bulk density and particle-size fractions) and biological (i.e., bacterial and fungal biomass) properties, and (ii) evaluate the relationships between soil physical and biological properties in a soil of the Mississippi Delta region in eastern Arkansas that is commonly used for rice and soybean production. We hypothesized that land leveling significantly alters the magnitude, spatial variability, and distribution of soil bacterial and fungal biomass, bulk density, and particle-size fractions. We also hypothesized that soil bacterial and fungal biomass are significantly correlated to soil physical properties, specifically that soil bacterial biomass concentration and content is significantly, positively correlated with the clay fraction of soil.
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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 was established in the study area. Grid 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 physical and biological properties. 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). The cut and filled areas of the grid were not considered experimental treatments.
Study Site Manipulations
Land leveling of approximately 5 ha of the 25-ha field occurred at the site in April 2002 resulting in a uniform slope throughout the study 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). Generally, composited 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 quantities 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, according to Warren and Fonteno (1993) composted poultry litter has a total porosity of approximately 78% and bulk density of around 0.5 Mg m-3.
Soil Sampling and Measurements
Soil samples were collected for selected physical and biological analyses before leveling (January 2002) and after leveling (May 2002). 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 upon sampling. Oven-dry soil was subsequently crushed and sieved to pass a 2-mm mesh screen for particle-size analysis using the hydrometer method (Arshad et al., 1996). Ten 2-cm diameter soil cores were collected and composited from the 0- to 10-cm depth below the poultry litter within a 20-cm radius surrounding each grid point. Samples were kept in a cooler in the field and stored at 4°C for microbial biomass determination.
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% 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 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.
Data Manipulation and Statistical Analyses
Soil microbial biomass concentrations, expressed on a µg (g dry soil)-1 basis, and bulk density were used to calculate microbial biomass contents. Microbial biomass contents are expressed on a g m-2 basis. Soil physical and biological properties are reported as mean values (± standard error [SE]), and coefficients of variation (CV) were also calculated.
Paired t tests were performed to determine the effect of land leveling on soil bulk density, particle-size fractions, bacterial and fungal biomass concentrations and content, and fungal/bacterial biomass concentration ratios (Minitab 13.31, Minitab Inc., State College, PA). Soil moisture was above field capacity and near saturation on both sampling dates; therefore water content was not considered a variable that affected pre- and post-leveling comparisons. Pearson linear correlations were performed to ascertain whether soil biological properties were correlated with soil physical properties, 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 between soil physical and biological properties. Analysis of covariance was used to determine if the slope and/or intercept differed between pre- and post-leveling linear relationships (SAS Version 8.1, SAS Institute, Inc., Cary, NC).
The effects of land leveling on the spatial variability of soil physical and biological properties studied 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 of the best-fitting (i.e., smallest residual sum of squares) of the spherical, exponential, or linear models were reported for pre- and post-leveling soil bulk density, particle-size fractions, bacterial and fungal biomass concentrations and contents, and fungal/bacterial biomass concentration ratios.
The effects of land leveling on the spatial distributions of soil physical and biological properties were determined by mapping pre- and post-leveling soil bulk density, particle-size fractions, bacterial and fungal biomass concentrations and contents, and fungal/bacterial biomass concentration ratios 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 between soil parameters.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Several places within the sampling 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 10-cm surface layer (Fig. 1) . Similarly, changes in the spatial distributions occurred for sand, silt, and clay contents (Fig. 2) . Silt content in the top 10 cm decreased over most of the sampling area, while the most pronounced change occurred with clay content. Before land leveling, the clay content in the top 10 cm of the sampling area ranged from 10 to 14%, but increased to between 16 and 22% after manipulation of the original top 10 cm of topsoil due to exposure of and mixing with subsoil with a higher clay content.
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Significant alterations of soil surface physical properties due to land leveling can create potential agronomic problems that cannot be ameliorated with increased nutrient inputs. 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 seedling emergence capability, soil water storage, crop water-use efficiency (Radford et al., 2001), crop growth characteristics (Lowery and Schuler, 1991), yield (Johnson et al., 1990; Radford et al., 2001), nutrient uptake (Lowery and Schuler, 1991), and root development and distribution (Taylor and Gardner, 1963; Unger and Kaspar, 1994). Soils compacted to any degree will have a smaller proportion of macro- than micropores and more non-connected pore space that will lead to fewer crop roots being able to penetrate compacted soil in comparison to non-compacted soil, consequently reducing the ability of roots to locate soil water and nutrients.
High surface bulk densities coupled with higher clay and/or sodium contents in recently exposed subsoils may also negatively impact growth and yield of non-rice crops in a rotation if sufficient soil moisture does not exist from either irrigation or natural rainfall to prevent surface crusting. The formation of a crust at the soil surface is caused by dispersion of soil structural units, which leads to compaction, and is regarded as a major form of soil degradation worldwide (Valentin and Bresson, 1998), especially in the south central and southeastern USA (Miller and Radcliffe, 1992). The potential negative effects of soil crusting are numerous and include reduced infiltration and increased runoff, reduced oxygen diffusion to seedlings, and restricted seedling emergence (Miller and Radcliffe, 1992; USDA-NRCS, 1996). However, the application of poultry litter following land leveling will minimize the potential for surface crusting to some degree because of the slightly increased water holding capacity associated with the new organic material. In addition, clay soils in eastern Arkansas generally require greater fertilizer additions to overcome diffusion constraints during plant uptake of soluble nutrients; thus the same would hold true when cropping exposed subsoils with a high clay content.
Biological Properties
In contrast to the physical properties, the biological properties in the top 10 cm of soil were more variable within the 0.36-ha sampling area before land leveling (Table 1). Soil bacterial biomass concentrations ranged from 163 to 473 µg g-1 and averaged 204 (SE ± 5.3) µg g-1, and soil fungal biomass concentrations ranged from 10 to 407 µg g-1 and averaged 137 (SE ± 14) µg g-1 before land leveling (Table 1). The mean fungal/bacterial biomass ratio was 0.68 (SE ± 0.07), which provides an indication of the vegetation-associated microbial community structure. A ratio of this magnitude is in the lower range reported for typical row-crop agricultural soils (e.g., 0.6 to 1.2; E.R. Ingham, personal communication, 2002). Similar to the microbial biomass concentrations, the microbial biomass contents were variable, where bacterial biomass contents ranged from 20 to 50 g m-2 and averaged 25.4 (SE ± 0.6) g m-2, and fungal biomass contents ranged from 1.3 to 51 g m-2 and averaged 17.0 (SE ± 1.8) g m-2. Fungal and bacterial biomass concentrations, fungal/bacterial biomass concentration ratio, and bacterial biomass content were spatially autocorrelated, while fungal biomass content displayed some level of spatial independency among samples within the study area.
Similar to pre-leveling soil physical properties, range parameters from exponential or spherical semivariogram models for pre-leveling soil biological properties were large, >98 m, except for fungal biomass contents in which spatial autocorrelation discontinued and sample independence was achieved at a range of 12.2 m (Table 2). The spatial component explained between 50 and 90% of the variation in soil biological properties.
Following land leveling, mean bacterial and fungal biomass concentrations and contents were significantly smaller (P 
0.001) than before land leveling (Table 1). On average, land leveling reduced the mean bacterial biomass concentration by 38%, fungal biomass concentration by 42%, bacterial biomass content by 36%, and fungal biomass content by 40%. However, mean fungal/bacterial biomass concentration ratio did not change significantly indicating that, even though the magnitude of microbial biomass significantly decreased, the structure of the microbial community remained unaltered (Table 1). These results indicate that biological properties, especially bacterial and fungal biomass, are highly sensitive to such extensive soil disturbance as results from land leveling. In addition, subsequent crop growth may be affected by altered soil biological properties, but restoring adequate biological functioning to the soil may be a challenge.
Collecting post-leveling soil samples approximately 1 wk after application of poultry litter to the study site may have introduced additional variability into the post-leveling results. The mode of litter application may have resulted in a non-uniform 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. However, since relatively little time elapsed between litter application and post-leveling sample collection, we feel that the poultry litter application did not compromise the results obtained to a significant degree. If litter application did have a significant affect, our results may have underestimated the actual effect of land leveling on soil biological properties. The addition of poultry litter (i.e., fresh organic material), plus warm, moist soil conditions during the week between litter application and post-leveling sample collection, may have stimulated growth of the indigenous fungal and bacterial biomass remaining in the recently disturbed soil.
The variability of soil biological properties was also significantly affected by land leveling. Coefficients of variation increased for bacterial biomass concentrations and contents, decreased for fungal biomass concentrations and contents, and did not change for fungal/bacterial biomass concentration ratios (Table 1). In contrast to soil physical properties, land leveling resulted in a significant decrease (P 0.001) in the variance of fungal biomass concentrations and contents, and a significant increase (P 0.01) in the variance of bacterial biomass contents, while the post-leveling variance for bacterial biomass concentrations and fungal/bacterial biomass concentration ratios did not differ significantly from their pre-leveling variance (Table 3). Similar to soil physical properties, land leveling also significantly affected the spatial variability of several soil biological properties.
Land leveling caused virtually no change in the spatial structure of bacterial biomass concentrations and contents, and fungal biomass contents as range parameters following land leveling were <2% different than before land leveling (Table 2). However, the range parameter for fungal biomass concentrations and fungal/bacterial biomass concentration ratios decreased by 87 and 30%, respectively, as a result of land leveling. As a result of land leveling and similar to pre-leveling fungal biomass contents, fungal biomass concentrations became spatially independent at a range of 11.2 m, which is also similar to the post-leveling range parameter for silt and clay fractions, 13.3 and 14.2 m, respectively, indicating that changes in the concentration of fungal biomass may be related to the disruption of the soil structure in conjunction with slight textural variations.
The amount of variation explained by the spatial component following land leveling decreased somewhat for bacterial biomass concentration and content, did not change for fungal/bacterial biomass concentration ratio, but increased from 50 to 93% for fungal biomass concentration, and from 76 to 88% for fungal biomass content (Table 2). Similar to soil physical properties, the spatial distributions of soil biological properties changed as a result of land leveling, but spatial patterns of physical properties did not correlate visually with spatial patterns of biological properties. This observation suggests that slight variations in soil texture do not account for changes in soil structure as a result of land leveling, which may help to explain the lack of visual correlation between the spatial distributions of soil physical and biological properties.
The spatial distributions of fungal biomass concentrations (Fig. 3) and contents (Fig. 4) were most noticeably affected by soil surface disturbance by land leveling activities. Several spots within the sampling area with relatively high fungal biomass concentrations or contents before leveling were removed creating a more uniform distribution after leveling. In contrast, several spots within the sampling area with relatively low fungal biomass concentration or content before leveling resulted in spots of comparatively higher concentration or content after leveling. Despite no change in the overall mean within the study area, the spatial distribution of the fungal/bacterial biomass ratios changed noticeably as a result of land leveling (Fig. 5) . Spatial distributions of bacterial biomass concentrations (Fig. 3) and contents (Fig. 4) were least affected by land leveling compared with the other soil biological properties, but some degree of spatial-distribution alteration was observed. The results of this study are consistent with that of previous studies that have reported microbial response to other soil disturbances, such as tillage (Doran, 1980; Karlen et al., 1994).
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Soil biological properties, such as fungal and bacterial biomass and their ratio, may have the potential to serve as indicators of soil quality (Dick, 1992). Growers in eastern Arkansas often attempt to improve soil quality and productivity of land-leveled soils by application of poultry litter. Despite favorable yield response in rice to the addition of poultry litter on leveled soils (Miller et al., 1990; 1991), numerous questions regarding the long-term effects of poultry litter additions to leveled soils on crop production and soil and environmental quality remain unanswered. For example, how will the addition of non-indigenous microorganisms found in poultry litter affect the severely depleted abundance of naturally occurring microbes remaining in the soil after leveling and what will the impact of introducing poultry litter containing potentially significant amounts of poultry-growth-enhancing pharmaceutical-type compounds (i.e., antibiotics and hormones) be to the soils, production systems, and surface waters in eastern Arkansas and other states within the Mississippi Delta region.
Changing the spatial variability of soil biogeochemical properties as a result of land leveling activities can negatively impact the bottom-line of maximum economic return from high yielding crops. Altered variability of soil physical and/or biological properties can affect the uniformity of crop growth, which is a constant goal for the majority of farmers in Mississippi Delta region of eastern Arkansas and beyond, and ultimately yield. In addition, increased variations in soil biogeochemical properties as a result of any soil-disturbance practice will make uniform field management (i.e., tillage, soil fertility, pest and disease control) difficult and may require the consideration of more advanced and expensive management practices such as precision agriculture technologies.
Relationships among Physical and Biological Properties
Land leveling was expected to impact soil physical and biological properties and that the changes among these properties would be correlated. However, different properties may be impacted by or respond to disturbances in different ways. For example, biological properties might be expected to respond faster than physical or chemical properties to management practices (i.e., tillage) that result in less physical perturbation than land leveling because of the dynamic nature of the soil microbial community.
In this study, land-leveling activities caused numerous changes in relationships among soil physical and biological properties. Before soil disturbance by land leveling, only soil bacterial biomass concentration was significantly correlated with bulk density (r = -0.30; P = 0.032). Following land leveling, bacterial biomass concentration was significantly correlated with the sand fraction (r = -0.41; P = 0.003), while bacterial biomass content was significantly correlated with both the sand (r = -0.38; P = 0.006) and silt fractions (r = 0.28; P = 0.046). The intercept parameters changed significantly as a result of land leveling for the linear relationships between soil bacterial biomass concentration and bulk density (P = 0.048) and clay fraction (P = 0.006) and between soil bacterial biomass content and clay fraction (P = 0.004), but the slope parameters did not differ significantly.
Neither pre- nor post-leveling microbial biomass concentrations or contents were significantly correlated with clay content, negating the original hypothesis of a significant correlation, indicating that perhaps soil structure and texture combined are more important determinants of microbial niches in the soil than texture alone. However, combining pre- and post-leveling data (i.e., n = 100), bacterial biomass concentrations and contents were significantly, but weakly, correlated with bulk density (r > -0.29; P 0.003) and sand (r > -0.47; P < 0.001), silt (r > 0.52; P < 0.001), and clay (r > -0.30; P 0.002) fractions. Combined pre- and post-leveling fungal biomass concentrations and contents were also significantly, but weakly, correlated with sand (r > -0.27; P 0.006), silt (r > 0.33; P 0.001), and clay (r > -0.22; P 0.027) fractions. These results are similar to Frey et al. (1999) who also reported no strong relationship between fungal biomass and soil texture. The slopes for all linear relationships between microbial biomass concentrations (Fig. 6)
and contents (Fig. 7)
and soil physical properties were significantly different (P 0.027) than zero, except for the relationship between fungal biomass and bulk density.
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0.12). These results suggest that particle-size fractions are related to, but by themselves are not controlling factors for microbial biomass. The arrangement of soil particles to yield soil structure may be more highly related to microbial biomass because it is the pore space within soil that microorganisms inhabit. Despite numerous significant correlations between soil physical and biological properties, only bulk density was significant (P = 0.015) in the multiple regression model using bulk density and sand, silt, and clay fraction to predict soil bacterial biomass concentration (r2 = 0.39 and P < 0.001 for multiple regression model). No soil physical properties were significant in multiple regression models used to predict fungal biomass concentrations and fungal and bacterial biomass contents and no model had a r2 > 0.42.
Previous findings, along with the results of this study, indicate that expected relationships between soil physical and biological properties are not simple. Soil structure, though difficult to quantify, undoubtedly plays a role in the magnitude and distributions of biological properties, such as fungal and bacterial biomass, in soil and some quantifiable combination of soil structure and texture may explain more variation in soil microbial biomass than either one alone.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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ACKNOWLEDGMENTS |
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Received for publication September 3, 2002.
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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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K. R. Brye, N. A. Slaton, M. Mozaffari, M. C. Savin, R. J. Norman, and D. M. Miller Short-Term Effects of Land Leveling on Soil Chemical Properties and Their Relationships with Microbial Biomass Soil Sci. Soc. Am. J., May 1, 2004; 68(3): 924 - 934. [Abstract] [Full Text] [PDF]
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CV) of pre- to post-leveling soil properties.
) of soil physical and biological properties from the 0- to 10-cm depth. Asterisks next to post-leveling values represent significant differences from Levene's tests in the variance of pre- and post-leveling soil properties.
