SHORT-TERM DISRUPTION OF SOIL PHYSICAL AND BIOLOGICAL PROPERTIES CAUSED BY LAND LEVELING

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1 RICE CULTURE SHORT-TERM DISRUPTION OF SOIL PHYSICAL AND BIOLOGICAL PROPERTIES CAUSED BY LAND LEVELING K.R. Brye ABSTRACT The spatial variability and distributions of soil physical and biological properties, and the relationships between them, are not well-characterized in agroecosystems that have been land-leveled. The objectives of this study were to characterize the short-term impacts of land leveling on the magnitudes, spatial variability, and spatial distributions of soil physical and biological properties in a Stuttgart silt loam (fine, smectitic, thermic Albaqultic Hapludalf) used for irrigated soybean [Glycine max (L.) Merr.] and rice (Oryza sativa L.) production in the Mississippi Delta region of eastern Arkansas. Bulk density, and sand and clay contents, significantly increased while silt content and fungal and bacterial biomass significantly decreased as a result of land leveling. Land leveling significantly changed the variance associated with soil biological properties, but the variances of soil physical properties were unaffected. The spatial distributions of soil fungal and bacterial biomass in the top cm were most noticeably altered by land leveling in comparison to pre-leveling distributions. These results indicate that variability in soil biological properties may play an important role in restoring productivity to precision-leveled soils. INTRODUCTION Land leveling is a relatively common agricultural practice in the south-central United States and is routinely performed in fields where irrigated crops will be grown, such as rice (Oryza sativa L.) and soybean [Glycine max (L.) Merr.]. The potential agronomic benefits of land leveling have been recognized for more than half a century 262

2 B.R. Wells Rice Research Studies 2 and include the ability to irrigate non-level soils, improved distribution of irrigation waters, soil and water conservation, and improved uniformity of crop growth and yield (Whitney et al., 19). However, equally as numerous as the positive effects of land leveling on crop production are the negative effects of growing crops in exposed subsoil. Previous research has generally 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. In addition, very little, if any, research has focused on the effects of land leveling on soil biological properties. 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). Therefore, the objective of this study was to 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 in a soil of the Mississippi Delta region in eastern Arkansas that is commonly used for rice and soybean production. PROCEDURES The study site consisted of a.36-ha area within a 25-ha field predominantly cropped to rice located in Arkansas County, Arkansas (34 6' N, 91 22' W) on a Stuttgart silt loam (fine, smectitic, thermic Albaqultic Hapludalf). Prior to land leveling, a - x 9-m sampling grid was established in the study area. Grid points were spaced evenly at -m apart for a total of grid points. Land leveling of approximately 5 ha of the 25-ha field occurred at the site in April 2 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 ~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, 1). Soil samples were collected for selected physical and biological analyses before leveling (January 2) and after leveling (May 2). A single 4.8-cm diameter soil core was collected from the - to -cm depth within a -cm radius surrounding each grid point, oven dried at 7 C for 48 hr, 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 - to -cm depth below the poultry litter within a -cm radius surrounding each grid point. Samples were kept in a cooler in the field and stored at 4 C for microbial biomass determination. 263

3 AAES Research Series 4 Soil physical and biological properties are reported as mean values [± standard error (SE)]. 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-to-bacterial biomass concentration ratios (Minitab 13.31, Minitab Inc., State College, PA). Homogeneity of variance was evaluated for pre- and postleveling soil properties using Levene s test (Minitab). The effects of land leveling on the spatial distributions of soil physical and biological properties were determined by mapping pre- and post-leveling soil properties using Surfer 7 (Golden Software, Inc., Golden, CO). RESULTS AND DISCUSSION Soil bulk density and particle-size fractions in the top cm were significantly affected by land leveling (Table 1). Soil microbial biomass in the top cm significantly decreased as a result of land leveling (Table 1). However, the relative proportion of fungal-to-bacterial biomass, hence the structure of the soil microbial community, remained unaffected. 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 -cm surface layer (Fig. 1). Similar changes in the spatial distributions occurred for sand, silt, and clay contents (data not shown). Significant alterations of soil surface physical properties due to land leveling can create potential agronomic problems that can not be ameliorated with increased nutrient inputs. Elevated bulk densities following land leveling indicate 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., 1); crop growth characteristics (Lowery and Schuler, 1991); yield (Johnson et al., 199; Radford et al., 1); 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 noncompacted soil, consequently reducing the ability of roots to locate soil water and nutrients. Land leveling did not significantly affect the variance of the soil physical properties evaluated (Table 2) but did significantly affect their spatial variability as indicated by the changes in bulk density (Fig. 1). In contrast to soil physical properties, land leveling resulted in a significant decrease (P.1) in the variance of fungal biomass concentrations and contents, and a significant increase (P.1) in the variance of bacterial biomass contents (Table 2). Similar to soil physical properties, land leveling significantly affected the spatial variability of soil bacterial (Fig. 2) and fungal biomass (Fig. 3). 264

4 B.R. Wells Rice Research Studies 2 SIGNIFICANCE OF FINDINGS The practice of land leveling significantly affected the magnitude and spatial variability of both soil physical and biological properties in the top cm of a Stuttgart silt loam. The results of this study showed that soil bacterial and fungal biomass decreased due to land leveling, which may negatively affect soil quality and productivity until restored, despite the fungal-to-bacterial-biomass ratio remaining unaffected. The results also 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, and restoring adequate biological functioning to the soil may be difficult. It is expected that the soil biological properties will continue to change and equilibrate to the new soil conditions created by the land leveling and to subsequent cropping systems that will be used. However, these changes are not predictable; thus long-term studies that evaluate the interrelationships between soil physical, chemical, and biological properties will be needed to define and/or refine management practices that expedite the restoration of productivity to precision-leveled soils. ACKNOWLEDGMENTS Mr. Sam Counce and his family are gratefully thanked for allowing this work to be conducted on their property. Also acknowledged are Jared Holzhauer, Jason Grantham, and Mandy Pirani for their field assistance and Dr. Rick Norman for his assistance in obtaining soil microbial biomass data. LITERATURE CITED Arshad, M.A., B. Lowery, and B. Grossman Physical tests for monitoring soil quality. p In: J.W. Doran and A.J. Jones (Eds.) Methods for assessing soil quality. SSSA Spec. Publ. 49. Soil Sci. Soc. Am., Madison, WI. Johnson, J.F., W.B. Voorhees, W.W. Nelson, and G.W. Randall Soybean growth and yield as affected by surface and subsoil compaction. Agron. J. 82: Lowery, B. and R.T. Schuler Temporal effects of subsoil compaction on soil strength and plant growth. Soil Sci. Soc. Am. J 55: Radford, B.J., D.F. Yule, D. McGarry, and C. Playford. 1. Crop responses to applied soil compaction and to compaction repair treatments. Soil Till. Res. 61: Slaton, N.A. (ed.). 1. Rice production handbook. University of Arkansas Cooperative Extension Service MP p. Taylor, H.M. and H.R. Gardner Penetration of cotton seedling taproots as influenced by bulk density, moisture content, and strength of soil. Soil Sci. 96:

5 AAES Research Series 4 Turco, R.F., A.C. Kennedy, and M.D. Jawson Microbial indicators of soil quality. pp In: J.W. Doran, D.C. Coleman, D.F. Bezdicek, and B.D. Stewart (Eds.) Defining soil quality for a sustainable environment. SSSA Spec. Publ. 35. Soil Sci. Soc. Am., Madison, WI. Unger, P.W. and T.C. Kaspar Soil compaction and root growth: a review. Agron. J. 86: Whitney, R.S., R. Gardner, and D.W. Robertson. 19. The effectiveness of manure and commercial fertilizer in restoring the productivity of subsoils exposed by leveling. Agron. J. 42: Table 1. The effect of land-leveling on soil physical and biological properties from the - to -cm depth. Soil parameter Pre-leveling Post-leveling z Physical properties: Bulk density (g cm -3 ) 1.25 (.1) y 1.29 (.1)*** Sand (%) 11. (.2) 12.7 (.2)*** Silt (%) 75.7 (.3) 71.4 (.5)*** Clay (%) 13.3 (.3) 15.8 (.5)*** Biological properties: Bacterial biomass ( g g -1 ) 4 (5.3) 127 (4.7)*** Fungal biomass ( g g -1 ) 137 (14) 79.4 (6.4)*** Fungal-to-bacterial-biomass ratio.68 (.7).68 (.7) Bacterial biomass (g m -2 ) 25.4 (.6) 16.3 (.6)*** Fungal biomass (g m -2 ) 17. (1.8).2 (.82)*** z y Asterisks next to post-leveling means represent significant differences (P.1) between pre- and post-leveling measurements caused by land-leveling. Numbers in parentheses are the standard error for the mean value. Table 2. Effect of land leveling on the variance (σ) of soil physical and biological properties from the - to -cm depth. Soil parameter Pre-leveling Post-leveling z (σ) Physical properties: Bulk density (g cm -3 ) Sand (%) Silt (%) Clay (%) Biological properties: Bacterial biomass ( g g -1 ) Fungal biomass ( g g -1 ) *** Fungal-to-bacterial-biomass ratio.. Bacterial biomass (g m -2 ) ** Fungal biomass (g m -2 ) ** z Asterisks next to post-leveling values represent significant differences (**.1 P >.1; *** P.1) from Levene s tests in the variance of pre- and post-leveling soil properties. 266

6 B.R. Wells Rice Research Studies 2 9 Bulk Density (g cm -3 ) Pre-leveling Post-leveling y-direction (m) Fig. 1. Pre- and post-leveling spatial distributions of soil bulk density from the - to -cm depth. The x-direction is north and the y-direction is west on the land surface. 267

7 AAES Research Series 4 y-direction (m) Bacterial Biomass (g m -2 ) Pre-leveling Post-leveling Fig. 2. Pre- and post-leveling spatial distributions of soil bacterial biomass contents from the - to -cm depth. The x-direction is north and the y-direction is west on the land surface. 268

8 B.R. Wells Rice Research Studies 2 y-direction (m) Fungal Biomass (g m -2 ) Pre-leveling Post-leveling Fig. 3. Pre- and post-leveling spatial distributions of soil fungal biomass contents from the - to -cm depth. The x-direction is north and the y-direction is west on the land surface. 269