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a Dep. of Soil, Water and Climate, Univ. of Minnesota, 1991 Upper Buford Circle, St. Paul, MN 55108
b Southwest Research and Outreach Center, Univ. of Minnesota, 23669 130th St., Lamberton, MN
* Corresponding author (jstrock{at}umn.edu)
Funding provided by USDA-CSREES.
Received 14 April 2005.
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ABSTRACT |
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 TOPEXECUTIVE SUMMARY ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: SSURGO, Soil Survey Geographic database • STATSGO, State Soil Geographic database
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INTRODUCTION |
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TOPEXECUTIVE SUMMARYABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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It is widely known that spatial variability exists among soil physical properties, including bulk density, water content, and saturated hydraulic conductivity. The extent of variability is most often represented by the coefficient of variation because it is a relative measure of variation and allows comparisons across studies (Warrick and Nielsen, 1980; Peck, 1983). In general, bulk density and saturated water content exhibit relatively small variation, with CVs between 7 and 10%, while water contents at various matric potentials exhibit CVs ranging from 10 to 100%. The greatest variation is reported for saturated hydraulic conductivity, with CVs >100% (Warrick and Nielsen, 1980; Peck, 1983). Large variations of soil property measurements have the potential to produce modeling errors.
While previous research has focused on soil spatial variability between landscapes and within soil series (Warrick and Nielsen, 1980; Wilding and Drees, 1983; Wilding, 1985; Mulla and McBratney, 2002), few investigations have compared soil characteristics on adjacent fields with similar soil series but varying cropping systems and management practices. Studying adjacent fields minimizes the possibility for different soil-forming factors, such as climate and landscape position, to influence soil variability (Reganold, 1988).
Land use and anthropogenic management practices can influence soil properties. This study examined the effects of alternative and conventional management practices and crop rotations on soil physical properties within three soil series. Alternative cropping systems included both organic management practices and practices with minimal inputs of inorganic fertilizer. Alternative and conventional farming systems differ in their soil and crop management, and these differences presumably will affect soil properties. While conventional farming systems aim to maximize crop yield, alternative farming systems focus on sustainability. Conventional management practices rely on the use of synthetic fertilizers, herbicides, and pesticides for crop production and elimination of weeds and insects, while alternative farming systems focus on natural processes, manures, and green manures for crop production and weed and pest control (National Research Council, 1989; Stockdale et al., 2001). Alternative farming systems consider the physical limitations of the land, such as soil characteristics and water availability, when designing crop rotations and focus on biological processes to enhance nutrient cycling.
Alternative farming systems incorporate crop rotations as a primary management tool. Crop rotations are designed to include legumes that fix N, disrupt weed and insect cycles present in conventional farming practices, and enhance soil structure and organic matter, thus eliminating synthetic fertilizers, herbicides, and pesticides from the system (Power, 1987; Granatstein, 1988; Lampkin, 1990; Stockdale et al., 2001). To enhance soil structure, deep-rooted plants often follow shallow-rooted plants in alternative crop rotation designs (Lampkin, 1990). Crop rotations that include perennial species have been shown to decrease soil erosion and NO3–N leaching from the soil (Randall et al., 1997).
Alternative management practices that can influence soil physical properties include tillage and diverse crop rotations. No-till and ridge-till practices, often utilized in alternative farming systems to attain long-term soil fertility, have been shown to reduce soil erosion (Lampkin, 1990). Legumes in crop rotations are often plowed under as green manures to provide nutrients to the soil. Green manure crops include hairy vetch (Vicia villosa Roth subsp. villosa), while alternative cash crops include wheat (Triticum aestivum L.), barley (Hordeum vulgare L.), oat (Avena sativa L.), and rye (Secale cereale L.) (Lampkin, 1990). Conventional crops such as corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] are often rotated with these crops. In a review article, Reganold (1995) reported that studies conducted around the world consistently found lower bulk densities in soils with alternative cropping systems when compared with conventional systems. Lower bulk densities often are related to increased porosity and saturated hydraulic conductivity of the soil. Soils in the Palouse region of Washington exhibited lower bulk densities, higher porosities, and significantly higher saturated hydraulic conductivity when managed with natural prairie versus conventional tillage and no-till (Fuentes et al., 2004). No-till soils had higher saturated conductivities than those managed with conventional tillage. Other studies have also shown that soils managed with alternative cropping systems and practices maintained higher porosity and infiltration rates when compared with conventional systems for various soil series (Metzger and Yaron, 1987; Reganold, 1995; Lotter et al., 2003). Lower bulk densities with alternative management practices compared with conventional practices also imply deeper root growth and easier root penetration for improved crop development (Reganold, 1995).
Increased porosity and infiltration rates reduce coarse-textured soils' ability to retain moisture; however, fine-textured silt loam soils with alternative cropping systems and management practices have been found to retain significantly more water when compared with conventional systems (Reganold et al., 1987; Reganold, 1988; Lotter et al., 2003). A study conducted on a loam soil in the Netherlands also found higher water content in soils under alternative cropping systems, but the results were not significantly different (Droogers and Bouma, 1996). When water is retained in the soil, less leaching of nutrients occurs. Less leaching of nutrients improves ground and surface water quality by reducing transport of nutrients that cause eutrophication and hypoxia.
The objective of this study was to examine the effects of long-term alternative and conventional farming practices on soil physical and hydraulic properties.
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MATERIALS AND METHODS |
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TOPEXECUTIVE SUMMARYABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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The alternative cropping systems included 48.5 ha of certified organic management. In this area N requirements of alternative crops were largely provided by additions of composted solid beef manure or plowdown of legume and cover crops. The remaining 16.5 ha of the alternative site received minimal inorganic fertilizer inputs. Primary tillage included chisel plow, moldboard plow, and no-till treatments for both management systems, excluding areas in the alternative cropping system planted to perennial crops. Moldboard plow was used following corn harvest somewhat more frequently in the alternative cropping systems (17% of total area) than in the conventional cropping systems (7% of total area). In the corn–soybean rotations under alternative management practices, secondary tillage occurred more often compared with conventional practices due to the need for nonchemical weed population control methods.
Soils at the site were formed in glacial till and included Webster (Typic Endoaquolls) and Revere (Typic Calciaquolls) clay loams, Okoboji (Cumulic Vertic Endoaquolls) silty clay loam, Ves (Calcic Hapludolls), Storden (Typic Eutrudepts), Linder (Aquic Hapludolls), and Normania (Aquic Hapludolls) loams, and Estherville (Typic Hapludolls) sandy loam. The Webster, Normania, and Ves soils covered a majority of the area (78–94%) at the study site (Table 1, Fig. 2 ). Webster soils are typically poorly drained and are located at bottom-slope positions. Normania soils are moderately well drained and are located on concave mid-slope areas, while Ves soils are well drained and are located on convex upper-slope areas.
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Soil analyses included soil texture, organic C content, soil moisture retention, vertical saturated hydraulic conductivity (Ks), and bulk density. The A, B, and C horizons were analyzed, with the B and C horizons divided into soils with and without carbonates when carbonates were present. The depths of the soil horizons are presented in Table 2. Soil texture and organic C content were analyzed at the University of Minnesota Research Analytical Laboratory. Soil texture was analyzed using the hydrometer method (Miller et al., 1997). Total organic C was analyzed on a 0.2- to 0.5-g ground and sieved sample by dry combustion and infra-red detection on a Primacs TOC Analyzer (Skalar, Inc., Norcross, GA). Analyses for soil moisture retention, Ks, and bulk density were conducted on undisturbed samples. Soil moisture retention was characterized with two replicate samples approximately 1 cm in height and 5 cm in diameter at 10-kPa matric potential using slight modifications of the hanging water column method and two replicate samples at 30- and 100-kPa matric potentials using the pressure chamber method (Berliner et al., 1980; Klute, 1986). Plastic sleeves used to store cores were cut and sized to fit around the samples to hold the soil in place. The soil cores were carefully cut to sample size to avoid structural disturbance. Moisture content at 0 kPa (
s) was determined by oven drying the Ks samples after Ks measurements were completed. Linear regression and log-log plots of moisture contents () for the 10-, 30-, and 100-kPa matric potentials (
) were used to estimate the b and air entry (e) parameters of the two-parameter (b, e) Campbell moisture retention model (Campbell, 1985) given by
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Statistical Analysis
Data analysis focused on two comparisons: (i) alternative versus conventional management practices for the entire site and (ii) alternative versus conventional management practices across the three major soil series (Normania, Webster, and Ves). The examination of management effects on soil properties across soil series was conducted to remove effects of natural differences in soil types across management systems. Each soil horizon was investigated separately. The Linder and Okoboji soils and the Storden–Estherville–Ves soils complex occurred primarily on the alternative field, while the Normania, Webster, and Ves soils occurred frequently on both the alternative and the conventional fields. For the two comparisons, the Kruskal–Wallis nonparametric test was performed on the medians using SYSTAT 11 (SYSTAT Software, Inc., Richmond, CA). Statistical significance was determined at the P < 0.01 and P < 0.05 levels for both comparisons.
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RESULTS AND DISCUSSION |
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TOPEXECUTIVE SUMMARYABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Subsurface soil Ks was higher than surface soil Ks under both alternative and conventional management practices. This difference was attributed to the presence of macropores that likely resulted from the shrinking and swelling of clay in these subsoils. Laboratory observations revealed that macropores, formed by soil fauna, root channels, or shrink–swell processes, were more prevalent in subsurface samples than surface samples for both alternative and conventional management practices.
In contrast to significantly higher Ks in surface soils, there were no significant differences in subsurface soils between Ks values under alternative management practices compared with conventional practices. It should be noted that significant differences were more difficult to detect for Ks than for the other soil properties, as the variability was up to 10 times higher.
Soil Moisture Retention Curves
Moisture retention curves express the relationship between soil matric potential and moisture content. The alternatively managed system had significantly smaller Campbell b values than the conventionally managed system in the A, B, and C horizons. A smaller b value indicates that as matric potential decreases, the soil releases more water than a soil with a larger b value, all other factors being equal. Since there were no significant differences in soil texture between the two management systems in the A horizon, differences in the b parameter are probably due to management effects. Differences in the b values for B and C horizons between the alternative and conventional systems are probably due mainly to the smaller clay contents in soils on the alternative versus the conventional system.
Air entry values were significantly larger in the A and C horizons of soils on the alternatively managed system than soils on the conventionally managed system. There were no significant differences in air entry potential in the B horizon. Larger air entry values in the A horizon of the alternatively managed system are consistent with its smaller bulk density and better soil structure in comparison with the conventionally managed system. Diverse cropping systems, additions of animal manure and differences in tillage on the alternative system could all contribute to smaller bulk density, better soil structure, and larger air entry values than in the conventional system.
The above results showed that management practices and crop rotations have beneficial effects on soil physical and hydraulic properties, particularly in the A horizon. These differences have traditionally been difficult to account for in water flow and water quality models, and the use of soil physical or hydraulic properties typical of conventional crop rotations in such models may not accurately represent the effects of alternative crop rotations on drainage and nutrient loss estimates.
Comparison of Alternative and Conventional Management Practices by Soil Series
Soil Texture and Organic Carbon Content
As stated above, soil texture and organic C content may influence soil moisture retention properties that are used by modelers to determine rates of water flow and nutrient fluxes through subsurface drainage. A comparison of alternative and conventional management practices across soil series was conducted to determine whether or not soil differences could explain more of the differences in soil physical properties than cropping system differences. In general, cropping systems explained more of the differences than soil series, since there were no significant differences in sand and silt content or organic C content between alternative and conventional management practices across soil series for any of the soil horizons (Table 5). However, clay content was significantly lower in the alternative management system compared with the conventional system for all horizons of the Normania soils.
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Saturated Hydraulic Conductivity
Saturated hydraulic conductivity within a soil series followed the same pattern as the comparison of alternative versus conventional management practices for the entire site, with the exception of the Ves subsurface soils. Saturated hydraulic conductivity was generally higher in soils in the A horizon with alternative management practices compared with conventional practices (Table 6). Significant differences in Ks between alternative (43.2 cm d–1) and conventional management practices (0.7 cm d–1) were observed in Webster A horizon soils, which are located at bottom-slope positions. There were no significant differences in Ks for Normania A horizon soils (mid-slope positions) or Ves A horizon soils (upper-slope positions) between alternative and conventional management systems. It should be noted that significant differences are difficult to detect with the high level of variability in Ks data (Warrick and Nielsen, 1980; Peck, 1983). Nevertheless, it appears that increases in Ks under alternative management practices are greatest at bottom-slope positions, and the differences in Ks between alternative and conventional management systems progressively decline going from bottom- to mid- to upper-slope positions.
There were no significant differences between soil texture or soil organic C in the Webster A horizon soils (Table 5); therefore, differences in Ks for Webster A horizons are most likely due to differences in soil structure affected by management practices. Higher Ks under alternative management practices would be expected to lead to greater infiltration and less surface runoff than with conventional management practices.
In subsurface soil horizons, Ks values were larger than values in the corresponding topsoil horizons, consistent with results from the earlier comparison between alternative and conventional management systems. There were no significant differences in Ks values for subsurface soils in the Normania, Webster, or Ves soils, although Ks values for a given soil were generally larger under conventional practices than under alternative practices in the Normania and Webster soils.
Soil Moisture Retention Curves
The b value of the Campbell moisture retention model was significantly smaller in all horizons of the Normania soil under alternative practices than conventional practices (Table 6). This is consistent with significantly smaller clay contents in all horizons of the Normania soil on the alternatively managed system in comparison to the conventionally managed system. Webster soil b values were significantly smaller in the A horizon under alternative management than conventional management, despite the fact that there were no significant differences in clay content. Thus, the differences in slope of the moisture retention curve in Webster A horizon soil can be attributed to management effects, whereas the differences in Normania soils are probably due to a combination of differences in soil texture as well as differences in management. There were no significant differences in slope of the moisture retention curve for Webster B and C horizons or for any horizon in the Ves soil under alternative management versus conventional management.
In general, air entry values were larger under alternative management than conventional management (Table 6), although the differences were only significant for the Normania and Webster C horizons. Thus, there was some evidence that alternative management produced a greater frequency of large pore sizes in comparison with conventional management, but additional study of this phenomenon is warranted.
The results of these comparisons showed that alternative management practices have beneficial effects on soil physical and hydraulic properties within a soil series. Because soil physical and hydraulic properties for water flow and water quality models are often extracted from soil databases based on fields in conventional management, simulations of flow and transport for alternative management systems that rely on these databases may be inaccurate. When applicable, modelers should adjust the soil properties for effects of alternative management practices to avoid these errors.
Water Flow Simulations in Webster Soil
The HYDRUS-1D model (Simunek et al., 1998) was used to illustrate the impacts on water flow of differences in soil physical and hydraulic properties for the Webster soil under alternative versus conventional management. Two initial and boundary condition scenarios are considered, each involving 1 d of water flow. The first involves constant potential at the soil surface ( = 0 kPa), free drainage, and an initial soil profile matric potential of = –50 kPa. The second involves a soil profile initially at static equilibrium with a water table located in the B and C horizons ( = 0 kPa at the top of the B horizon) with tile drainage (spacing 55 m, depth 1.3 m) and a zero flux upper boundary condition. For each scenario, median values of measured soil physical and hydraulic properties for the Webster soil A, B, and C horizons (Table 6) under alternative versus conventional management were used as input to the model (after converting to the van Genuchten form of the moisture retention curve). Neither of these scenarios considers the effects of cropping system on evapotranspiration.
After 1 d of infiltration ( = 0 kPa) and free drainage from an initially dry soil profile ( = –50 kPa), 30 cm of water infiltrated into the Webster soil under alternative management, whereas only 0.5 cm of infiltration occurred under conventional management. These large differences in infiltration are largely due to the significantly larger saturated hydraulic conductivity in the A horizon of the Webster soil under alternative management versus conventional management. By the end of 1 d of infiltration, water flux at the bottom of the Webster soil profile reached 13 cm under alternative management, whereas no water flow at the lower boundary occurred under conventional management. Based on this scenario, there should be significant effects of alternative management practices on infiltration and free drainage in comparison with conventional management.
In the second scenario, a Webster soil in static equilibrium with a water table at the top of the B horizon had cumulative tile drain water fluxes of 0.25 versus 0.35 cm after one day of drainage for alternative versus conventional practices, respectively. These differences in drainage between management practices can be attributed to differences in saturated hydraulic conductivity in the Webster B and C horizons (tile drains are located in the C horizon). Drainage under alternative practices may decrease even further in comparison to these results if differences in evapotranspiration and height of the water table between the alternative and conventional practices are considered. Randall et al. (1997) reported that during a 6-yr period, drainage from row-crop systems was 1.6 times higher than drainage from perennial grasses and alfalfa because the greater season-long evapotranspiration with perennial species and alfalfa resulted in greater water uptake and less drainage.
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CONCLUSIONS |
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TOPEXECUTIVE SUMMARYABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Overall, alternative management practices improved soil physical properties and should provide environmental benefits to improve water quality compared with conventional management practices. The results of this study imply that computer simulations of drainage and nutrient leaching under alternative management practices could be improved by taking into account the beneficial effects of alternative management practices on soil bulk density and saturated hydraulic conductivity.
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ACKNOWLEDGMENTS |
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REFERENCES |
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TOPEXECUTIVE SUMMARYABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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This article has been cited by other articles:
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  K. A. Oquist, J. S. Strock, and D. J. Mulla Influence of Alternative and Conventional Farming Practices on Subsurface Drainage and Water Quality J. Environ. Qual., June 27, 2007; 36(4): 1194 - 1204. [Abstract] [Full Text] [PDF]
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 D. L. Corwin, J. Hopmans, and G. H. de Rooij From Field- to Landscape-Scale Vadose Zone Processes: Scale Issues, Modeling, and Monitoring Vadose Zone J., March 8, 2006; 5(1): 129 - 139. [Abstract] [Full Text] [PDF]
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