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SOIL & WATER MANAGEMENT & CONSERVATION

Regional Assessment of Soil Compaction and Structural Properties under No-tillage Farming

Carbon Management and Sequestration Center, FAES/OARDC School of Environment and Natural Resources, The Ohio State Univ., 412C Kottman Hall, 2021 Coffey Rd., Columbus, OH 43210-1085

^* Corresponding author (blanco.16{at}osu.edu).

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
No-tillage (NT) farming is a proven technology for soil and water conservation, but its impacts on soil compaction and structure development are soil- and site-specific. We conducted a regional assessment of long-term (>5-yr) NT farming impacts on soil compaction, structure, and aggregate-associated soil organic carbon (SOC) concentration across 13 contrasting but representative soils in the eastern USA, each within a Major Land Resource Area (MLRA: 111C, 98, 114B, and 122 in Indiana; 111A, 111B, 111B2, 99, 111D, 124, and 126 in Ohio; and 147 and 127 in Pennsylvania). Each MLRA comprised NT, chisel plow (CP), and woodlot (WL) land uses. Impacts of NT management were moderate on soil compaction, small on soil structural properties, and nonsignificant on aggregate-associated SOC concentration. No-tillage soils had higher cone index (CI) and shear strength than CP in nine out of the 13 MLRAs, and they had the highest CI (2 MPa) and shear strength (>180 kPa) within MLRAs 122 and 124. Bulk density (_b) in NT was higher than in CP soils only in 111B (1.31 vs. 1.18 Mg m^–3) and 127 (1.37 vs. 1.17 Mg m^–3). No-tillage farming increased the mean weight diameter (MWD) of aggregates by a factor of 1.6 in MLRA 99, by 3.0 in 124, and by 5.3 in 111A, and reduced their tensile strength (TS) in 114B, 126, and 111B by a factor of 2.5. Macroaggregates (>1 mm) contained 15 to 100% more SOC than microaggregates. Woodlot soils had the lowest _b and TS and the highest MWD and aggregate-associated SOC concentration. The MWD increased with increasing SOC concentration. Overall, the impacts of no-tillage farming on soil compaction and structure were small compared with plow tillage.

Abbreviations: CI; cone index • CP, chisel plow • MLRA, Major Land Resource Area • MWD, mean weight diameter • NT, no-tillage • SOC, soil organic carbon • TS, tensile strength • WL, woodlot

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
No-tillage agriculture is among the top options in the portfolio of technologies to reduce tillage costs, conserve soil and water, increase soil organic carbon (SOC) pools, and reduce net CO₂ emissions, which contribute to global warming. By eliminating intensive soil plowing and leaving crop residues on the soil surface, no-tillage (NT) agriculture generally improves soil properties, increases nutrient content and recycling, and moderates fluxes of water, air, and heat through the soil, resulting in improved agronomic productivity. Although the area under NT is, at present, only about 5% of the cultivated land (1379 Mha) in the world, NT technology is gaining wide acceptance, particularly in North and South America (Lal et al., 2004). In the USA, the area under NT, mostly in the Corn Belt region and Northern Plains, has increased from 5 Mha in the 1980s to about 22 Mha in 2000, which represents 18% of the total cultivated land area. No-tillage farming is expected to increase to about 75% in the USA by the year 2020 (Lal et al., 1998, p. 18–21). Adopting NT technology is advocated not only to meet the requirements of soil conservation programs but also to sustain crop production.

Despite their numerous benefits, however, various questions still remain about the potential of NT systems for improvement of soil physical quality (Blanco-Canqui et al., 2004) and long-term SOC sequestration (Baker et al., 2006). In fact, NT may not always improve the physical and mechanical properties of soils (Arshad et al., 2004), for example, its role in alleviating soil compaction is unclear. Soil compaction often increases with the conversion of plow tillage into NT systems from the lack of transient soil loosening by tillage operations (Drury et al., 2003; Seybold et al., 2003; Bueno et al., 2006). Machinery traffic during planting, harvesting, manuring, and weed and pest control can adversely affect soil strength properties, especially in soils under highly mechanized NT agriculture such as in the U.S. Corn Belt region. Excessive compaction in NT can retard root growth and reduce crop yields compared with chisel plow (CP) practices and create mixed reactions to NT adoption. This is the reason why some farmers, although NT advocates, favor occasional tillage (e.g., deep tillage) to ameliorate compaction problems in soils under NT that are susceptible to natural reconsolidation. In contrast, in some soils, NT management may not always increase soil compaction (Cassel et al., 1995; Arshad et al., 1999). Indeed, it can even decrease soil compaction due to the addition of organic amendments (e.g., animal manure) and enhanced biological processes (e.g., earthworm activity) (Logsdon and Karlen, 2004; Gregory et al., 2005; Blanco-Canqui et al., 2005a). The impacts of NT management on soil structural properties such as stability and strength of aggregates are also variable. Knowledge of properties of soil aggregates, structural units, is indispensable, but they have not been widely characterized to understand the macroscale structural behavior of NT soils (Horn, 1990). For example, quantification of aggregate-associated C distributions across contrasting NT management systems is necessary to determine the dynamics of aggregation and turnover rates of SOC. Moreover, the magnitude of soil compaction and structural parameters may (Blanco-Canqui et al., 2005a) or may not (Karlen et al., 1994) be affected by changes in SOC pools in long-term (>10-yr) NT systems.

The inconsistent response of soil compaction and structural properties to NT systems warrants further research. To date, most studies on soil physical quality in NT have been restricted to research on small plots or point measurements. A regional study involving a range of environments is required to gain a broader understanding of the impacts of NT technology by integrating information across a large geographic spectrum of NT management scenarios under contrasting soil types and topographic and management conditions. Research data from NT practices on farmers' fields would provide a better understanding of soil processes under real-world conditions. Information on a regional scale is especially needed when large areas of croplands are being gradually converted into long-term NT systems. A regional assessment, involving synthesis of data from contrasting soils, can lead to a broad generalization of changes in soil compaction and structural properties in relation to SOC pools.

The Midwest Regional Carbon Sequestration Partnership (MRCSP) initiative, established in 2003 as one of seven partnerships in the U.S. Department of Energy's Carbon Sequestration Program to assess the potential of C sequestration and strategies for mitigating CO₂ emissions, offers an excellent opportunity for a regional assessment of NT farming implications on soil physical quality in the eastern Corn Belt region. On-farm studies of soil processes within each Major Land Resource Area (MLRA) allow an elucidation of the uncertainties of NT impacts on soil compaction in relation to changes in SOC pools. Thus, the objectives of this study were to assess (i) the impacts of NT farming on soil compaction, soil structure development, and aggregate-associated SOC distributions compared with CP and forest management systems, and (ii) the relationships of soil compaction and structural parameters to management-induced changes in SOC, if any, across 13 selected MLRAs in the eastern USA.

	MATERIALS AND METHODS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Description of the Study Sites
This study was conducted within the MRCSP framework, whose primary goal is to characterize the potential of terrestrial ecosystems for sequestering SOC and reducing CO₂ emissions. Paired fields under long-term (>5-yr) NT and CP systems were selected within 13 representative MLRAs across Indiana, Ohio, and Pennsylvania in the eastern USA (Table 1 ). The selected MLRAs were: 111C, 98, 114B, and 122 in Indiana; 111A, 111B, 111B2, 99, 111D, 124, and 126 in Ohio; and 147 and 127 in Pennsylvania (Fig. 1 ). These MLRAs were selected on the basis of their large area and high potential for SOC sequestration. A woodlot (WL) or forest site, adjacent to each paired cropped soil, was included in the study for comparison. Detailed information on soil and management for each selected site for the 13 MLRAs is presented in Table 1. Soil textural classes included sandy loam, silt loam, silty clay loam, and clay loam, with silt loam being the most common. Slope gradient ranged from 1 to 6% and the parent materials differed among the MLRAs. Soils in Indiana (111C, 122, 98, and 114B) and central and western Ohio (111A, 111B2, IIID, 99, and 111B) formed on lowland glacial deposits including dense and deep glacial till and lake sediments, whereas those in Pennsylvania (127 and 147) and eastern Ohio (124 and 126) are unglaciated and developed from weathered sedimentary deposits of shale, siltstone, and sandstone (NRCS, 2007). While the same land use and management systems were selected for all MLRAs, cropping systems and the duration of tillage management were not identical for all sites (Table 1). In a few MLRAs, NT and CP fields were not adjacent to each other but 0.16 to 8 km apart. Wooded sites were, however, always adjacent to either NT or CP farms. It was not always possible to locate adjoining NT and CP paired sites. Three soil strength parameters, cone index (CI), shear strength, and bulk density (_b), were determined to evaluate the level of soil compaction, while two structural parameters, mean weight diameter (MWD) and tensile strength (TS) of aggregates, were quantified to discern differences in soil structural development among the three management and land use scenarios.

View larger version (7K): [in this window] [in a new window]   Fig. 1. Map of the three states (Indiana, Ohio, and Pennsylvania) showing the soil sampling points within each of the 13 Major Land Resource Areas.

Determination of Structural Properties and Aggregate-Associated Soil Carbon
A bulk soil sample of about 1 kg was collected from each field and MLRA in spring 2006 from the 0- to 5-cm depth for the determination of TS and stability of aggregates. The bulk samples were air dried at about 20°C for 72 h, gently crushed, and dry sieved to obtain aggregates in the 4.75- to 8-mm size range. Water-stable aggregates (WSA) were characterized by the wet-sieving procedure (Nimmo and Perkins, 2002). Fifty grams of 4.75- to 8-mm aggregates were saturated by capillarity on top of a nest of sieves of 4.75-, 2-, 1-, 0.5-, and 0.25-mm mesh, and vertically oscillated in water at 30 cycles min^–1 for 30 min using a sieving device. Soil retained in each sieve was transferred to preweighed beakers, oven dried at 50°C, and weighed to compute the percentage of WSA and MWD of aggregates (Nimmo and Perkins, 2002). The soil fraction of <0.25 mm was obtained by collecting the sediment after decanting the water, and determining the oven-dry weight. Aggregate-size fractions between 0.25 and 8 mm were classified as macroaggregates and those <0.25 mm as microaggregates (Tisdall and Oades, 1982). The TS of the 4.75- to 8-mm aggregates was determined using the crushing method (Dexter and Watts, 2001). Nine aggregates per treatment were used for the TS tests to account for the expected high variability.

A portion of these samples dried at 50°C from each aggregate-size fraction was ground and passed through a 0.25-mm sieve for the determination of the aggregate-associated SOC concentration by the dry combustion method (900°C) using a CN analyzer (Vario Max, Elementar Americas, Mount Laurel, NJ) (Nelson and Sommers, 1996). In addition, total SOC concentration was determined on the bulk soil sampled in fall 2006. This study focuses only on soil physical properties, aggregate-associated SOC concentration, and relationships between SOC concentrations and soil physical properties. A one-way ANOVA model was used to test whether differences in soil physical properties and aggregate-associated SOC among the three land use and management systems by MLRA were significant. In this study, differences in soil properties among the three treatments were tested by site or MLRA to account for the high variability of soils among sites. Correlation analyses of SOC concentration vs. soil physical properties were performed with and without the WL data to assess relationships in both cropped soils and across all management systems, respectively. All statistical analyses were conducted using SAS software (SAS Institute, 2007).

	RESULTS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Soil Compaction Parameters
There were no significant differences in _v between NT and CP practices at any of the MLRAs (Fig. 2A ). Although NT soils tended to have greater _v than CP soils in eight out of 13 MLRAs, differences were not significant due to the high variability of _v values. Both NT and CP soils had, however, greater _v than wooded sites in the majority of MLRAs, and this is explained by the lower _b of forest soils (Fig. 2B). While _g under WL management was typically higher than that for NT and CP systems, the opposite was true for _v.

View larger version (34K): [in this window] [in a new window]   Fig. 2. Mean volumetric water content and bulk density by management for each Major Land Resource Area (MLRA) across Indiana, Ohio, and Pennsylvania. Bars with the same letters within each MLRA are not significantly different at the 0.05 probability level.

Differences in _g between CP and NT management were not significant for any of the sites and MLRAs. Moreover, regression fits between _g and CI and shear strength across all cultivated systems and MLRAs were not significant. Thus, no adjustment in measured CI and shear strength values to account for the possible confounding effect of _g on soil strength properties was performed (Busscher et al., 1997; Blanco-Canqui et al., 2005a). Differences in CI (Fig. 3A ) and shear strength (Fig. 3B) between NT and CP management were more significant than differences in _b. Similar trends in CI and shear strength under CP and NT soils indicated the strong interdependence between these parameters. Soils under NT had higher CI and shear strength than those under CP in nine out of the 13 MLRAs, and also had the highest values of CI (2 MPa) and shear strength (>180 kPa) in two MLRAs (122 and 124). The CI in CP soils was consistently lower than that in NT soils except in 114B, where it was higher by 30%. The CI values across the whole region ranged from 0.44 to 2.13 MPa in NT soils and from 0.20 to 0.76 MPa in CP soils. In contrast, the shear strength values ranged from 12 to 54 kPa in NT soils and from 8 to 41 kPa in CP soils. Both CI and shear strength values were lower in WL soils than in NT soils but did not differ from those in CP soils except in MLRAs 98, 114B, 111B2, and 147.

View larger version (22K): [in this window] [in a new window]   Fig. 3. Mean cone index and shear strength by management for each Major Land Resource Area (MLRA) across Indiana, Ohio, and Pennsylvania. Bars with the same letters within each MLRA are not significantly different at the 0.05 probability level.

View larger version (31K): [in this window] [in a new window]   Fig. 4. Geometric mean tensile strength and mean weight diameter of soil aggregates under three management systems across 13 Major Land Resource Areas (MLRAs) in Indiana, Ohio, and Pennsylvania. Bars with the same letters within each MLRA are not significantly different at the 0.05 probability level.

	DISCUSSION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

Increases in soil compaction by NT management relative to CP within each MLRA were not always correlated with changes in SOC concentration. The fact is that while adoption of NT practices consistently increased soil compaction, it did not increase SOC concentration for any aggregate size compared with CP practices at any of the MLRAs (Table 3). The higher CI and lower SOC concentration in NT soils compared with those in CP soils for MLRAs 111B2, 127, and 98 implies that a lower SOC concentration probably contributed to an increase in soil compaction levels in NT systems. In contrast, NT management had higher SOC concentration and soil compaction levels than CP in MLRA 124. These data show that relationships between SOC concentration and soil compaction parameters were soil specific.

Similar to the effects on soil compaction, the impacts of NT farming on soil structural properties were small and site specific (Fig. 4A and 4B). Soils under CP management reduced aggregate stability slightly compared with those under NT, but drastically when compared with those under WL management. This trend may be attributed to frequent disruption of aggregates by tillage in CP systems. The lack of disturbance in association with increased biological activity in NT and WL systems promotes aggregation (Shukla et al., 2003). The higher aggregate strength in CP than in NT soils for MLRAs 126 and 111B did not result in higher aggregate stability in CP soils because, during wet sieving, these aggregates slaked rapidly and differences in aggregate stability between CP and NT soils were not generally significant. These results are in accord with those reported from long-term cultivated watersheds by Shukla et al. (2003) and Blanco-Canqui et al. (2005b), who observed that CP soils had significantly higher aggregate strength but lower stability than long-term (>35-yr) NT on an unglaciated Rayne silt loam in Ohio. These trends are attributed to the fact that aggregates formed by organic binding agents, abundant in NT soils, are more water stable than those in soils under a CP system. The strong aggregate stability vs. SOC concentration relationship across the cropped soils indicates that increases in SOC concentration had an overall positive influence on macroaggregation, in accord with similar data reported for the region (Blanco-Canqui et al., 2006). The higher SOC concentration in macro- than in microaggregates indicates that the former are important to the sequestration and retention of SOC.

Aggregate-associated SOC concentration was not increased by the adoption of NT systems for any aggregate size at any of the MLRAs (Table 2). These results are not surprising, however, given that data from on-farm testing are affected by many interactive management factors compared with those from experimental research plots or small watersheds managed under controlled conditions. This on-farm study across a wide range of soil and management conditions shows that NT technology may not always increase SOC concentrations at the aggregate level relative to CP management. These findings have important implications for assessing the potential of NT farming for sequestering C and offsetting CO₂ emissions. We hypothesize that, in some soils, C gains in NT aggregates from higher input of biomass C may be offset by greater losses of C as greenhouse gases (e.g., CO₂ and CH₄) due to higher soil water content and favorable temperature in summer, thereby reducing any SOC gains in NT relative to CP systems (Baker et al., 2006; Venterea et al., 2006).

The inconsistent changes in soil physical properties and their relationships with SOC concentration with NT farming within each MLRA may be explained by differences in (i) soil attributes, (ii) management duration, (iii) cropping systems, and (iv) crop residue management. Within a generic NT system, farmers used a wide range of practices. Unlike in studies on small research plots, in this study factors such as slope gradient and cropping systems between NT and CP fields were not always identical in each MLRA. For example, soil textural and topographic characteristics varied among the 13 MLRAs (Table 1). The soils were developed under different parent materials and textural classes ranging from sand loam to clay. These differences probably influenced the magnitude of NT farming impacts on soil physical quality and aggregate-associated SOC concentration. For example, in a clayey soil (MLRA 99) in Ohio, the relationship between SOC concentration and soil physical properties was not significant, whereas in silt loams (MLRAs 114B and 124) the correlations were strong (P < 0.01). In some MLRAs, slope gradient significantly differed even within the same soil series for NT and CP systems.

The duration of NT and CP management also differed among all MLRAs. The NT management duration ranged between 5 and 30 yr. Improvements in soil structure and increases in SOC pools are a function of the duration of the NT system. The 35-yr NT management in MLRA 124 increased aggregate size by a factor of 6, whereas the 5-yr NT management in MLRA 111D did not have any effect on aggregate size when compared with the CP system. The 35-yr NT management compacted soil more than the 5-yr NT management, contrasting with the perception that long-term NT management often ameliorates compaction problems as the soil structure restores from initial stresses toward a steady-state equilibrium with time (Wilkins et al., 2002).

While some of the NT farms received a complete return of crop residues after harvest, others, for example under corn (Zea mays L.) for silage, received only partial or none. Complete residue return is vital to buffer wheel traffic and raindrop impacts, reduce soil compaction, and increase both aggregate stability and aggregate-associated SOC concentration in NT systems. Some of the fields received animal manure, which often has positive effects on soil physical quality and SOC pools (Mosaddeghi et al., 2000). Blanco-Canqui et al. (2005a) reported that CI and shear strength in cultivated watersheds under NT soils without manure were about twice as high as those in NT with manure. In this study, soil compaction parameters increased in MLRAs 126 and 127 in spite of frequent manure application. Cropping systems and the length of crop rotations also differed with MLRAs and tillage management. While a corn–soybean [Glycine max (L.) Merr.] rotation was the dominant system, there were other rotations such as corn–alfalfa (Medicago sativa L.) (MLRAs 124 and 127) and corn–soybean–wheat (Triticum aestivum L.) (MLRA 99). Soil aggregates were more stable and the aggregate-associated SOC concentration was relatively higher in NT under a corn–alfalfa rotation than in CP continuous corn in MLRA 124.

Cropping (CP and NT) increased soil compaction and reduced soil structural stability and aggregate-associated C in all aggregates size fractions compared with soils under WL management, which were less compact, more stable, and had higher aggregate-associated SOC concentrations than cropped soils. Inclusion of soils under WL in the correlation analyses greatly increased the significance of correlation coefficients between SOC concentration and soil physical properties.

	CONCLUSIONS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
The data presented show that NT management induced moderate changes in compaction across a broad range of soils in the eastern USA. The moderate compaction is not likely to adversely impact crop production because the values are below the high thresholds levels of compaction. The impacts of NT on soil structural properties are similarly small and site specific. Chisel-plowed soils reduced aggregate stability moderately compared with those under NT systems and drastically compared with wooded land. No-tillage farming appeared not to increase soil aggregate-associated SOC concentration compared with plow tillage in any of the soils studied. Differences in soil attributes, NT duration, cropping systems, and management of crop residues among the soils studied may explain the variable impacts of NT on soil properties. On a regional basis, improvement in aggregate stability is positively correlated with increases in SOC concentration. Cropping compacted and degraded soil structure and reduced aggregate-associated SOC concentration compared with wooded land. Further monitoring of spatial and temporal changes of the measured soil physical properties and aggregate-associated soil organic C concentration across these and other representative NT soils on a regional scale is recommended to make comparisons based on multiyear data.

	ACKNOWLEDGMENTS

 
This research was supported by the U.S. Dep. of Energy through the Midwest Regional Carbon Sequestration Partnership led by the Battelle Memorial Institute, Columbus, OH. Thanks are due to the following farmers for allowing us access to their crop fields: Keith Kemp, Gary Davis, Dale Profit, Rob Rettig, Eldon Cohrs, Michael Clark, Brent Porteus, Dave Biehl, and Brent Zimmer in Ohio; Lynn Teel, Bruce Brown, Scott Fritz, Kevin Podell, Phil Carter, Eldon McCullough, Mike Flock Harrison, and Chris Sanders in Indiana; and Lucas Criswell, Bill Baker, Ross Orner, and Mike Kennis in Pennsylvania. We also thank Sheila Christopher for selecting the study sites and collecting the bulk soil samples for the aggregate analyses in spring 2006.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
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Received for publication February 3, 2007.

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