Saturated Hydraulic Conductivity and Porosity within ...

Published January, 2005

Reproduced from Soil Science Society of America Journal. Published by Soil Science Society of America. All copyrights reserved.

Saturated Hydraulic Conductivity and Porosity within Macroaggregates Modified by Tillage Eun-Jin Park and Alvin J. M. Smucker* ABSTRACT

(Horn et al., 1995; Wan and El-Swaify, 1998). Wetting and drying cycles induce hydration pressures within aggregates, causing unstable aggregates to collapse (Ghezzehei and Or, 2003). Similar forces contribute to the expansion of pores within stable aggregates (Horn et al., 1995). There is a general concept that tillage decreases aggregate stability by increasing mineralization of organic matter and exposing aggregates to additional raindrop impact energies (Balesdent et al., 2000; Ame´zketa, 1999). However, reports of tillage modifications on macropore structure and hydraulic properties at the field scale are often contradictory (Mahboubi et al., 1993; Coutadeur et al., 2002; Heard et al., 1988; Lal and Van Doren, 1990) and there is no comparative experimental data on intraaggregate porosity and associated hydraulic properties modified by tillage. Intraaggregate domains are considered separately from the preferential flow pathway of macropores surrounding aggregates and are considered to be a less permeable region, where water is stored and solutes move by diffusive exchange (Gerke and Kohne, 2002; Cote et al., 1999). Many studies have discussed convective fluxes of water and solutes associated with interaggregate macropore networks (Booltink and Bouma, 1991; Hart and Lowery, 1996; Coppola, 2000; Gerke and van Genuchten, 1993; van Genuchten and Dalton, 1986), whereas few studies have investigated hydraulic properties within individual macroaggregate domains (Gerke and Kohne, 2002). McKenzie and Dexter (1996) demonstrated that microchamber methods could be constructed to accurately measure the permeability through individual aggregates, especially when entrapped air has been removed. Youngs et al. (1994) analyzed water uptake by aggregates with or without air entrapment during wetting of stabilized clay aggregates. Gerke and Kohne (2002) reported differences in the hydraulic conductivities between interior centers and exterior layers of macroaggregates and suggested that discontinuities of pores within aggregates resulted from clay accumulations at soil aggregate surfaces retarding water movement between inter- and intraaggregate domains. Two objectives of this study were to identify the effects of tillage on the saturated hydraulic conductivity (Ks) through intraaggregate pore space and to identify the total porosities and their distributions within differentsized macroaggregates from native forest (NF), conventional tillage (CT), and no tillage (NT) agroecosystems.

Greater knowledge of intraaggregate porosity modifications by tillage conveys new information for identifying additional hydrologic, ion retention, and aggregate stability responses to specific management practices. Macroaggregates, 2 to 4, 4 to 6.3, and 6.3 to 9.5 mm across, were separated into multiple concentric layers and their porosities were determined. Saturated hydraulic conductivity (Ks) of multiple aggregate fractions from two soil types subjected to conventional tillage (CT), no tillage (NT), and native forest (NF) soils were measured individually to identify the effects of tillage on aggregate structure, porosity, and Ks. Intraaggregate porosities were the highest in NF aggregates. Greater porosities were identified in exterior layers of soil aggregates from all treatments. Lowest intraaggregate porosities were observed in the central regions of CT aggregates. Soil aggregates, 6.3 to 9.5 mm across, had the greatest total porosities, averaging 37.5% for both soil types. Long-term CT reduced intraaggregate porosities and Ks within macroaggregates, of the same size fraction, from both the Hoytville silty clay loam and Wooster silt loam soil types. Values for Ks of NF aggregates, 5.0 ⫻ 10⫺5 cm s⫺1, were reduced 50fold by long-term CT treatments of the Hoytville series. The Ks values through Wooster aggregates from NF, 16.0 ⫻ 10⫺5 cm s⫺1, were reduced 80-fold by long-term CT treatments. The Ks values through NF and NT aggregates were positively correlated with their intraaggregate porosities (R2 ⫽ 0.84 for NF and R2 ⫽ 0.45 for NT at P ⬍ 0.005). Additional studies are needed to identify rates at which pore geometries within macroaggregates are degraded by CT or improved by NT.

T

otal porosity within soil aggregates and their connectivities with interaggregate pore spaces influence the movement and retention of solutes, chemical processes, aeration, erosion, and biological activity (Revil and Cathles, 1999). Consequently, spatial distributions of soil organic carbon, solutes, and microbial communities within aggregates depend on the pore development within aggregates (Chenu et al., 2001). Porosity and microstructure within aggregates change with aggregate turnover (i.e., breakdown and reformation), altering the biological activities associated with plant roots and soil fauna (Czarnes et al., 2000; Hussein and Adey, 1998). Tillage enhances the decomposition of soil organic matter and often increases wetting–drying cycles of soils, disrupting soil aggregates in a manner that leads to structural deteriorations including reductions in the stability and collapse of intraaggregate pores (Beare et al., 1994; Paustian et al., 1997). Tillage-induced aggregate breakdown and reformation disrupts the formation of biogeochemical gradients and associated microbial communities that promote the stabilization of macroaggregates

MATERIALS AND METHODS Department of Crop and Soil Sciences, Michigan State University, East Lansing, MI 48824. Received 17 Dec. 2003. *Corresponding author ([email protected]).

Preparation of Soil Aggregate Samples Soil aggregates were sampled at the 0- to 5-cm soil depth in three field replicates from CT and NT continuous corn (Zea

Published in Soil Sci. Soc. Am. J. 69:38–45 (2005). © Soil Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA

Abbreviations: CT, conventional tillage; Ks, saturated hydraulic conductivity; NF, native forest; NT, no tillage.

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PARK & SMUCKER: Ks AND INTRAAGGREGATE POROSITY

mays L.) production systems and adjoining native forest at both Wooster and Hoytville research sites of the Ohio Agricultural Research and Development Center in October 1998. Continuous corn production systems were established in 1962 and 1963, respectively. The Hoytville soil is a poorly drained, silty clay loam that contains higher soil organic carbon contents ranging from 1.9 to 4.1%, at 0- to 5-cm depth, compared with the Wooster silt loam soil, which contains lower soil organic carbon contents ranging from 1 to 2.8% at the same depth (Martens and Dick, 2003). Additional physical and chemical characteristics of these two soil series have been published by Collins et al. (2000). A block of soil (15 ⫻ 15 ⫻ 5 cm) was excavated using a flat spade, wrapped in foam-bubble packing, and transported in a rigid plastic bucket to the laboratory. Moist soil blocks were manually broken along planes of weakness while being air-dried. Air-dried aggregates were very gently sieved, manually, into six size fractions (⬍1, 1–2, 2–4, 4–6.3, 6.3–9.5, and ⬎9.5 mm across) and stored in rigid plastic containers.

Saturated Hydraulic Conductivity Microflow cells, each containing a single aggregate (Fig. 1), were constructed to measure the Ks of individual macroaggregates, 2 to 4, 4 to 6.3, and 6.3 to 9.5 mm across. Cylindricallike aggregates without obvious macropores were placed in the microflow cells and the most longitudinal axis was oriented parallel to the flow cell. Multiple aggregates were chosen for their cylindrical geometries and selected for their similar sizes. One aggregate was installed into each cylinder of microflow cell by filling with a parawax mixture of one part paraffin to six parts of petroleum jelly, by weight. Melting temperature of this parawax mixture was maintained at 55 to 60⬚C. Parawax penetration into the aggregate surface was minimized by adding thin coatings of the melted parawax onto the surface of a cooler aggregate. Sealed aggregates with solidified parawax were installed into the microcell chamber and voids were filled with molten parawax. Maximum diameters of openings were developed by removing the parawax and some of the soil at both ends of the encapsulated aggregate to form a soil column diameter. Soil aggregate length (L ) and diameters (d ) at both ends were measured using vernier calipers with a 0.1-mm resolution and the average cross-sectional area was calculated. Pipette tips, modified by heating and attached to both ends of the cylinder body by parawax, fully enclosed the aggregate (Fig. 1). The flow cell was sealed by encapsulating the entire external surface with a more durable 1:1 parawax mixture. Following solidification of the parawax seal, the entire cham-

ber was encapsulated and sealed by a 1- to 2-mm layer of silicon glue. Individual Ks flow cells were connected to a constant head reservoir and oriented in a vertical position. The inlet was on the lower side of the flow cell preventing air entrapment within the enclosed aggregate during initial hydration (Fig. 2). Aggregates were hydrated slowly, at 0 hydraulic head, using degassed 5 mM CaSO4 solutions to avoid clay dispersion during Ks measurements (Klute and Dirksen, 1986). Then the hydraulic head was slowly increased to 50 cm in three steps at intervals of 20 min, and equilibrated for 1 h before replicated measurements of the flow rates were evaluated gravimetrically. Measurement of very slow flow rates through small aggregates was completed by using a 50-␮L capillary tube (0.91-mm diameter) connected to the outflow of each microchamber. Changes in length of the meniscus were measured in the exit capillary tube at various time intervals. The term Ks was calculated by the following equation:

Ks ⫽ 4vL/(⌬H␲d 2)

[1] 3

⫺1

where v is the flow rate through each aggregate (cm s ), L is the length of the embedded cylindrical soil aggregate (cm), ⌬H is the hydraulic head (50 cm), and d is the mean diameter of two cross-sectional areas measured at each end of soil aggregate. Differences in cross-sectional areas between the inflow and outflow surfaces had a negligible influence on the Ks estimation, as well as the imperfections in the cylindrical form of soil aggregates. We determined Ks for five replicates from each size fraction and each field replicate.

Intraaggregate Porosity Porosities of intact whole aggregates were calculated from bulk density (␳b) and particle density (␳s) measurements for each of the three aggregate size fractions (2–4, 4–6.3, and 6.3–9.5 across), each of the three treatments, and both soil series. Ten uniform spherical to elliptical-shaped aggregates without obvious macropores were selected from each size fraction of each field replicate to determine ␳b by the Sarancoated clod method (Blake and Hartge, 1986a). Individual aggregates were coated with Saran resin (Dow Saran F-310; Dow Chemical Company, Indianapolis, IN) and weighed in air and in water at 23.5⬚C room temperature to calculate their respective volumes and the densities. Particle density was determined by the pycnometer method (Blake and Hartge, 1986b). Duplicate 10-g subsamples of air-dried crushed soil aggregates were used for each size fraction and the weights were corrected for water contents.

Fig. 1. Diagrammatic representation of the microflow cell for measuring saturated hydraulic conductivity (Ks). The term L is the length of the embedded cylindrical soil aggregate, and d is the mean diameter of two cross-sectional areas measured at each end of soil aggregate.


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SOIL SCI. SOC. AM. J., VOL. 69, JANUARY–FEBRUARY 2005

Fig. 2. Diagram of constant head reservoir and microflow cell connections for measurement of saturated hydraulic conductivities (Ks). The term ⌬H is the hydraulic head (50 cm).

Total porosity gradients within aggregates 4 to 6.3 and 6.3 to 9.5 mm across for the Wooster series and 6.3 to 9.5 mm across for the Hoytville series were calculated from bulk densities of aggregates and partial aggregates after removal of onethird, by weight, and two-thirds of the outer layer by the soil aggregate erosion (SAE) abrasion method. These were analyzed for ␳b by the Saran coating method described earlier. A single air-dried aggregate was placed inside the SAE chamber (Fig. 3), secured to a rotary shaker platform, and rotated at 250 rpm to abrade single particles from the aggregate surface. Gentle frictional forces overcome particle bonding strengths on the surface of each eroding aggregate. Air-dry aggregates are very rigid and do not deform or compact during the rotational peeling process. Sometimes the rotational energies break weaker aggregates, which were discarded. Rotational and centrifugal energies peeled concentric layers ranging from less than 1 mm thick to several millimeters, as desired, from aggregate surfaces. Eroded soil fragments were separated from the partially eroded aggregate by a 352-␮m screen and collected for further analyses. The ␳b of each layer was calculated from the bulk densities of aggregates with 0, one-third, and two-thirds of their layers removed by the SAE chamber method, using the following calculations:

␳i ⫽ ␳2 ␳t ⫽ ␳1␳2/(2␳2 ⫺ ␳1) ␳e ⫽ ␳0␳1␳2/(3␳1␳2 ⫺ 2␳0␳2)

[2]

where ␳i, ␳t, and ␳e are the bulk densities of interior, transitional, and exterior layer, respectively. The terms ␳0, ␳1, and ␳2 are the bulk densities for soil aggregates that are whole, one-third peeled, and two-thirds peeled. The porosity of each layer was calculated using ␳b of each layer and ␳s determined for each size fraction of CT, NT, and NF assuming constant ␳s within aggregates.

Statistical Analyses Comparisons of intraaggregate porosities among aggregate size fractions and concentric layers across tillage treatments

were accomplished using ANOVA-GLM analyses of the SAS system (SAS/STAT; SAS institute, 1999) with a significance level of P ⬍ 0.05. The power law relationship between intraaggregate porosity and Ks for each treatment was analyzed using linear regression analysis of the SAS system.

RESULTS AND DISCUSSION Saturated Hydraulic Conductivity through Individual Soil Aggregates Saturated hydraulic conductivities through individual whole aggregates sampled from NF soils were the highest. They ranged from 9.4 ⫻ 10⫺7 to 5.0 ⫻ 10⫺5 cm s⫺1 for Hoytville and 2.0 ⫻ 10⫺6 to 1.6 ⫻ 10⫺4 cm s⫺1 for Wooster soil series. The average Ks of the three aggregate size fractions analyzed in this study could be ranked as follows: NF ⬎ NT ⬎ CT (Fig. 4). The Ks values increased rapidly with increasing aggregate sizes for NF and NT treatments, which contained greater total porosities. Increasing aggregate sizes from 2 to 4 to 4 to 6.3 mm did not affect the Ks of CT aggregates primarily because there were fewer intraaggregate pores. Smaller aggregates, 2 to 4 mm across, had the lowest Ks with no differences among CT, NT, and NF. However, the differences in Ks values became larger as aggregate sizes increased. The Ks values of CT aggregates, 6.3 to 9.5 mm across, were only 6% of NF soil aggregates for both soil series. Saturated hydraulic conductivities through soil aggregates are generally lower and the retention times of water and solutes within soil aggregates are longer compared with bulk flow through soil macropores because soil aggregates have lower pore connectivities and finer and more tortuous pores than bulk soils (Horn, 1990). Mahboubi et al. (1993) reported 13-fold greater Ks values through NT bulk soils than through CT bulk soils



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Fig. 3. Soil aggregate erosion chamber for mechanically abrading thin concentric layers from single aggregates. (A) Erosion chamber with knurled walls and screen at base with 352-␮m openings (left) and base for retaining peeled materials (right). (B) Concentric layers at the exterior, transitional, and interior centers of soil aggregate.

in the Wooster soil series. Average intraaggregate Ks values reported in this study were 7.3 ⫻ 10⫺6 and 4.7 ⫻ 10⫺6 cm s⫺1 for all three aggregate size fractions of NT and CT in Wooster. These intraaggregate Ks values are substantially smaller than bulk flow values of 1.1 ⫻ 10⫺2 cm s⫺1 reported to flow through NT soils of Wooster silt loam. Increased Ks through bulk soils and through intraaggregates in the NT management system suggest greater diffusive exchange and possibly longer retention times of soil solutions. In contrast, much lower Ks within CT aggregates implies that diffusive exchanges of nutrients may be limited to exterior regions of macroaggregates. Smucker et al. (1998) reported that more C and N were identified in the exterior layers of CT macroaggregates than within their interior centers. Greater porosities and higher Ks rates through the more stable macroaggregates from NF and NT management systems may be one reason for greater quantities of C stored in NT aggregates (Lal and Van Doren, 1990). Greater storage of ions within aggregates leads to more stable aggregates by increasing both the number and strength among organomineral bonds (Angers et al., 1997; Chenu and Guerif, 1991; Jastrow, 1996; Kogel-Knabner, 2002).

Intraaggregate Porosities Total porosities increased from smaller to larger aggregates in aggregates sampled from CT, NT, and NF

Fig. 4. Saturated hydraulic conductivities of soil aggregates, 6.3 to 9.5, 4 to 6.3, and 2 to 4 mm across, from the (A) Hoytville and (B) Wooster soil series. Bars are standard errors of three field replicates. CT, conventional tillage; NF, native forest; NT, no tillage.

soils (Fig. 5). The largest intraaggregate total porosity was identified in NF of Wooster. However, there were no differences between NT and NF aggregates for any of the aggregate fractions from the Hoytville soils (Fig. 5A). Particle densities (␳s) and bulk densities (␳b) of aggregates used to calculate intraaggregate porosities for the three aggregate size fractions are reported in Table 1. Intraaggregate total porosities for aggregates from the Hoytville silty clay loam soils ranged from 0.25 to 0.40 cm3 cm⫺3 and Wooster silt loam soils from 0.29 to 0.45 cm3 cm⫺3. Conventional tillage dramatically reduced intraaggregate porosities for all three macroaggregate size fractions measured for both soil series. Replacing long-term moldboard practices by NT, 35 yr ago, significantly improved intraaggregate soil porosities of similar aggregate size fractions. Our observations that intraaggregate porosities increased with increasing aggregate size, except for the Wooster CT aggregates, appear to correspond with the “porosity exclusion principle” explaining that larger aggregates will have greater porosity because they contain


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2000). More frequent turnover rates of CT macroaggregates appear to prohibit continuous development of internal pores during repeated wetting–drying cycles. It has been reported that total clay and dispersible clay contents increase with tillage (Rhoton, 2000) while water stabilities of aggregates decrease and the tensile strengths of dried aggregates increase (Munkholm et al., 2002). Greater tensile strengths are attributed to soil densification by aggregate coalescence as soil aggregates rejoin with the buildup of capillary forces and clay cohesions during drying (Grant et al., 2001; Or and Ghezzehei, 2002).

Contrasting Regions of Different Porosities within Macroaggregates Significant porosity reductions were observed in interior centers of all soil macroaggregates, 4 to 6.3 and 6.3 to 9.5 mm across, sampled from NT and CT treatments of both Hoytville and Wooster soil series (Fig. 6). This suggests that central regions of macroaggregates, compressed by repeated tillage and relatively free of pores, retain less porous regions for up to 35 yr after CT soil management systems are replaced by NT. Porosities of the central regions of CT and NT aggregates were 12 and 15% lower than their exterior layers of Hoytville soils (Fig. 6A). Exterior layers of NT aggregates from the Hoytville site had greater porosities than transitional layers and interior centers, even surpassing the porosities of exterior layers of NF soils. These spatial gradients of porosity from exterior to interior centers of macroaggregates should enhance the diffusion of soil solutions into aggregate centers during subsequent wetting and drying cycles. Smaller yet significantly different porosity gradients were also observed between exterior layers and the interior centers of NF soil aggregates from the Hoytville series (Fig. 6A). Wooster aggregates, 6.3 to 9.5 mm across, had higher intraaggregate porosities, yet the highly variable porosities within exterior layers did not significantly contrast with porosities of interior centers within these macroaggregates (Fig. 6C). Significantly lower porosities in central regions of smaller aggregates (Fig. 6B) were observed for all management systems of the Wooster soils. Greater porosities were limited to the exterior one-third regions of CT aggregates, whereas no porosity gradients were observed between the exterior and transitional concentric layers in Wooster aggregates, 4 to 6.3 mm across, whether sampled from the NT or NF management systems (Fig. 6B). Lower porosities of interior

Fig. 5. Total intraaggregate porosities of whole aggregates subjected to reducing amounts of tillage for soil aggregates, 6.3 to 9.5, 4 to 6.3, and 2 to 4 mm across, sampled from (A) Hoytville and (B) Wooster soil series. Uppercase letters indicate significant differences among aggregate size fractions and lowercase letters indicate significant differences among management systems at P ⬍ 0.05. Bars designate standard errors of three field replicates. CT, conventional tillage; NF, native forest; NT, no tillage.

pore networks between the small aggregates when aggregate hierarchies exist (Dexter, 1988). Lower porosities in CT aggregates are attributed to the compression and breakage of CT aggregates, illuviation, and accumulation of greater quantities of clay due to lower water stability (Horn et al., 1995; Smucker et al., 1998; Rhoton,

Table 1. Particle density (␳s) and bulk density (␳b) of soil aggregates from Hoytville and Wooster soil series subjected to conventional tillage (CT), no tillage (NT), and native forest (NF) ecosystem managements.† Hoytville 2–4 mm CT

NT

Wooster

4–6.3 mm NF

CT

NT

6.3–9.5 mm NF

CT

NT

2–4 mm NF

4–6.3 mm

6.3–9.5 mm

CT

NT

NF

CT

NT

NF

CT

NT

NF

2.61 (0.01) 1.79 (0.03)

2.57 (0.01) 1.69 (0.05)

2.56 (0.02) 1.57 (0.06)

2.58 (0.01) 1.74 (0.02)

2.53 (0.02) 1.58 (0.01)

2.51 (0.03) 1.43 (0.02)

2.55 (0.01) 1.82 (0.01)

2.56 (0.03) 1.55 (0.04)

2.51 (0.01) 1.38 (0.02)

cm⫺3

␳s ␳b

2.58 (0.01) 1.95 (0.02)

2.53 (0.02) 1.76 (0.04)

2.40 (0.03) 1.63 (0.01)

2.64 (0.01) 1.84 (0.01)

2.60 (0.02) 1.74 (0.02)

2.44 (0.01) 1.60 (0.02)

2.64 (⬍0.01) 1.69 (0.02)

2.64 (0.01) 1.55 (0.01)

g 2.50 (0.01) 1.51 (0.03)

† Values in the parentheses are the standard errors of three field replicates.



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centers strongly suggest that the centers of macroaggregates are disturbed less frequently than their exterior layers. Lower porosities in the centers of aggregates combined with the presence of porosity gradients in both soil series suggest that outer layers of aggregates are subjected to hydration energies that develop additional pore networks toward their interior centers. Additionally, water flow through intraaggregate domains apparently decreases dramatically toward aggregate centers having limited porosities. However, aggregates with higher Ks and total porosities adsorb more water into interior centers of aggregates, leading to higher pore pressures during rehydration, resulting in subsequent expansions of porosities within aggregate interior centers. Aggregates from NF and NT treatments of both soil series contained higher total porosities and there were smaller differences between porosities in their exterior and transitional layers or interior centers (Fig. 6).

Relationship between Saturated Hydraulic Conductivity and Porosity of Aggregates and Its Implication The significant differences in intraaggregate porosities among management systems and aggregate size fractions greatly amplified Ks values within macroaggregates (Fig. 7). Water flow is probably limited to interaggregate macropores among CT aggregates. Intraaggregate domains conduct greater portions of the soil solution fluxes, contributing to greater solute flow and retention within aggregates from NF and NT soils. We observed a power law relationship between Ks and porosity for NT and NF soil aggregates but not for CT aggregates (Fig. 7). Hydraulic conductivities through NF aggregates were more dependent on porosity changes than NT aggregates, indicating differences in their pore geometries. Revil and Cathles (1999) analyzed relationships between intrinsic permeability and porosity for porous media with different effective interconnected porosities. According to their analyses, greater porosity and greater dependency of Ks on the porosities of NF aggregates suggests there may be more large pores interconnected with narrow throats and the interconnections increase with increasing porosity. In contrast, no response of Ks to porosity change in CT aggregates indicates there were no increases in the effective interconnected porosities within CT aggregates. Tillage decreases aggregate stability and enhances wetting–drying cycles. As pneumatic pressure energies during rapid rehydration exceed the aggregate tensile strength, less stable aggregates break down, decreasing further expansion of intraaggregates porosities. Under negative tension wetting or nebulizing mists, in which the soil aggregates attain saturated conditions without slaking, it is suggested that internal pores develop along the pathways of entrapped air as the pneumatic pressures increase within dead-end pores, forming additional internal pore networks that interconnect more of the developing pores as suggested by Revil and Cathles (1999). Albee et al. (2000), while examining three-dimensional computer microtomographic (CMT)

Fig. 6. Total intraaggregate porosities among three concentric layers within whole aggregates from (A) Hoytville soils, 6.3 to 9.5 mm across, (B) Wooster soils, 4 to 6.3 mm across, and (C) Wooster soils, 6.3 to 9.5 mm across. Uppercase letters indicate significant differences among layers and lowercase letters indicate significant differences among management systems at P ⬍ 0.05. Bars designate the standard errors of three field replicates. CT, conventional tillage; NF, native forest; NT, no tillage.

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quent yet relatively mild exposures to less destructive forces associated with slower rates of wetting.

CONCLUSIONS In summary, Ks through single aggregates was much greater for macroaggregates subjected to NF and NT ecosystem management. We observed greater porosities in exterior layers of soil aggregates from all treatments. The interior centers of CT aggregates were less porous compared with those of NT or NF aggregates. Water permeability through aggregates decreased with decreasing intraaggregate porosity and the relationships between porosities and Ks imply differences in pore connectivities within aggregates from CT, NT, and NF soils. The differences in intraaggregate porosity and spatial distribution of porosity within aggregates, and hydraulic properties of aggregates from CT, NT, and NF soils suggest a hypothesis of pore development during rehydration of stable aggregates, which results in greater internal pore connectivities. ACKNOWLEDGMENTS Fig. 7. Power law relationships between the saturated hydraulic conductivity (Ks) and the total porosities of macroaggregates ranging from 2 to 9.5 mm across for no tillage (NT) and native forest (NF) soils from Wooster and Hoytville. The solid line is a regression for NF and the dashed line is for NT.

reconstructions of soil aggregates, observed greater expansions and probable connectivities of “ink bottle– like” pores at the terminus ends of pores extending inward from outer layers of soil aggregates. Images are available at www.smucker.msu.edu (verified 7 Oct. 2004). They suggested that micropore development continued to increase as pore pressures in dead-end pores increased within existing pore networks of aggregates subjected to multiple wetting–drying cycles and associated movement of soluble soil carbon compounds, microbes, and cations into aggregate centers. In our study, more stable aggregates from NF soils appear to maintain structure continuities during wetting–drying cycles and hence develop greater porosities in the central regions. However, the more unstable aggregates, associated with CT management systems, did not resist repeated wetting–drying cycles and broke up before additional wetting and drying cycles could aid in the construction of extended intraaggregate porenetworks. The high intraaggregate porosities of NF soil aggregates for both soil series may be related to decreases in wetting energy due to the hydrophobic properties of organic matter. Soil organic matter stabilizes soil aggregates by acting as cementing materials (Tisdall and Oades, 1982) and their hydrophobic properties mitigate the destructive wetting forces by slowing the rates of internal hydration (Chenu et al., 2000). Therefore, the more stable NF soil aggregates, containing larger quantities of hydrophobic soil organic matter, may develop greater internal pores without disintegration during fre-

Financial support was provided by the NSF/LTER program at KBS, the Michigan Agricultural Experiment Station, and CASMGS (USDA/CSREES Project no. S03057). We are grateful to C.J. Dell, Research Associate at MSU, now Research Scientist, USDA-ARS, State College, PA, and Karin Adtjandra, NSF/LTER/REU (NSF Grant DEB98-10220) for their laboratory assistance. Soil samples provided by W. Dick, Ohio State University, OARDC, from Hoytville and Wooster, OH, are greatly appreciated.

REFERENCES Albee, P.B., G.C. Stockman, and A.J.M. Smucker. 2000. Extraction of pores from microtomographic reconstructions of intact soil aggregates. ANL/MCS-P790-0100. Mathematics and Computer Sci. Div., Argonne Natl. Lab., Argonne, IL. Ame´zketa, E. 1999. Soil aggregate stability: A review. J. Sustainable Agric. 14:83–151. Angers, D.A., S. Recous, and C. Aita. 1997. Fate of carbon and nitrogen in water-stable aggregates during decomposition of 13C 15 N-labeled wheat straw in situ. Eur. J. Soil Sci. 48:295–300. Balesdent, J., C. Chenu, and M. Balabane. 2000. Relationship of soil organic matter dynamics to physical protection and tillage. Soil Tillage Res. 53:215–230. Beare, M.H., M.L. Cabrera, P.F. Hendrix, and D.C. Coleman. 1994. Aggregate-protected and unprotected organic matter pools in conventional- and no-tillage soils. Soil Sci. Soc. Am. J. 58:787–795. Blake, G.R., and K.H. Hartge. 1986a. Bulk density. p. 363–375. In A. Klute (ed.) Methods of soil analysis. Part 1. 2nd ed. Agron. Monogr. 9. ASA and SSSA, Madison, WI. Blake, G.R., and K.H. Hartge. 1986b. Particle density. p. 377–382. In A. Klute (ed.) Methods of soil analysis. Part 1. 2nd ed. Agron. Monogr. 9. ASA and SSSA, Madison, WI. Booltink, H.W.G., and J. Bouma. 1991. Physical and morphological characterization of bypass flow in a well-structured clay soil. Soil Sci. Soc. Am. J. 55:1249–1254. Chenu, C., and J. Guerif. 1991. Mechanical strength of clay minerals as influenced by an adsorbed polysaccharide. Soil Sci. Soc. Am. J. 55:1076–1080. Chenu, C., J. Hassink, and J. Bloem. 2001. Short-term changes in the spatial distribution of microorganisms in soil aggregates as affected by glucose addition. Biol. Fertil. Soils 34:349–356. Chenu, C., Y. Le Bissonnais, and D. Arrouays. 2000. Organic matter influence on clay wetability and soil aggregate stability. Soil Sci. Soc. Am. J. 64:1479–1486.



Collins, H.P., E.T. Elliott, K. Paustain, L.G. Bundy, W.A. Dick, D.R. Huggines, A.J.M. Smucker, and E.A. Paul. 2000. Soil carbon pools and fluxes in long-term cornbelt agroecosystems. Soil Biol. Biochem. 32:157–168. Coutadeur, C., Y. Coquet, and J. Roger-Estrade. 2002. Variation of hydraulic conductivity in a tilled soil. Eur. J. Soil Sci. 53:619–628. Coppola, A. 2000. Unimodal and bimodal descriptions of hydraulic properties for aggregated soils. Soil Sci. Soc. Am. J. 64:1252–1262. Cote, C.M., K.L. Bristow, and P.J. Ross. 1999. Quantifying the influence of intra-aggregate concentration gradients on solute transport. Soil Sci. Soc. Am. J. 63:759–767. Czarnes, S., P.D. Hallett, A.G. Bengough, and I.M. Young. 2000. Root- and microbial-derived mucilages affect soil structure and water transport. Eur. J. Soil Sci. 51:435–443. Dexter, A.R. 1988. Advances in characterization of soil structure. Soil Tillage Res. 11:199–238. Gerke, H.H., and J.M. Kohne. 2002. Estimating hydraulic properties of soil aggregate skins from sorptivity and water retention. Soil Sci. Soc. Am. J. 66:26–36. Gerke, H.H., and M.T. van Genuchten. 1993. A dual-porosity model for simulating the preferential movement of water and solutes in structured porous media. Water Resour. Res. 29:1225–1238. Ghezzehei, T.A., and D. Or. 2003. Pore-space dynamics in a soil aggregate bed under a static external load. Soil Sci. Soc. Am. J. 67:12–19. Grant, C.D., D.A. Angers, R.S. Murray, M.H. Chantigny, and U. Hasanah. 2001. On the nature of soil aggregate coalescence in an irrigated swelling clay. Aust. J. Soil Res. 39:565–575. Hart, G.L., and B. Lowery. 1996. Partitioned flow domains of three Wisconsin soils. Soil Sci. Soc. Am. J. 40:203–207. Heard, J.R., E.J. Kladivko, and J.V. Mannering. 1988. Soil macroporosity, hydraulic conductivity and air permeability of silty soil under long-term conservation tillage in Indiana. Soil Tillage Res. 11:1–18. Horn, R. 1990. Aggregate characterization as compared to soil bulk properties. Soil Tillage Res. 17:265–289. Horn, R., T. Baumgartl, R. Kayser, and S. Baasch. 1995. Effect of aggregate strength on strength and stress distribution in structured soils. p. 31–52. In K.H. Hartge and B.A. Stewart (ed.) Soil structure. Its development and function. CRC Press, Boca Raton, FL. Hussein, J., and M.A. Adey. 1998. Changes in microstructure, voids and b-fabric of surface samples of a Vertisol caused by wet/dry cycles. Geoderma 85:63–82. Jastrow, J.D. 1996. Soil aggregate formation and the accrual of particulate and mineral-associated organic matter. Soil Biol. Biochem. 28:665–676.

45

Klute, A., and C. Dirksen. 1986. Hydraulic conductivity and diffusivity: Laboratory methods. p. 687–734. In A. Klute (ed.) Methods of soil analysis. Part 1. 2nd ed. Agron. Monogr. 9. ASA and SSSA, Madison, WI. Kogel-Knabner, I. 2002. The macromolecular organic composition of plant and microbial residues as inputs to soil organic matter. Soil Biol. Biochem. 34:139–162. Lal, R., and D.M. Van Doren, Jr. 1990. Influence of 25 years of continuous corn production by three tillage methods on water infiltration for two soils in Ohio. Soil Tillage Res. 16:71–84. Mahboubi, A.A., R. Lal, and N.R. Faussey. 1993. Twenty-eight years of tillage effects on two soils in Ohio. Soil Sci. Soc. Am. J. 57: 506–512. Martens, D.A., and W. Dick. 2003. Recovery of fertilizer nitrogen from continuous corn soils under contrasting tillage management. Biol. Fertil. Soils 38:144–153. McKenzie, B.M., and A.R. Dexter. 1996. Methods for studying the permeability of individual soil aggregates. J. Agric. Eng. Res. 65: 23–28. Munkholm, L.J., P. Schjønning, K. Debosz, H.E. Jensen, and B.T. Christensen. 2002. Aggregate strength and mechanical behaviour of a sandy loam soil under long-term fertilization treatments. Eur. J. Soil Sci. 53:129–137. Or, D., and T.A. Ghezzehei. 2002. Modeling post-tillage soil structural dynamics: A review. Soil Tillage Res. 64:41–59. Paustian, K., H.P. Collins, and E.A. Paul. 1997. Management controls on soil carbon. p. 15–49. In E.A. Paul et al. (ed.) Soil organic matter in temperate agroecosystems. CRC Press, Boca Raton, FL. Revil, A., and L.M. Cathles. 1999. Permeability of shaly sands. Water Resour. Res. 35:651–662. Rhoton, F.E. 2000. Influence of time on soil response to no-till practices. Soil Sci. Soc. Am. J. 64:700–709. Smucker, A.J.M., D. Santos, Y. Kavdir, and E.A. Paul. 1998. Concentric gradients within stable soil aggregates. In Proceedings of the 16th World Congress of Soil Science, Montpellier, France [CDROM]. 20–26 Aug. 1998. CIRAD, Paris. Tisdall, J.M., and J.M. Oades. 1982. Organic matter and water-stable aggregates in soils. J. Soil Sci. 33:141–163. Van Genuchten, M.T., and F.N. Dalton. 1986. Models for simulating salt movement in aggregated field soils. Geoderma 38:165–183. Wan, Y., and S.A. El-Swaify. 1998. Sediment enrichment mechanisms of organic carbon and phosphorus in a well-aggregated Oxisol. J. Environ. Qual. 27:132–138. Youngs, E.G., P.B. Leeds-Harrison, and R.S. Garnett. 1994. Water uptake by aggregates. Eur. J. Soil Sci. 45:127–134.