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Published online 6 July 2006
Published in J Environ Qual 35:1576-1583 (2006)
DOI: 10.2134/jeq2005.0225
© 2006 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
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Use of Physical Properties to Predict the Effects of Tillage Practices on Organic Matter Dynamics in Three Illinois Soils

Gayoung Yooa,*, Todd M. Nissenb and Michelle M. Wanderc

a Korea Environment Institute, 613-2 Bulgwang-Dong, Eunpyeong-Gu, Seoul, 122-706 Korea
b Office of Multilateral Trade Affairs, Room 3828 HST, Department of State, Washington, DC 20520
c Department of Natural Resources and Environmental Sciences, University of Illinois, S406 Turner Hall MC 047, 1102 South Goodwin Avenue, Urbana, IL 61801

* Corresponding author (gyyoo{at}kei.re.kr)

Received for publication June 7, 2005.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
This work builds on a previous study of long-term tillage trials that found use of no-tillage (NT) practices increased soil organic carbon (SOC) sequestration at Monmouth, IL (silt loam soil) by increasing the soil's protective capacity, but did not alter SOC storage in DeKalb, IL (silty clay loam), where higher clay contents provided a protective capacity not affected by tillage. The least limiting water range (LLWR), a multi-factor index of structural quality, predicted observed soil CO2 efflux patterns. Here we consider whether LLWR can predict sequestration trends at a third site, Perry, IL (silt loam soil) where SOC content is lower and bulk density is higher than in previously considered sites, and determine whether pore size characteristics can help explain the influence use of NT practices has had on SOC sequestration at all three locations. At Perry, LLWR was again related with differences in specific soil organic carbon mineralization rates (RESPsp) (2000–2001). Reduced RESPsp rates explain increases in SOC storage under NT management observed only after 17 yr. Trends in RESPsp suggest use of NT practices only enhance physical protection of SOC where soil bulk density is relatively high (approximately 1.4 g cm–3). In those soils (Monmouth and Perry), use of NT management reduced the volume of small macropores (15–150 µm) thought to be important for microbial activity. Physical properties appear to determine whether or not use of NT practices will enhance C storage by increasing physical protection of SOC. By refining the functions used to compute the LLWR and our understanding of the interactions between management, pore structure, and SOC mineralization, we should be able to predict the influence of tillage practices on SOC sequestration.

Abbreviations: CT, conventional tillage • LLWR, least limiting water range • NT, no tillage • RESPsp, specific soil organic carbon mineralization rate • SOC, soil organic carbon


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
DESPITE MANY REPORTS that use of no-tillage (NT) practices sometimes fails to increase soil organic carbon (SOC) sequestration relative to conventionally tilled (CT) counterparts (Dick et al., 1991; Paustian et al., 1995; Angers et al., 1997; Needleman et al., 1999), efforts to understand why tillage impacts on SOC storage vary by sites are relatively rare. In many fine-textured soils, use of NT practices has increased SOC contents in the surface at the expense of SOC stored within the rooting zone (Wander et al., 1998; Kay and VandenBygaart, 2002). This observation is common especially when C inputs from crops are reduced under NT management as a result of lowered yield or when soil erosion is not a major concern (Hussain, 1997; Alvarez et al., 1998; Yang and Wander, 1999). When crop yield does not account for differences in the SOC content of soils maintained under NT and CT management, studies have considered the effects of tillage on SOC mineralization (Alvarez et al., 1995; Dick et al., 1991; Angers et al., 1997; Karunatilake et al., 2000). Tilled soils are generally reported to have greater CO2 evolution rates than soils under NT management because tillage modifies the decomposition environment by aerating the soil, breaking up soil aggregates, and incorporating residues into the soil profile (Beare et al., 1994; Fortin et al., 1996; Lee et al., 1996; Prior et al., 1997; Six et al., 2002). However, soil CO2 evolution from the NT soils has been found to be equal to, or greater than, that from the CT soils during the growing season where the accumulation of SOC in the surface of NT soils has enhanced infiltration and water holding capacity (Blevins, 1984; Linn and Doran, 1984; Hendrix et al., 1988; Follett and Schimel, 1989; Alvarez et al., 1995; Franzluebbers et al., 1995).

Specific carbon mineralization rates (RESPsp: CO2 mineralized expressed per unit SOC) can be used to evaluate how readily SOC is mineralized. The quantity and quality of organic substrates and the surrounding environment will all influence this variable. Soil RESPsp has been used to predict the protective capacities of soils when the soils being compared have had similar contents of labile SOC (Franzluebbers and Arshad, 1997). Protective capacity has also been used to explain elevated RESPsp in soils where labile C stocks are relatively reduced. For example, Rice and Garcia (1994) observed higher RESPsp in soil obtained from a burned than from an unburned prairie. They associated lower RESPsp with greater physical protection of C in the unburned prairie and attributed this to structural integrity. Franzluebbers et al. (1995) observed higher RESPsp from a conventionally tilled soil than from a soil maintained under no-tillage management. Again, lower RESPsp was associated with greater physical protection of SOC.

Soil RESPsp may be used as a tool to help predict SOC sequestration capacity. Yoo and Wander (2006) investigated two sites where the use of CT and NT practices for over a decade had previously been reported (Wander et al., 1998) to have had variable effects on SOC sequestration. In the site where use of NT management increased SOC contents relative to the CT treatment, seasonal mean SOC mineralization rate and RESPsp were greater from soils under CT than NT management. In the site where use of NT practices failed to increase SOC storage, there was no difference in mean SOC mineralization rate or RESPsp of the CT and NT treatments. Interactions between soil structure and water contents could account for the inconsistent effects that tillage practices had on SOC mineralization and storage. The least limiting water range (LLWR), which is a multi-factor index of structural quality developed by Da Silva and Kay (1997), successfully explained soil CO2 efflux at both sites. When the LLWR was small, C mineralization was restricted. The LLWR integrates the effects of clay content, bulk density, and SOC and soil water contents on the physical environment influencing biological growth and activity. Yoo and Wander (2006) found the LLWR and RESPsp to be positively correlated and speculated that measures like the LLWR, that describe soil water-structure interactions, might help us predict whether or not the adoption of NT practice can increase SOC sequestration. Alterations of practice to reduce C mineralization may have less benefit to SOC sequestration in fine textured soils that have relatively low LLWRs and thus have high protective capacities for SOC.

Greater knowledge of soil pore characteristics might improve our understanding of structural controls over SOC decay. In general, the total porosity of surface soils maintained under NT is relatively reduced compared to CT counterparts with the exception of the top few centimeters of some NT soils (Kay and VandenBygaart, 2002). While bulk density and total porosity provide general information about soil compaction, knowledge about the distribution of volume among different pore size classes may provide additional insight. The relationship between total porosity and biotic activity is not well understood. Consolidation or compaction of soil does not necessarily degrade the soil's capacity to support biotic activity including plant growth or microbial or faunal activity (Logsdon et al., 1992; Kaspar et al., 1995; Shestak and Busse, 2005). Moderate soil compaction can increase crop yield, especially during dry years, by improving seed–soil contact and pore continuity contributing to capillary rise of water to the root zone (Johnson et al., 1990; Lipiec and Simota, 1994).

The relationship between bulk density and biological activity was refined by De Neve and Hofman (2000) who proposed a threshold value of 1.6 Mg m–3 for bulk density above which microbial processes were negatively affected by soil compaction. We speculate that the threshold value for bulk density above which SOC mineralization is reduced will be related to changes in the volume of selected pore classes. While the criteria used to define pore classes are variable and somewhat arbitrary, pores are commonly divided into macro-, meso-, and micropores (Taboada et al., 1998; Arshad et al., 1999; Schjonning and Rasmussen, 2000; Kay and VandenBygaart, 2002). Larger macropores, pores greater than approximately 100 to 500 µm, constitute free spaces where plant roots can grow or earthworm activity prevails (Kay, 1990). Pores with diameters ranging between 15 and 30 and 100 to 500 µm are considered small macropores which are largely responsible for soil aeration and rapid drainage (Taboada et al., 1998). Mesopores have an equivalent diameter of 0.2 to 30 µm, and are particularly important for the storage of water for plant growth (Kay and VandenBygaart, 2002). Micropores with a diameter of <0.2 µm retain water generally not available to plants and their small diameter restricts microbiological activity (Hill et al., 1985).

This work builds on previous work conducted in Monmouth and DeKalb, IL (Yoo and Wander, 2006) and considers a third site, located in Perry, IL, at the Orr Agricultural Research and Demonstration Center, where, as was true for previous sites, tillage practices had no consistent effect on crop yield. Here we test the applicability of previous findings by measuring RESPsp and LLWR at the Perry site, and further explore soil structural controls over SOC dynamics at all three sites by quantifying soil pore characteristics.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Experimental Sites
The experimental trials are located at three University of Illinois Agronomy Research Centers in DeKalb (north), Monmouth (middle), and Perry (south), IL. Soils at these locations are Drummer silty clay loam (poorly drained, fine-silty, mixed, mesic Typic Haplaquoll), Muscatine silt loam (somewhat poorly drained, fine-silty, mixed, mesic Aquic Hapludoll), and Herrick silt loam (somewhat poorly drained, fine, montmorillonitic, mesic, Aquic Argiudoll), respectively. This study was conducted in the corn phase of a corn–soybean [Zea mays L. and Glycine max (L.) Merr.] rotation. The main experimental treatments were established in 1985, and they were conventional fall tillage (CT) and no tillage (NT). The CT treatment consisted of moldboard plowing after corn and chisel plowing after soybean. At each experiment location, main treatments were laid out in a completely randomized block; there were three replicate blocks per location and each plot had two sub-plots.

Soil Organic Carbon Sequestration, Soil Carbon Dioxide Evolution, and Specific Soil Organic Carbon Mineralization Rate
Soil carbon sequestration influenced by tillage practice was assessed by measuring total SOC concentration in the 0- to 30-cm depth for samples collected in spring 2003. Soil samples were taken with a splittable core sampler (4.7-cm i.d., 30-cm length; Forest Supply, Jackson, MS). Two cores were taken from each sub-plot. Each core was divided into three depths (0–5, 5–15, and 15–30 cm), and samples from each core were combined by depth in a plastic bag in the field and transported on ice to the lab. Bulk density was determined by the core method (Krzic et al., 2000). After grinding soil samples using a disc mill, total C content was measured by combustion analysis using a Model NA 1500 C/N analyzer (Carlo Erba, Milan, Italy). The amount of SOC storage was expressed on an equivalent volume in the depth of 0 to 30 cm by multiplying depth-weighted average of SOC concentration (g C kg–1 soil) by depth-weighted average of bulk density.

Soil CO2 evolution rates were measured biweekly during the growing season (from May to September) in 2000 and once a month from May to October in 2001 using a Model 6400 instrument (Li-Cor, Lincoln, NE). The soil chamber was sealed by a ring made of PVC (10-cm i.d., 4.5-cm height) installed approximately one inch deep into, and extending approximately one inch above, the soil surface in inter-rows. Specific SOC mineralization rate (RESPsp) (µCO2 s–1/µg SOC) was calculated as the amount of CO2 evolved divided by total mass of C in the soil profile (0–30 cm). The total mass of C was computed using depth-weighted averages of total SOC concentrations and bulk densities in the 0- to 30-cm depth.

Soil Temperature and Gravimetric Water Contents
On the dates that soil CO2 evolution was measured, soil temperature and gravimetric water content were determined at 0 to 5, 5 to 15, and 15 to 30 cm. Soil temperature in the 0- to 5-, 5- to 15-, and 15- to 30-cm depths was determined using a PVC tube (1.5-cm i.d., 25-cm length) with copper thermocouples placed in the holes at 2.5, 10, and 22.5 cm and fixed with paraffin (Meshkat et al., 1998). A tube containing three thermocouples had been installed from spring after planting to fall before tillage in the inter-row position of each sub-plot and the thermocouples were read with a digital thermometer (Model HH501 DK; Omega, Standford, CT). Two subsamples for determining gravimetric soil water content were taken from each sub-plot from the inter-row position with an auger (1.8-cm i.d., 25-cm length). Samples were divided into three depths (0–5, 5–15, and 15–25 cm) in the field. The subsamples were combined in a plastic bag and transported on ice to the lab. In the lab, approximately 10 g of soil were taken from each plastic bag and dried in the oven at 105°C for 24 h to calculate gravimetric soil water contents.

Soil Bulk Density, Pore Size Distribution, and Least Limiting Water Range
Soil samples to determine bulk density were taken twice a year in 2000 and 2001 by the core method (Krzic et al., 2000), immediately after planting and after harvest. A splittable core sampler (4.7-cm i.d., 30-cm length; Forest Supply, Jackson, MS) was used to collect soil samples driven by a hydraulic soil probe (Giddings Machine Co., Fort Collins, CO) mounted on a truck. Two cores were taken from each sub-plot. After each core was divided into three depths (0–5, 5–15, and 15–30 cm), samples from each core were combined in a plastic bag in the field.

To determine pore size distribution, intact cores (0–30 cm) were collected from each sub-plot in spring 2001 as described above. Intact cores were then transferred to splittable acrylic tubes (4.7-cm i.d., 30-cm length) and gently wrapped with plastic wrap in the field to prevent sample drying. In the laboratory, intact cores were stored in the refrigerator until analysis. Soil-water retention curves were collected using a tension table within the pressure potential range of –0.02 to –0.002 MPa. Pore size distribution was determined using the relationship between pore size and capillary water retention (Townsend et al., 2000):

Formula 1[1]
where d is the diameter of the smaller pores drained (µm) and h is the soil water pressure potential (kPa) at 20°C.

We used 150 and 15 µm as the two boundaries for pore size classes with pores of >150 µm defined as large macropores, pores from 15 to 150 µm as small macropores, and pores of <15 µm as meso- and micropores. These size classes generally follow criteria used by Taboada et al. (1998) who used 100 and 20 µm to investigate tillage impacts on pore size distribution. Total pore volume was calculated by summing the pore volume of the three different pore size classes.

The procedure used to calculate LLWR was explained in detail in Yoo and Wander (2006). Briefly, the minimum range of water contents where there are no limits on biological activity was calculated by taking the smaller value of factors that limit biological activity when the soil is wet and the larger value of factors that limits activity when the soil is dry to determine the range within which soil biological activity is not limited by water availability or soil compaction. Values of limits were calculated from soil moisture release and soil resistance curves. The moisture release curve was constructed using the pedotransfer function from Da Silva and Kay (1997). From the soil moisture release curve, the volumetric water contents at field capacity ({theta}fc) and wilting point ({theta}wp) were calculated and they were considered as one of two wet and dry limits, respectively. The other dry limit was calculated from the soil resistance curve, which was also constructed by using the pedotransfer function from Da Silva and Kay (1997) and it was the volumetric water content when soil resistance is 2 MPa ({theta}sr). The other wet limit was volumetric water content at 10% air filled porosity ({theta}afp), which was calculated by subtracting 0.1 from total porosity. Pedotransfer functions required clay contents (%), SOC contents (%), and bulk density (g cm–3). To calculate LLWR in Perry, we measured clay content by the hydrometer method using soil samples collected in the spring of 2000 (Sheldrick and Wang, 1993). Twenty grams of soil were dispersed in 150 mL of sodium hexametaphosphate (50 g L–1) for 1 h with shaking on a reciprocal shaker. The suspension was diluted to 1 L in a cylinder with distilled water and mixed thoroughly. Hydrometer readings were taken at 40 s and 6 h after completion of stirring. The SOC contents used for calculation of LLWR in Perry site were 10.6 and 11.4 g C kg–1 soil for CT and NT soils, respectively, which were reported in Wander et al. (1998).

Statistical Analysis
Analysis of variance was performed using the MIXED procedure of SAS (SAS Institute, 2001) on seasonal mean CO2 evolution rates, specific SOC mineralization rates (RESPsp), and depth-weighted averages of soil temperature, soil water contents, and bulk density representing the 0- to 30-cm depth. We did not consider the depths separately for soil temperature, water content, and bulk density because these parameters were related to soil CO2 efflux measured at the soil surface rather than CO2 production within each soil depth. Tillage, site, and date were the fixed effects. Year, block(site), year x block(site), and year x tillage were the random effects. To assess site-based differences in SOC storage, total pore volume, large macropores, small macropores, and meso- and micropores, analysis of variance was performed using the GLM procedure of SAS (SAS Institute, 2001). For tillage-based differences, Students's t test was performed using PROC TTEST (SAS Institute, 2001) within each site.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Soil Carbon Dioxide Evolution Patterns and Soil Organic Carbon Storage
To allow comparison among sites and provide consistency in presentation, data related to SOC mineralization is presented for all sites even though data for two of the sites was reported in Yoo and Wander (2006). Seasonal trends in CO2 evolution rates differed slightly among sites (Table 1). In DeKalb, soil CO2 evolution rate peaked in July and then decreased throughout the rest of the growing season before increasing slightly in September (data not shown). In Monmouth, the CO2 efflux rate was the highest in June and decreased until the last sampling date. A similar trend in CO2 efflux pattern as in Monmouth was observed in Perry except that at Perry, efflux rates peaked in July (data not shown). Overall mean soil CO2 evolution rates did not differ among soils under NT and CT management in DeKalb or Perry, but did differ in Monmouth, where mean soil CO2 evolution rates were significantly higher from CT than NT soils (Fig. 1a). RESPsp ranked DeKalb < Monmouth < Perry (Fig. 1b). Although RESPsp did not differ among NT and CT soils in DeKalb, RESPsp rates were reduced in NT soils in Monmouth and Perry. Soil C sequestration considered in the 0- to 30-cm depth on an equivalent volume basis was highest in DeKalb, intermediate in Monmouth, and least in Perry (Fig. 2). There was no difference in the SOC contents of NT and CT soils in DeKalb, whereas NT soils had higher SOC than CT soils in Monmouth and Perry (Fig. 2).


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  		Table 1. Variance analysis of mean CO2 evolution rate, specific mineralization of soil organic carbon (RESPsp), soil temperature, gravimetric water contents, and bulk density.

 

Figure 1
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  		Fig. 1. Seasonal mean CO2 evolution rates (a) and specific mineralization rates of soil organic carbon (RESPsp) (b) from no-tillage (NT) and conventional-tillage (CT) soils in DeKalb, Monmouth (from Yoo and Wander, 2006), and Perry, IL. If the letters above data from each location are not the same, then means were significantly different at P < 0.05.

 

Figure 2
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  		Fig. 2. Effect of conventional (CT) and no-tillage (NT) practices on soil organic carbon (SOC) sequestration on an equivalent volume basis in the 0- to 30-cm depth. If the uppercase letters above data are not the same, then site-based means were significantly different at P < 0.05. Means with different lowercase letters are significantly different at P < 0.05 within each site.

 
Soil Temperature, Gravimetric Water Contents, Bulk Density, and Clay Content
The depth-weighted average of soil temperature differed by sites and sampling date, but tillage practices did not alter soil temperature at any site (Table 1). Soil temperature at Perry was highest and the mean temperature was greater in DeKalb than in Monmouth (Table 2). Soil water contents did not vary with tillage practices in a consistent manner across the three sites (Table 1). Soil water contents were highest in DeKalb and soils in Monmouth were wetter than those in Perry. At Perry, soil water contents were higher in NT than in CT soils, while there was no difference in soil water contents of NT and CT soils at DeKalb and Monmouth (Table 2). Adoption of NT practice increased bulk density at all three sites (Table 2), with bulk density ranked DeKalb < Monmouth < Perry. Clay content measured at Perry was 17.5% regardless of tillage practice and it was used for calculation of LLWR.


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  		Table 2. Site and tillage (CT, conventional tillage; NT, no tillage) based differences in seasonal mean of soil temperature, gravimetric water content, and bulk density.

 
Pore Size Distribution and the Least Limiting Water Range
Total pore volume was greatest in DeKalb, followed by Monmouth and Perry and higher in CT than NT soils in all sites (Table 3). Pore volume was mainly concentrated in the meso- and micropore range (<15 µm) in all three sites (Table 3). The volume of small macropores was similar at all sites and the volume of large macropores and meso- and micropores ranked DeKalb > Monmouth > Perry. At DeKalb and Monmouth, soils under CT management had a higher volume of large macropores (>150 µm) than soil under NT. Large macropore volume did not vary with tillage practice at Perry. At Perry and Monmouth, CT soils had a greater volume of small macropores (15–150 µm) than did soils under NT management. The volume of small macropores was similar in CT and NT soils at DeKalb. The volume of meso- and micropores (<15 µm) did not vary among tillage treatments at any site.


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  		Table 3. Site and tillage (CT, conventional tillage; NT, no tillage) effects on total pore volume, volume of large macropores (>150 µm), small macropores (15–150 µm), and meso- and micropores (<15 µm).

 
Again, for comparison among sites and consistency with other data, previously reported LLWRs from DeKalb and Monmouth (Yoo and Wander, 2006) are shown together with data from Perry. The LLWR in the NT soil at Perry was reduced compared to soils under CT management. The dry limits for LLWR in Monmouth and Perry were based on limits associated with soil resistance, whereas the dry limit in DeKalb was related to wilting point (Table 4).


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  		Table 4. Least square means of least limiting water ranges (LLWR) and values of limits for LLWR in DeKalb and Monmouth (Yoo and Wander, 2006) and Perry, IL.

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
CONCLUSIONS
 REFERENCES
 
The mean CO2 evolution rates observed at Perry were not consistent with tillage-based differences in SOC storage seen after 17 yr. Differences in the RESPsp (NT < CT) observed at this site, however, were consistent with increased SOC storage (0–30 cm) in NT soils. The amount of labile C (particulate organic matter C) was reported to be greater in NT than CT soils at Perry (Wander et al., 1998). Thus, reduced RESPsp in the NT soils cannot be explained by substrate quality. Lower RESPsp in the NT soils at Perry suggests there is increased physical protection of SOC. This is consistent with results from Monmouth where RESPsp was higher in the CT than NT soils. Agreement between the RESPsp and SOC storage observed at these three sites suggests that the influence SOC sequestration is largely controlled by SOC mineralization.

Trends in soil temperature and soil water contents, which are the most important factors influencing SOC mineralization, did not successfully explain tillage impacts on mean SOC mineralization rates at any site. Higher water contents in the NT than CT soils at Perry were probably related to surface plant residues that were more abundant in soils under NT management (Tindall and Crabtree 1980; Stott 1991). The similar soil water contents of NT and CT soils at DeKalb and Monmouth are probably due to the fact that residues disappeared rapidly in the higher fertility sites (Linden and Clapp, 1998; Yang and Wander, 1999; Wander and Yang, 2000). Soil bulk density increased as organic matter levels declined along a north to south gradient (DeKalb < Monmouth < Perry). Trends were consistent with findings that bulk density is dependent on soil texture and SOC contents (Soane et al., 1981; Chen et al., 1998). The increase in bulk density we observed in NT soils at all sites has also been noted in several other studies (Wu et al., 1992; Gregorich et al., 1993; Kay and VandenBygaart, 2002). The increase in soil compaction resulting from use of NT practices was not sufficient to reduce RESPsp at all sites considered (Fig. 3a). At DeKalb, where soil bulk density was ≤1.4 g cm–3, RESPsp was not reduced whereas in the other two locations where bulk density was greater, RESPsp was reduced under NT management. There may be a threshold value for bulk density that must be exceeded before pore-dependent processes are constrained. The influence of tillage practices on particular pore classes might be important to consider. Reductions in the volume of large macropores (>150 µm) that occurred in the NT soils at DeKalb and Monmouth do not account for trends in RESPsp but are consistent with results of Taboada et al. (1998) who studied loamy and silty clay loam soils in Argentina. The volume of small macropores (15–150 µm) was reduced in the NT soils at the Monmouth and Perry sites and this may help explain the decrease of RESPsp in these sites. Microbial activity might have been negatively influenced by reductions in the volume of this pore size class. This notion is supported by the findings of Strong et al. (2004) who observed rapid decomposition of C in the pores with neck diameters between 15 and 60 µm. Similar RESPsp values recorded in DeKalb's NT and CT treatments are consistent with our finding that the volume of small macropores (15–150 µm) was not affected by tillage practices applied. Physical thresholds affecting SOC mineralization might be related to changes in the volume of pores in this size class (15–150 µm). Kay and VandenBygaart (2002) reported that much of the decrease in total porosity associated with the use of NT practice is due to reductions in the volume fraction of pores 30 to 100 µm. Tillage practices did not alter the volume of meso- and micropores at any site. This is consistent with the results of Hill et al. (1985) and Kay and VandenBygaart (2002). Efforts to develop threshold values for physical controls over SOC mineralization might focus on characterization of the small macropore range (15–150 µm) instead of on bulk density or total porosity.


Figure 3
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  		Fig. 3. The effect of depth-weighted bulk density (a) and least limiting water range (LLWR) size (b) on specific soil organic carbon (SOC) mineralization rates in the no-tillage (NT) and conventional-tillage (CT) soils.

 
Efforts to develop predictive tools might also consider the LLWR, which is computed using easy-to-measure properties. Yoo and Wander (2006) suggested that LLWR might be used to predict whether or not use of NT practices could increase SOC sequestration. Results presented here support that idea as the computed LLWR explained trends in RESPsp well (Fig. 3b). The dry limits for LLWR in Perry suggest that soil compaction limits SOC mineralization at that site. The same observation was previously made for the Monmouth site.

Increases in SOC sequestration and soil bulk density under NT management that were previously not observed (Wander et al., 1998) at Perry were apparent after 17 yr. Soils at this site may have resisted compaction for a time as a result of texture and climate. According to Imhoff et al. (2004), soil compressibility is positively correlated with clay up to contents of 33%. Thus, with clay contents of 17.5%, the Perry soil may not have been highly predisposed to compact. This, and the generally dry site conditions experienced during the experiment, may have slowed the rate of change in soil characteristics resulting from the adoption of NT practices. During summer, soil water contents recorded were generally below the 20% level above which soils are vulnerable to compression (Sanchez-Giron et al., 2001).


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
REFERENCES
 
This work built on previous findings that suggested that physical properties (LLWR and bulk density) and RESPsp can explain the inconsistent effects of tillage practices on C mineralization and SOC sequestration by considering a new site. We also determined whether pore size characteristics could explain the influence of NT practices on SOC sequestration. Soil RESPsp, which declines with increasing physical protection of SOC, was reduced in NT soils at Perry, IL. This was also true in Monmouth, IL, where use of NT practices increased SOC sequestration. While use of NT practices increased soil bulk density at all three sites, this was not accompanied by reductions in RESPsp at DeKalb where bulk density is relatively low. There may be a threshold in bulk density beyond which soil microbial activity starts to be influenced. The notion that soil compaction is associated with changes in SOC is supported by observed shifts in pore size distribution. The volume of small macropores (15–150 µm), which are thought to be important for microbial activity, was reduced in NT soils at Monmouth and Perry along with RESPsp. At DeKalb, neither RESPsp nor pore volume were altered by tillage practices.

Both pore size distribution and LLWR may help us predict whether or not adoption of NT practice can change C dynamics (RESPsp) and ultimately SOC sequestration. Pore size distribution may provide useful information about the mechanistic relationship between soil structure and microbial activity. While conceptually complex, LLWR would be easier to determine than soil pore structure as long as appropriate pedotransfer functions, which rely on soil-water release and soil resistance curves, are available. Future research should explore how indices of soil physical status, like the LLWR, can help us decide where to promote the adoption of NT practices to increase SOC sequestration.


   ACKNOWLEDGMENTS
 
Many thanks are due to Crops Department field station managers Eric Adee, Mike Voss, Glenn Raines, and Lyle Paul for their care and maintenance of these long-term trials and to the Crops Department for use of their facilities.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 




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G. Yoo and M. M. Wander
Tillage Effects on Aggregate Turnover and Sequestration of Particulate and Humified Soil Organic Carbon
Soil Sci. Soc. Am. J., May 1, 2008; 72(3): 670 - 676.
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