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Published in Soil Sci. Soc. Am. J. 68:1394-1402 (2004).
© 2004 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA

DIVISION S-6—SOIL & WATER MANAGEMENT & CONSERVATION

Persistence of Soil Organic Carbon after Plowing a Long-Term No-Till Field in Southern Ontario, Canada

A. J. VandenBygaarta,* and B. D. Kayb

a Eastern Cereal and Oilseed Research Centre, Agriculture & Agri-Food Canada, KW Neatby Building, 960 Carling Ave., Ottawa, ON, Canada K1A 0C6
b Dep. of Land Resource Science, Univ. of Guelph, Guelph, ON, Canada, N1G 2W1

* Corresponding author (vandenbygaarta{at}agr.gc.ca).


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 SUMMARY AND CONCLUSIONS
 REFERENCES
 
There is abundant evidence that minimizing soil disturbance reduces mineralization of organic matter and can result in larger storage of soil organic carbon (SOC) relative to conventional tillage. However, little is known about the persistence of SOC when no-till lands are plowed periodically. This study set out to determine the change in SOC when a long-term (22 yr) no-till field in southern Ontario, Canada, was plowed once. Four plots were located within three textural classes [sandy loam (SL), sandy clay loam (SCL), and silty clay loam (SiCL)] within two hydrologic conditions (well- and poorly drained) in the field. The plots were sampled before and three times after (3 d, 7 mo, and 18 mo) the no-till field was moldboard plowed. The single tillage event homogenized the SOC through the profile and reduced the stratification. When calculated on an equivalent mass basis beyond the plow depth, there was no significant change in SOC 18 mo after plowing the SCL, SiCL, and SL high SOC plots. However, in the SL plot with low SOC, there was a loss of about 3 Mg SOC ha–1 after 18 mo, and the loss occurred primarily between the 15- and 30-cm depths in the profile. The loss may have accounted for as much as two-thirds of the SOC gained from no-tillage. This study also emphasized the need for additional care to account for changes in bulk density when comparing the quantity of soil constituents, such as SOC, after plowing.

Abbreviations: SCL, sandy clay loam • SiCL, silty clay loam • SL, sandy loam • SOC, soil organic carbon • SLHC, sandy loam, high carbon • SLLC, sandy loam, low carbon


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 SUMMARY AND CONCLUSIONS
 REFERENCES
 
MINIMIZING SOIL DISTURBANCE by tillage can reduce mineralization of SOC and thus can aid in sequestering carbon from the atmosphere (West and Post, 2002; Six et al., 2002b). Consequently, some countries have considered no-tillage as a strategy to offset other sources of CO2, to aid in fulfilling international commitments to reduce greenhouse gas emissions (Lal et al., 1999; Paustian et al., 2000). However, little is known of the persistence of the SOC that is gained under long-term no-till if the field is plowed in a single season. This knowledge is required since little is known about the year-to-year soil management practices of an individual farmer. For example, long-term no-till can result in unfavorable conditions for crop growth such as compaction (Coote and Malcolm-McGovern, 1989; Chen et al., 1998) or weeds (Wicks et al., 1988; Kettler et al., 2000) that may only be feasibly ameliorated by a tillage event. Furthermore, many decisions made by an individual farmer are greatly influenced by economics, such that soil and crop management practices can be manipulated on a yearly basis.

The persistence of SOC that has been gained under long-term no-till following a single tillage event may be related to soil texture and the forms of SOC. A portion of SOC is chemically stabilized in soil by association with silt and clay-sized particles (Six et al., 2002a). Six et al. (2002b) confirmed the conclusions of Hassink (1997), showing that the quantity of clay and silt-size particles is positively correlated with soil organic matter in the same size fractions across a broad range of climates and soil types. However, this fraction of SOC would not be expected to be responsive to a single tillage event. Another portion of the SOC is physically protected from mineralization within macro- and microaggregates. Because of a lack of disruption, physical protection of SOC has been documented in no-till soils (Angers et al., 1993; Six et al., 2002b). Texture has an effect on aggregation of soils (Angers, 1998; Six et al., 2002b), and there is evidence that the extent of physical protection of SOC increases with increasing clay content (Six et al., 2002a). However, Yang and Kay (2001a) found that <30% of the increase in SOC in a clay loam soil after 19 yr of no-till was occluded particulate C (>60% of the increase was associated with the mineral fraction). Tillage would be expected to expose some of the protected SOC, making it susceptible to mineralization, although the influence of texture on the amount of C mineralized is unknown.

The purpose of this study was to (i) assess the effect of plowing a soil under no-till for 22 yr on the stability of SOC in southern Ontario, and (ii) determine whether texture affects the stability of SOC after plowing.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 SUMMARY AND CONCLUSIONS
 REFERENCES
 
Plot Design and Plowing
This study was performed on a cash-crop farm in southern Ontario, in which the farmer had maintained a portion of a field in no-till for 22 yr. Yang and Kay (2001a) determined that there was significant SOC storage in the no-till portion by comparing with an adjacent conventionally tilled section of the field. However, they concluded that the extent of the difference between the two tillage treatments varied because of soil texture as well as the drainage history. In a poorly drained section of the field, tile drains were installed in 1972, and, as a result, SOC levels were still declining, but they were decreasing much more quickly under conventional tillage. At a higher elevation with better drainage, the clay loam no-till soil sequestered about 7 Mg ha–1 within an equivalent mass of 1400 Mg soil (i.e., {approx}10 cm of soil; Yang and Kay, 2001a).

We located four plots within the field that represented three soil texture classes and two hydrologic situations (Fig. 1). Two plots were located on a SL, but had different drainage conditions, which influenced the SOC level [sandy loam, high carbon (SLHC) and sandy loam, low carbon (SLLC) plots]. The SLHC (Mollic Aqualf) was situated in a part of the field that had tile drains installed in 1972, while the SLLC (Typic Hapludalf) plot was situated in a well-drained part of the field. The remaining two plots were also located in well-drained positions in the field [SCL (Typic Hapludalf) and SiCL (Typic Hapludalf); Fig. 1]. Each plot was laid out in three transects with 10 cores in each transect oriented in the direction of tillage and planting (Fig. 1). Our initial premise was to have the 30 cores represent replications in each plot. Nonetheless, we designed the sample protocol so as to be able to reposition soil core locations in each successive sampling period across time to obtain spatial control for the cores. In May 2001, we extracted 30 soil cores (minimum 50-cm depth; 4.6-cm diam.) in each of the four plots using a hydraulic tractor-mounted coring device. The device allowed for extraction of soil cores in plastic tubes which could be capped at both ends and transported easily to the laboratory for subsequent analyses. We then made a single pass with a moldboard plow through each of the plots at a depth of approximately 20 cm one day following the first soil sampling. A light disc harrow was used to level the soil. Three days following tillage, 30 soil cores were again collected from each of the four plots. The second set of cores was collected approximately 5 cm from the pretillage core sampling in order not to resample the same position in the plot. After harvest in late fall 2001, 7 mo after the tillage event, a third set of 30 soil cores was collected from each of the four plots with a similar displacement in an alternative direction than the previous sampling (Fig. 1). A final set of cores was collected after harvest in the fall 2002, 18 mo after tillage of the no-till soil.



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  		Fig. 1. Locations of plots and plot layouts in a long-term no-till field in southern Ontario. The silty clay loam (SiCL), sandy clay loam (SCL), and sandy loam, low carbon (SLLC) plots were well drained, while the sandy loam, high carbon (SLHC) was poorly drained. Note: Not to scale.

 
Bulk Density, Soil Texture, and Soil Organic Carbon
After cores were collected from the field, they were stored in a refrigerator at 4°C until they were ready for segmentation. The cores were carefully subsampled into seven depth increments (0–5, 5–10, 10–15, 15–20, 20–30, 30–40, and 40–50 cm) and the bulk density of each increment was determined.

Textural analysis was performed on 15 cores (every second core on each transect) from each plot for the preplow coring event. Subsamples of soil were taken from the 0- to 5-, 5- to 10-, 10- to 15-, and 15- to 20-cm layers of each core and combined to create a composite for 0 to 20 cm. Percentages of sand, silt, and clay were determined for the 0- to 20- and 20- to 30-cm layers in each core using the hydrometer method (Sheldrick and Wang, 1993). Table 1 summarizes the textural analysis for the plots in the study before plowing the no-till soil.


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  		Table 1. Textural analysis before plowing the no-till soil in the four plots of the study (values in parentheses are 1 SD of the mean; n = 15).

 
Total carbon in the soil was determined by a Leco CS444 analyzer (Leco Corp., St. Joseph, MI). Samples were placed in a furnace overnight at 475°C to determine inorganic C, and organic C was determined after subtraction from the total carbon content (Yang and Kay, 2001a, 2001b). Total SOC in each soil core was calculated on an equivalent mass basis using the method outlined by Ellert and Bettany (1995).

Statistical Analysis
Because of the presence of clear patterns and high variability of SOC across plots, testing for significance between sampling periods using a paired t test was most appropriate. In this case, we assumed that each soil core and cores sampled at successive times at each core location in a plot were the same soil, but sampled at different times. This is appropriate for our purposes since successive cores in each plot were situated within 5 cm of the initial sample core location (Fig. 1). More specifically, the paired t test assesses whether or not the differences between the two measurements are significantly different than zero. As a result, we increase the statistical power of our analysis by minimizing the within-plot variability of total SOC (Yanai et al., 2003). We calculated the total SOC in each profile on an equivalent mass basis from the surface to beyond the plowing depth in each plot and tested the differences using paired t tests.

Of the pretillage cores on which textural analysis was performed, total SOC was related to sand, silt, and clay content by simple correlation analysis in each plot. For these statistical analyses we used the paired t test and correlation analysis components within the Analysis ToolPak add-in of Microsoft Excel (Microsoft Corp., Redmond, Washington).


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 SUMMARY AND CONCLUSIONS
 REFERENCES
 
Changes in Bulk Density
Bulk density changed significantly after tillage, and responded differently through time in each of the plots. In the SCL plot, there was a significant decrease in bulk density within the top 15 cm (Table 2). The 5- to 15-cm bulk density recovered after 18 mo to the initial bulk density before tillage. In the SiCL plot, there was a large decrease in bulk density from 5 to 30 cm after plowing, and the bulk density continued to approach the pretillage value through time (Table 2). However, bulk density did not recover to the pretillage value as in the SCL plot. In the SLHC plot, there was an increase in bulk density in the 5- to 15-cm layer, but a decrease in the top 5 cm immediately following tillage (Table 2). The increase in the 5- to 15-cm layer may have been due to incorporation of sandy subsoil (Table 1), as sand content has been shown to be positively related to bulk density (Chen et al., 1998). Although highly variable, it appeared that the bulk density in the 5- to 30-cm depth increased until the last sampling date 18 mo after tillage (Table 2). A similar trend occurred in the SLLC plot, whereby the bulk density continued to increase through the last sampling date in the 5- to 30-cm layer, although again the variability was considerable across the plot. This is in contrast to the expected result of a decrease in bulk density upon plowing a soil, in particular one that has been under no-till for an extended period of time.


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  		Table 2. Changes in bulk density in the top 30 cm after plowing the no-till soil based on unpaired t test. Values in parentheses are standard deviations of the mean (n = 30).

 
Soil Organic Carbon Concentration with Depth and Changes after Plowing
The SLHC plot had the highest concentration of SOC, while the SLLC plot had the lowest (Table 3). Each of the plots, with exception of the SLHC plot, had preplow SOC distributions typical of other long-term no-till studies in which SOC is stratified with the greatest concentration in the top 5 cm of the soil profile (Kay and VandenBygaart, 2002). It is not clear why a similar trend was not observed in the SLHC plot where the distribution was relatively uniform with depth in the A horizon. However, Yang and Kay (2001a) suggested that the differences in SOC between tillage treatments on this soil were due not to an increase in the sequestration of SOC, but to a slower loss of SOC induced by the no-till, since this location in the field had tile drains installed in 1972 and the SOC levels are likely still declining at present. Any expected increase in SOC near the soil surface in the no-till condition may be masked by the high SOC due to the drainage management history at this location in the field.


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  		Table 3. Changes in soil organic carbon concentration in the top 30 cm after plowing the no-till soil based on unpaired t tests. Values in parentheses are standard deviations of the mean (n = 30).

 
In all four plots, there was a homogenization of the SOC distribution with depth after a single tillage event (Table 3). There was a significant decrease in SOC concentration within the top 5 cm of the soil profile, which was countered by an increase at a lower depth in SLLC, SLC, and SiCL plots (Table 3). In the SLHC plot, the inversion did not occur until the 15- to 20-cm layer, where the SOC concentration was significantly higher after tillage (P < 0.05; Table 3).

A careful examination of Table 3 indicates that there were temporal changes in the distribution of SOC within the top 20 cm after plowing (Table 3). To assess these changes, we tested the differences with depth between the 18-mo postplow and the 3-d postplow SOC concentrations (significance test not shown in Table 3). At the 15- to 20-cm depth in the SLLC (Table 3), the SOC concentration decreased significantly from about 0.8 to 0.6% (P < 0.005), and was significantly greater (P < 0.01) at the 0- to 5-cm depth (about 1.2 vs. 1.0%) after 18 mo from the tillage event. This indicates that even after two cropping seasons, the stratification of SOC was approaching the preplow pattern associated with no-till from the previous 22 yr (Table 3). This temporal change was also occurring in the SCL and SiCL plots (Table 3). In the SCL plot, the SOC increased from about 2.0 to 2.1% in the 0- to 5-cm layer (P < 0.05) after 18 mo, while in the 10- to 15-cm layer, the SOC concentration decreased from about 2.1 to 2.0% (P < 0.05; Table 3). In the SiCL plot the SOC concentration increased from 2.0 to about 2.3% in the top 5 cm (P < 0.0001), while it decreased in the 15- to 20- cm layer from about 2.2 to about 2.1% (P < 0.10; Table 3).

There were clear differences in SOC concentrations and bulk density before and after plowing and with depth. This is useful for understanding the effects of tillage management on the vertical distribution of SOC and bulk density. However, to assess if there were any significant changes in total SOC stock after plowing the no-till soils, we require the determination of the total SOC within the soil profile on an equivalent mass basis by incorporating both bulk density and SOC concentration.

Within-Plot Variability and Total Soil Organic Carbon Stock
The total SOC in 4400 Mg of soil ({approx}30-cm depth) before tillage was highly variable (Fig. 2). The variability in SOC may be due, in part, to variation in texture, topography, or hydrology. Textural variation within the plots (Table 1) accounted for part of the variation in SOC. The SOC content was positively correlated with clay content in 4400 Mg of soil (r = 0.62) and negatively correlated with sand content (r = –0.46). When initializing the sampling design, we attempted to locate the plots in the flattest, most uniform positions in the field. Measurements of elevation were not conducted on the plots, but even slight changes in elevation, and consequently hydrology, may also have contributed to the variability in total SOC even though the area of each plot was as small as 50 m2 (Fig. 2).



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  		Fig. 2. Total soil organic carbon (SOC, Mg ha–1) in 4400 Mg ha–1 equivalent mass at each sample location in the plots before plowing the no-till soil (see Fig. 1 for core locations) (A) sandy loam, high carbon (SLHC); (B) sandy loam, low carbon (SLLC); (C) sandy clay loam (SCL); and (D) silty clay loam (SiCL). See text for explanation of annotation in (A).

 
When the change in SOC between sample periods was compared, it was evident that the single tillage event not only resulted in redistribution of SOC vertically (Table 3), but laterally as well. We calculated the net change in SOC relative to pretillage for each of the plots of each sampling event on an equivalent mass basis of 4400 Mg of soil (about 30 cm) (Fig. 3). There appeared to be a lateral redistribution of SOC by the plow, since the patterns of net change relative to the pretillage sampling were generally consistent for each of the posttillage sampling events (Fig. 3). Another way to interpret Fig. 3 is to picture the values for the preplow cores plotted directly on the x axis at 0 Mg ha–1. Consequently, it was more appropriate to assess the net change due to tillage on samples taken after tillage to avoid having the effects of lateral soil redistribution influencing the analysis and interpretation of the results; thus, we assumed that there was no significant loss of total SOC within 3 d after the tillage of the no-till soils (i.e., when the first posttillage sampling occurred). Rochette and Angers (1999) showed that fall moldboard plowing of fresh maize residues into the soil increased CO2 losses by 170 kg C ha–1 during a period of 4 d. However, much of the total flux of CO2 for the 4-d period occurred within the first few hours after tillage from the degassing of disrupted soil pores and not from decomposition of incorporated materials or SOC within the soil matrix. Thus, we were confident that there would be minimal change in SOC between the pretillage and the 3-d posttillage sampling events.



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  		Fig. 3. Net difference in SOC from preplow SOC (Mg ha–1 in 4400 Mg ha–1 equivalent mass) in (A) sandy loam, high carbon (SLHC); (B) sandy loam, low carbon (SLLC); (C) sandy clay loam (SCL); and (D) silty clay loam (SiCL). See Fig. 1 for core locations.

 
There were significantly greater amounts of SOC within 7 mo in 1400 Mg soil ha–1 (about 10 cm of soil) in the SiCL and within 18 mo in 700 Mg soil ha–1 (about 5 cm of soil) in the SCL and SiCL soils (Table 4). This increase was likely a consequence of crop residues concentrating at the soil surface since the soil was not plowed again. However, this cannot be regarded as sequestration of SOC, as the increase at the surface was counteracted by a decrease at depth; the total SOC within the top 4400 Mg of soil (or about 30 cm of soil) was not significantly different between the 3-d postplow, and the 7- and 18-mo sampling events in the SCL and SiCL plots (Table 4).


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  		Table 4. Mean soil organic carbon (SOC) storage with cumulative equivalent mass depth to 6000 Mg ha–1, and significance of differences in total SOC 7 and 18 mo after plowing the no-till soil using two-tailed paired t tests. Values in parentheses are mean differences between pairs where there were significant differences (n = 30).

 
There were no significant changes in SOC contents from 3 d after plowing up to 18 mo after plowing for equivalent soil masses of 2900 Mg soil ha–1 or greater on three of the four plots (Table 4). However, in the SLLC plot, in an equivalent mass of 2900 Mg of soil ha–1 (about 20 cm of soil) to 6000 Mg of soil ha–1 (about 40 cm of soil), there was a decrease in SOC, with the decrease ranging up to 3.5 Mg C ha–1. The loss occurred between 7 and 18 mo from the tillage of the no-till soil (Table 4). An examination of the distribution of SOC concentration (Table 3) suggests that a substantial part of this loss occurred between 10 and 30 cm. The SOC that was originally situated in the top 5 cm under no-till (Table 3) was inverted by plowing and resulted in greater decomposition than had it remained nearer the soil surface. The average SOC in 2900 Mg equivalent mass in this plot was 27 Mg C ha–1, while the loss of SOC in the same mass of soil was about 2.5 Mg ha–1 (Table 4), and represents about a 10% relative decrease in SOC.


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
SUMMARY AND CONCLUSIONS
 REFERENCES
 
Bulk Density and Soil Organic Carbon Stock
Changes in bulk density with time have a critical impact on how SOC changes are calculated and interpreted. In the SCL plot, there was a significant decrease in bulk density immediately after tillage, but the bulk density recovered after 18 mo in the 5- to 15-cm layers (Table 2). If we were to have sampled to a fixed depth of 20 cm in each plot, the total mass of the soil would have changed significantly through time resulting from a change in bulk density. Sampling to a fixed depth would have resulted in less soil being collected from the top 20 cm in the sampling time immediately following tillage relative to the final sample period. As a result, the SOC mass per volume calculation would not be made on an equivalent mass of soil, and could result in erroneous interpretations of the differences through time. The total mass of soil on a volumetric basis to depth of 20 cm in the SCL plot (Fig. 4) clearly shows that comparisons between sampling periods would result in erroneous interpretations since the mass of soil would be different. The total SOC measured to the 20-cm depth in each plot would vary as a function of the differences in bulk density, and therefore total SOC should be assessed by expressing it on an equivalent mass basis, as was demonstrated by Ellert and Bettany (1995).



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  		Fig. 4. Cumulative mass of soil (Mg ha–1) to the 30-cm depth in the sandy clay loam (SCL) plot. See Fig. 1 for core locations.

 
Interestingly, there were also considerable differences in bulk density and thus cumulative mass to 30 cm between transects within sampling events in the SCL plot (Fig. 4). The first transect (Core Locations 1 through 10) clearly had lower masses to 30 cm than the other two transects (Core Locations 11 to 20 and 21 to 30) before plowing the no-till soil. However, after a single tillage event, each of the transects appeared to converge to a similar cumulative mass. The differences in cumulative mass between transects within the same plot would greatly affect the calculation of SOC storage even at the same sampling event, further enhancing the variability within the same plot unless the SOC stock was determined on an equivalent mass basis.

Within-Plot Trends in Soil Organic Carbon Stock
Even after accounting for equivalent soil mass, there were obvious trends of SOC stock between each of the three transects in each plot. For example, if core locations in the SLHC plot (Fig. 2a) were pooled for each of the three transects, Transect A (containing Cores 1 through 10) had about 23 Mg ha–1 (P < 0.001) lower SOC than Transect B (containing Cores 11 through 20), and about 13 Mg ha–1 (P < 0.001) lower than Transect C (containing Cores 21 through 30), even though the distance between adjacent transects was only about 2 m.

Clearly, one must use caution when interpreting results from experiments designed in a side-by-side manner in which it is assumed that both transects had the same initial SOC. If a tillage experiment were to be designed side-by-side between Transects A and B without knowing the initial SOC in the SLHC plot (Fig. 2a), the difference in total SOC between the transects would not be due to tillage management, but rather due to the lateral variation of SOC in the field.

Change in Soil Organic Carbon Stock
The only soil to have a significant loss of SOC after plowing the long-term no-till was the SLLC plot (Table 4). In a study on the same field, Yang and Kay (2001a) showed that the SOC was about 15% greater in no-till relative to conventional tillage in an equivalent mass of 2900 Mg of soil of similar texture as the SLLC plot. However, the SOC contents were three times greater in the location of the soil in the Yang and Kay (2001a) study. If we assume that the relative changes in SOC contents in the two parts of the field with a SL texture can be compared, then the loss of SOC due to plowing would represent two-thirds of the SOC gained under 22 yr of no-till.

Stockfisch et al. (1999) showed a significant decline of about 13 Mg C ha–1 5 mo after a single moldboard tillage event of a silt loam soil in no-till for 20 yr in Lower Saxony, Germany. According to the authors, this represented all of the SOC which had been gained under no-till soil management. However, Stockfisch et al. (1999) did not measure the bulk density on the same samples that were collected for SOC. They used bulk density values from an adjacent moldboard plowed soil, and assumed no change occurred through time after tillage. The present study shows that significant changes in bulk density can occur after plowing a no-till soil (Table 2), and this would influence the total SOC calculation when not considering equivalent soil masses.

Kettler et al. (2000) showed that a single tillage event on a long-term no-till silt loam soil in Nebraska resulted in a redistribution of SOC in the top 30 cm. However, there was no significant change in SOC storage 5 yr after plowing the no-till soil. This is consistent with results of Pierce et al. (1994), who showed that there was also a redistribution of SOC in the top 15 cm after plowing a no-till loam soil in Michigan. However, annual variability in SOC after 4 yr in all treatments appeared to override any effects of the single tillage event.

Failure to observe any change in SOC contents in the SCL or the SiCL, in contrast to the SLLC, may reflect differences in forms of C that may have been sequestered during the no-till period. Yang and Kay (2001a) showed that about 62% of the increase in SOC under no-till was in the humified fraction of the SCL soils (Huron clay loam) of this field. This fraction would be less susceptible to short-term decomposition due to a single tillage event. Another cause could be that a single soil inversion event by the moldboard plow was not sufficient to stimulate the breakdown of soil organic matter protected within aggregates (Six et al., 2002a). Six et al. (2000) showed that about half of the macroaggregate weight (>250 µm) in a no-till soil in Nebraska consisted of microaggregates (<250 µm), compared with only 27% in soils under conventional tillage. Furthermore, Six et al. (2002b) showed that there was about three times as much easily decomposable particulate organic matter in intramicroaggregate locations in no-till relative to conventionally tilled soils. As such, a lack of continuous tillage (i.e., just a single tillage event) may have not had a sufficient influence to break apart micro- and macroaggregates that would expose intramicroaggregate particulate organic matter to decomposition (Six et al., 2000) in the finer-textured soils of the field.

In contrast, the coarser texture of the SLLC plot likely contributed to a lower proportion of microaggregates than the finer-textured plots (Angers, 1998; Carter et al., 2003). This lower aggregation potential could allow less protection of the particulate soil organic matter fraction from microbial decomposition. Yang and Kay (2001a) noted that almost 70% of the gain in SOC under no-till in this soil had been in the occluded particulate fraction. This fraction would be expected to be more susceptible to increased mineralization following tillage than fractions that were biochemically protected or protected by adsorption onto surfaces of the silt and clay fraction. In addition, there is recent evidence that the lower rate of C mineralization in no-till soils is not only due to a lack of physical disruption by the plow, but also due to soil aggregates being protected from continual exposure to wet–dry and freeze–thaw cycles at the surface (Denef et al., 2001; Six et al., 2002b). These effects may have been more important in the SLLC plot than those that were finer-textured.


   SUMMARY AND CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 SUMMARY AND CONCLUSIONS
REFERENCES
 
Little is known of the stability of the SOC in no-till under conditions where there is an increase over conventional tillage. This study set out to determine the net change in SOC storage in four plots with three different textures within two hydrologic conditions, after plowing a long-term no-till field. No significant losses of SOC occurred in the SLHC, SCL, and SiCL plots 18 mo after plowing when compared on an equivalent mass with about 30 and 45 cm. This suggests that the disturbance event was not significant enough to alter the SOC dynamics in these parts of the field. However, in the SLLC plot there was a decrease of about 3 and 3.5 Mg ha–1 in equivalent masses to 30 and 45 cm, respectively, after 18 mo, and this loss occurred at a depth between 15 and 30 cm in the profile.

This study also highlighted how interpreting changes in SOC is highly dependent on how the SOC stock is calculated and the variability of these properties across a field. When there is a large change in bulk density, such as that which occurs after plowing a long-term no-till soil, additional care must be taken to be certain that comparisons of soil constituents such as SOC are made on equivalent masses of soil, as concluded by Ellert and Bettany (1995).


   ACKNOWLEDGMENTS
 
The study was supported by the Climate Change Funding Initiative in Agriculture (CCFIA) of Agriculture and Agri-Food Canada through the Canadian Adaptation and Rural Development (CARD) II program.

Received for publication November 25, 2003.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 SUMMARY AND CONCLUSIONS
 REFERENCES
 




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J. A. Quincke, C. S. Wortmann, M. Mamo, T. Franti, and R. A. Drijber
Occasional Tillage of No-Till Systems: Carbon Dioxide Flux and Changes in Total and Labile Soil Organic Carbon
Agron. J., June 26, 2007; 99(4): 1158 - 1168.
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A. S. Grandy, G. P. Robertson, and K. D. Thelen
Do Productivity and Environmental Trade-offs Justify Periodically Cultivating No-till Cropping Systems?
Agron. J., October 3, 2006; 98(6): 1377 - 1383.
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R. T. Venterea, J. M. Baker, M. S. Dolan, and K. A. Spokas
Carbon and Nitrogen Storage are Greater under Biennial Tillage in a Minnesota Corn-Soybean Rotation
Soil Sci. Soc. Am. J., August 22, 2006; 70(5): 1752 - 1762.
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A. S. Grandy and G. P. Robertson
Aggregation and Organic Matter Protection Following Tillage of a Previously Uncultivated Soil
Soil Sci. Soc. Am. J., June 21, 2006; 70(4): 1398 - 1406.
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R. A. Omonode, A. Gal, D. E. Stott, T. S. Abney, and T. J. Vyn
Short-term Versus Continuous Chisel and No-till Effects on Soil Carbon and Nitrogen
Soil Sci. Soc. Am. J., February 2, 2006; 70(2): 419 - 425.
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