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Published online 15 February 2008
Published in Soil Sci Soc Am J 72:507-513 (2008)
DOI: 10.2136/sssaj2006.0417
© 2008 Soil Science Society of America
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SOIL BIOLOGY & BIOCHEMISTRY

Residue Carbon Stabilization in Soil Aggregates of No-Till and Tillage Management of Dryland Cropping Systems

Gabe P. Olchina,*, Stephen Oglea, Serita D. Freyb, Timothy R. Filleyc, Keith Paustiana and Johan Sixd

a Natural Resource Ecology Lab. and Dep. of Soil and Crop Sciences, Colorado State Univ., Fort Collins, CO 80523
b Dep. of Natural Resources, Univ. of New Hampshire, Durham, NH 03824
c Dep. of Earth and Atmospheric Science, Purdue Univ., West Lafayette, IN 47907
d Dep. of Plant Sciences, Univ. of California, Davis, CA 95616

* Corresponding author (golchin{at}nrel.colostate.edu).


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Tillage events have an important influence on residue incorporation into soil profiles and soil aggregate disruption, and ultimately influence the net C gain or loss in soils. Thus, our objective was to evaluate tillage-induced influences on aggregate structure, residue-derived C stabilization, and the subsequent efficiency of C stabilization in aggregates of no-till (NT) and tillage management (TM) practices at different depth increments of the soil profile. Uniformly 13C-labeled wheat residues were added to incubation cores representing soils under NT and TM during a year-long in situ incubation at a dryland agriculture experiment site. Residue was added directly onto the surface of NT cores, while residues were incorporated into the 0- to 5-, 5- to 15-, and 15- to 30-cm depth increments of the TM cores. We found that residue additions did not have a significant effect (P > 0.05) on aggregate dynamics in either NT or TM, but NT management did result in the greatest stabilization of residue-derived C (11.2 ± 2.4 g residue C kg–1 soil kg–1 residue C added, P < 0.05) in the macroaggregate (>250-µm) fraction of the 0- to 15-cm increment. Residue-derived C stabilization was significantly greater (P < 0.05) in the 0- to 30-cm increment than in the 0- to 15-cm increment of the TM management cores. Overall, our results indicate that, within a plow depth of 15 cm, limiting the tillage-induced disruption of aggregates has a stronger influence on the efficiency of C stabilization than residue incorporation into the profile via tillage. When residues are distributed to a 30-cm depth, however, the negative impact of aggregate disruption through tillage appears counterbalanced, with similar efficiencies of C stabilization between the NT and TM practices, possibly due to slower decomposition of residues deeper in the profile.

Abbreviations: cPOM, coarse particulate organic matter • minM, silt plus clay within macroaggregates • mM, microaggregate-sized fraction within macroaggregates • NT, no-till • SOC, soil organic carbon • SOM, soil organic matter • TM, tillage management


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
No-till management can increase soil organic C (SOC) levels (Lal and Kimble, 1997; Paustian et al., 2000). This is an important management decision because terrestrial C sequestration has been suggested as a potential strategy for greenhouse gas mitigation (Council for Agriculture Science and Technology, 2004, p. 120). Terrestrial C sequestration is achieved through SOC stabilization by physical occlusion within aggregates (Tisdall and Oades, 1982; Elliott, 1986; Six et al., 2000b), chemical interactions with clay minerals (Sørensen, 1972; Christensen, 1996; Hassink, 1997), and biochemical recalcitrance (Leavitt et al., 1996; Krull et al., 2003). These three mechanisms are influenced in part by the spatial variance of soil properties and C inputs (e.g., residue additions or root exudates) to the soil, as well as management decisions.

The manner by which crop residues are introduced to the soil matrix differs dramatically between NT and TM. Under NT management, crop residues are left on the surface of the soil after harvest, whereas residues are mechanically incorporated into the soil during tillage. Stable isotopes have shown that the majority of C from corn residues is concentrated in the upper 5 cm under NT management, but a more uniform vertical distribution throughout the plow depth is achieved during tillage (Balesdent et al., 1990; Angers et al., 1995).

Recently decomposed crop residues and soil organic matter (SOM) are central to the aggregate hierarchy model of Tisdall and Oades (1982). They proposed that transient forms of SOM could act as binding agents, causing microaggregates (53–250 µm) to form stable macroaggregates (>250 µm). Furthermore, Puget et al. (1995) proposed that recently introduced crop residues are preferentially incorporated into aggregates, aiding in soil stability. Similarly, Six et al. (1999) suggested that the addition of new residues in NT management promote organic matter stabilization through the binding of primary soil particles and old microaggregates into new macroaggregates. Fragmented crop residues (i.e., particulate organic matter [POM]) can form the nuclei for new microaggregates that can be bound together by transient, labile organic matter to form new macroaggregates, or new microaggregates may form within the larger macroaggregates around POM (Golchin et al., 1994; Six et al., 1998). Several studies have shown that incorporation of residue-derived C into aggregates increased with increasing aggregate size (Jastrow et al., 1996; Six et al., 2000b; Puget and Drinkwater, 2001). Consequently, macroaggregate formation could be enhanced by residue additions throughout the entire plow layer with TM. In contrast, under NT management, residues are concentrated near the soil surface so that a smaller soil volume is exposed to fresh residues, which may limit new aggregate formation.

Tillage has a strong influence on soil aggregation and SOM dynamics by increasing macroaggregate turnover and reducing microaggregate formation compared with NT (Six et al., 1999). The disruption of aggregates releases physically protected POM, increasing its susceptibility to decomposition (Jastrow and Miller, 1997). In addition, intraaggregate POM is less susceptible to decomposition relative to free POM in the soil matrix (Besnard et al., 1996; Six et al., 1999) due in part to the physical and chemical protection that aggregates provide. Tillage also enhances the decomposition of total SOC, however, by mixing plant residues with soil (Holland and Coleman, 1987), disrupting aggregates (Beare et al., 1994a), and enhancing dry–wet and freeze–thaw cycles (Balesdent et al., 2000). Consequently, physical disturbances resulting from tillage and other environmental phenomena may destabilize macroaggregates, thereby offsetting some or all of the organic matter stabilization potential from macroaggregate formation. Thus, the stabilization of SOM may be enhanced in NT management because macroaggregates that are formed are less susceptible to disruption from tillage-induced physical disturbances.

Our objectives were to (i) investigate the influence of residue incorporation on aggregate formation, (ii) evaluate the stabilization of residue-derived C in aggregate fractions between NT and TM of dryland cropping systems, and (iii) evaluate the efficiency of C stabilization between NT and TM. Here we have defined efficiency of C stabilization as the percentage of residue-derived C that is associated with soil aggregate fractions at the end of a given time frame (e.g., 1 yr in our study). Quantifying residue-derived C stabilization between contrasting tillage management practices allowed us to determine the relative importance of residue addition vs. aggregate disruption at different depths in the profile, and consider the consequences for C sequestration.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Study Site and Experiment Design
This field incubation study took place at the Sustainable Dryland Agroecosystem Management Project located in Sterling, CO (40°22'12'' N, 103°7'48'' W). The soil is a fine-silty, mixed, mesic Aridic Argiustoll (Peterson et al., 1993). At the start of the experiment, 32 stainless steel incubation cores (9.0-cm diam.) were installed in the summit position of a NT wheat (Triticum aestivum L.)–corn (Zea mays L.)–fallow rotation (under fallow during the incubation) established in 1985. Cores were arranged in four randomized blocks. Within each block there were eight cores representing TM and an additional eight cores for NT. Four TM and NT cores were amended with 13C-labeled residues. Each treatment core had a corresponding control core with no residue added to evaluate a change in the isotopic signature relative to the background SOM during the incubation period (1 yr). Each amended core and corresponding control was randomly placed within the block, ensuring at least 50 cm between individual cores.

Labeled wheat enriched in 13C ({delta}13C = 797.55{per thousand}) was grown to maturity within a closed growth chamber with continuous labeling of 13CO2. The labeled wheat residue was cut into 1- to 2-cm pieces and added to a closed system (i.e., an incubation core) at the Sterling site, with a soil having a contrasting background {delta}13C signature ({delta}13C = –17.71{per thousand}), a similar approach to the methods of Aita et al. (1997).

The stainless steel cores were driven into the ground for both treatments to minimize differences in initial disturbance effects. Soil cores for the NT treatment were taken from the wheat–corn–fallow NT plot in the field experiment, whereas the soil cores for the TM treatment were transplanted from an adjacent field (same texture and summit slope position) that had previously been under long-term tillage (stubble mulch) management. Soil was removed from each core and thoroughly mixed with a hand trowel. Labeled wheat residues, 2.5, 5, and 5 g, were incorporated by hand into the 0- to 5-, 5- to 15-, and 15- to 30-cm depth increments, respectively, for the amended TM treatment cores. The soil from each depth increment was then placed in the core, gently compressed to the same volume, and moved to the study plot (NT management, fallow cover during the year of the incubation). No-till cores received 2.5 g of labeled wheat residues on the surface of the soil within the core. Thus, the NT and TM cores received 2.5 and 12.5 g of the labeled wheat residue, respectively. Residue addition rates were designed to add equal amounts of residue to the surface layer of the NT and TM treatments and similar amounts of residue per unit volume of soil across the depth increments of the TM treatment. Furthermore, more residues were added to the deeper depth increments (5–15 and 15–30 cm) of the TM cores to ensure that the 13C signature would be detected. Accordingly, the focus of this study was to compare relative stabilization of residue-derived C between tillage treatments and, therefore, data were normalized on a per gram of added residue basis. The tops of all the cores were covered with aluminum mesh to minimize losses of residue or contamination from outside sources. Aluminum mesh was also placed over control cores to minimize differences.

Soil Sampling
Two randomly selected pairs of amended and unamended cores for NT and TM were collected from each block at the start of the experiment (July 2004), immediately after residue addition. Initial incubation cores were collected to determine the baseline 13C content of the soil and to measure any contamination effects of adding fresh residues on the different SOC fractions. Final samples were collected (July 2005) after 1 yr of incubation under field conditions at the Sterling, CO, site. Soil cores remained intact and cool during transport to the laboratory and were refrigerated for a maximum of 1 to 2 d before initial preparation. Intact soil cores were weighed and subsamples taken for moisture analysis. No-till incubation cores were then divided into two depth increments (0–5 and 5–15 cm) for lab analysis, while the TM cores had an additional 15- to 30-cm increment.

Field-moist soils were gently passed through an 8-mm sieve, removing large rocks, recognizable surface litter, residues >8 mm, and root material. The remaining soil was then air dried and stored at room temperature. Bulk density was calculated based on the soil mass and core volume after adjusting for the moisture content estimated from drying a subsample at 105°C for 24 h.

Aggregate Soil Organic Matter Fractionation
The physical fractionation (Fig. 1 ) of air-dried, whole soil subsamples was accomplished through wet sieving (Elliott, 1986) and further fractionation using a microaggregate isolator (Six et al., 2000a). For wet sieving, a 100-g subsample was subjected to a slaking period of 5 min in deionized water. Floating material, considered the free light fraction, was aspirated from the surface and transferred to an aluminum pan during the slaking process. Samples were manually wet sieved on 250- and 53-µm sieves with 50 vertical, 3-cm strokes in a 2-min period. Macroaggregates were collected from the 250-µm sieve and free microaggregates from the 53-µm sieve by backwashing the material into aluminum pans. Material <53 µm, which is the silt plus clay fraction, was centrifuged and the pellet transferred to an aluminum pan. All isolated fractions were dried at 60°C in a forced air oven. Mass yield after fractionation was 97.4 ± 1.1% (mean ± standard deviation) (data not shown).


Figure 1
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  		Fig. 1. Fractionation scheme for the isolation of aggregate size classes by wet sieving and macroaggregate disruption (Six et al., 2000a).

 
A 10-g subsample of the macroaggregates was then further fractionated with the microaggregate isolator (Six et al., 2000a). The isolator gently dispersed the macroaggregate subsample using 50 glass beads (4-mm diam.) under a continuous flow of deionized water. Microaggregates and silt- and clay-sized particles were flushed through the 250-µm mesh screen as the macroaggregates were disrupted. Three fractions were collected: (i) >250-µm coarse particulate organic matter (cPOM), (ii) 53- to 250-µm microaggregate-sized fraction within macroaggregates (mM), and (iii) <53-µm silt plus clay within macroaggregates (minM). Mass yield after macroaggregate fractionation was 98.6 ± 2.4% (mean ± standard deviation; data not shown).

Carbon and Isotopic Analyses of Soil Organic Carbon Fractions
Before isotopic analysis, all fractions were pulverized and placed in a HCl fumigation chamber for 30 to 45 min to remove any inorganic C in the form of CaMg(CO3)2 or CaCO3 (Harris et al., 2001). Note that the larger residues and the free light fraction were not analyzed because they are not considered part of the SOM. Organic C content and isotopic analysis for all fractions were measured using a Carlo Erba NA 1500 CN Analyzer (Carlo Erba, Milan, Italy) coupled to a GV Isochrom mass spectrometer (GV Instruments, Manchester, UK). Organic C recovery was 98.2 ± 6.6% (mean ± standard deviation) and 95.7 ± 7.1% for the wet-sieving and microaggregate isolation procedures, respectively (data not shown). Results were expressed as

Formula 1[1]
where (13C/12C)reference is the international Pee Dee Belemnite standard. The proportion of C derived from the wheat residue was calculated according to a simple mixing model (Cerri et al., 1985):

Formula 2[2]
where {delta}13CI is the isotopic ratio of the soil at the beginning of the experiment, {delta}13CS is the isotopic ratio of the soil or aggregate sample after the incubation, and {delta}13CR is the isotopic ratio of the labeled residue. The incorporation value, f, is the proportion of residue-derived C present in the physically isolated fractions or whole soil. Residue-derived C concentrations for the individual fractions are a product of the incorporation value and the C concentration (g C kg–1 soil) for each sample. Total residue-derived C stabilization stocks (on a whole-soil basis) were calculated as a function of f, bulk SOC concentration, depth increment, and soil bulk density.

Statistical Analysis
Data were analyzed using ANOVA with main effects for depth (0–5, 5–15, and 15–30 cm), treatment (TM or NT), and aggregate fraction (SAS Institute, Cary, NC). We specifically compared the initial and final samples to account for differences in the initial conditions between the tilled field and the NT experimental plot. Separate analyses were conducted for the fractions derived from wet sieving (macroaggregates, microaggregates, and silt plus clay) and the fractions isolated from the macroaggregates (cPOM, mM, and minM). Analyses were conducted for residue-derived C stabilization and relative mass distributions of the aggregate size classes. Due to the unbalanced design of depth increments in this experiment, analyses comparing the two tillage treatments included only the first two depths. A separate analysis was conducted evaluating differences across the three depths in the TM treatment. Pairwise comparisons were made using the Bonferroni method with {alpha} = 0.05.


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Soil Structure and Influence of Residue Incorporation
At the onset of the incubation, there were significantly more macroaggregates under NT for the 0- to 5-cm layer compared with the tillage treatment (Fig. 2 ). This result is consistent with previous findings demonstrating that the transient binding agents of macroaggregates are disrupted by tillage-induced disturbance (Elliott, 1986; Besnard et al., 1996; Six et al., 2000b).


Figure 2
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  		Fig. 2. Initial and final aggregate distributions for the first two depths (0–5 and 5–15 cm) of the tillage management (TM) and no-till (NT) treatments at Sterling, CO. The macroaggregate (macros), microaggregate (micros), silt plus clay (S+C), coarse particulate organic matter (cPOM), microaggregates within macroaggregates (mM), and silt plus clay within macroaggregates (minM) fractions are expressed in terms of grams per kilogram whole soil. Error bars represent ± one standard error of the mean. Different letters represent significant differences (Bonferroni's adjustment, {alpha} = 0.05) within each sieving class among tillage treatment and depth increment. Uppercase and lowercase letters correspond to the TM and NT treatments, respectively. * Significant differences between the two tillage treatments within each aggregate class and depth combination.

 
The proportion of macroaggregates did not increase during the incubation under the NT treatment, where aggregate dynamics may have reached steady state during the long-term (20-yr) continuous NT management at the experiment site. The aggregate proportions of the tillage treatment converged with the proportions measured under NT following 1 yr of field incubation without further tillage events (Fig. 2). These findings are similar to an analogous incubation experiment by Coppens et al. (2006), who found that there were no significant differences in the mean weight diameter of soils receiving surface-applied or incorporated (over a 10-cm depth) residues after a 33-wk laboratory incubation with labeled residues.

Though not significant, the macroaggregate and minM fractions showed increases in the tillage treatment (0–5 cm) corresponding with decreases in free microaggregates, which is consistent with the aggregate hierarchy model of Tisdall and Oades (1982). Final aggregate proportions for both TM and NT decreased in the following order for the upper two depths (0–5 and 5–15 cm): microaggregates > silt plus clay ~ macroaggregates. Within the macroaggregates, the proportions decreased from mM ~ cPOM > minM. The binding of primary particles into microaggregates by persistent organic matter, in addition to macroaggregate turnover, could explain why free microaggregates (53–250 µm) dominate the aggregate distributions in each treatment and depth increment because microaggregates are inherently more stable than macroaggregates. Few significant differences were measured among the macroaggregate-derived fractions within a tillage treatment and also for comparisons between tillage treatments (Fig. 2).

Differences in aggregate class distributions after 1 yr of field incubation were measured when comparing across the three depth increments of the TM treatment. Significant statistical differences occurred among fractions (within a given depth increment) and among depth increments for a given fraction (Fig. 3 ). Microaggregates consistently dominated the aggregate distributions, as observed in other experiments with similar slaking treatments and textures that varied from sandy loam to silty clay loam (Cambardella and Elliott, 1993; Jastrow et al., 1996; Six et al., 2000b). In the deepest increment (15–30 cm) of the tillage treatment, there were significantly more macroaggregates and fewer microaggregates compared with the upper two increments (Fig. 3). Presumably, the reduction of free microaggregates was a result of new macroaggregate formation, as confirmed by the significantly greater proportion of mM and minM in the 15- to 30-cm increment. Across depth increments, the proportions of mM and minM within macroaggregates increased while the proportion of cPOM decreased. This suggests that new macroaggregates were formed in the deepest increment from the binding of microaggregates with transient organic matter from decomposition processes of cPOM.


Figure 3
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  		Fig. 3. Final aggregate distributions for the tillage management (TM) treatment for each depth increment (0–5, 5–15, and 15–30 cm). The macroaggregate (macros), microaggregate (micros), silt plus clay (S+C), coarse particulate organic matter (cPOM), microaggregates within macroaggregates (mM), and silt plus clay within macroaggregates (minM) fractions are expressed in terms of the whole soil. Error bars represent ± one standard error of the mean. Aggregate proportions followed by a different uppercase letter within the depth increment or isolation method are significantly different; aggregate proportions followed by a different lowercase letter within an aggregate class are significantly different (Bonferroni's adjustment, {alpha} = 0.05).

 
We also evaluated the direct effects of residue addition on aggregate dynamics relative to the unamended cores. These comparisons revealed no significant (P > 0.05) differences among aggregate distributions between the control and amended cores for either tillage treatment (data not shown). This suggests that binding agents from the decomposition of incorporated residues are not an important mechanism for macroaggregate formation in the semiarid environment of our field incubation during an annual cycle. Prior research has suggested, however, that the decomposition of organic matter provides binding agents and a nucleus for aggregate formation (Oades, 1984; Beare et al., 1994b). In shorter time frames, buried residues may induce an initial increase in microbial decomposition, increasing the formation of transient binding agents, countering the disruption of aggregates by tillage through the formation of new macroaggregates (Bossuyt et al., 2002). The aggregate distributions in our study were analyzed during a much longer period, spanning 1 yr, where an initial increase in macroaggregate formation could not have been detected. Our incubation took place in situ at a semiarid dryland agriculture system, whereas incubations are typically conducted under laboratory conditions. The presumed lower moisture content of our soil cores, compared with laboratory conditions, during the incubation period probably hindered the decomposition of residues and macroaggregate formation, as described above.

Residue Carbon Stabilization Related to Tillage Disturbance
A significantly greater amount of residue-derived C (10.2 ± 2.2 g residue C kg–1 soil kg–1 residue C added, P < 0.05) was stabilized in the macroaggregate fraction of the NT treatment in the 0- to 5-cm increment (Fig. 4A ). The difference in residue C stabilization in the 0- to 5-cm increment was due in part to the significantly more residue C in the cPOM fraction (P < 0.05, Fig. 4A) of the NT treatment. Additionally, fresh residues have been shown to be preferentially incorporated into larger aggregates under NT management (Cambardella and Elliott, 1993; Jastrow et al., 1996), similar to our results for the NT treatment. This result confirms the critical physical protection afforded to cPOM by macroaggregates and the differences between tillage regimes on the stabilization of SOM.


Figure 4
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  		Fig. 4. Residue-derived C stabilization for the 0- to 5- and 0- to 15-cm depth increments of the tillage management (TM) and no-till (NT) treatments at Sterling, CO. The macroaggregate (macros), microaggregate (micros), silt plus clay (S+C), coarse particulate organic matter (cPOM), microaggregates within macroaggregates (mM), and silt plus clay within macroaggregates (minM) fractions are all expressed in terms of grams residue-derived C per kilogram soil per kilogram residue-derived C added. Error bars represent ± one standard error of the mean. Different letters represent significant differences (Bonferroni's adjustment, {alpha} = 0.05) among each aggregate class within a sieving method and tillage treatment for each depth. Uppercase and lowercase letters correspond to the TM and NT treatments, respectively. * Significant difference between TM and NT within each aggregate class.

 
Not surprisingly, significantly (P < 0.05, data not shown) more C was stabilized in the aggregate fractions of the TM treatment in the 5- to 15-cm layer because litter was directly added into this layer as part of the experimental design. In contrast, litter was only placed on the surface of the NT treatment. Gale et al. (2000) also reported that surface-applied residues play a minor role in macroaggregate formation deeper in the soil profile under NT management.

Residue-derived C stabilization in the 0- to 15-cm increment (Fig. 4B) was similar to stabilization in the 0- to 5-cm increment (Fig. 4A). More C was stabilized in a majority of the aggregate fractions under NT, with significant differences in the macroaggregate and cPOM fractions, suggesting a faster stabilization rate and greater residue stabilization efficiency under NT. It is important to note that the differences were not significant for all of the individual aggregate classes (Fig. 4B), but stabilization was significantly greater at the whole-soil level for the NT treatment (Table 1 ). This finding is similar to the results of Six et al. (1999), who reported that although similar concentrations of crop-derived C were initially stabilized within the macroaggregate fraction under NT and conventional tillage (i.e., TM), there were significant differences in the total amount of C stabilized under each tillage treatment.


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  		Table 1. Stabilized residue-derived C stocks and subsequent C stabilization efficiency for the two treatments and depth increments.

 
Therefore, as originally proposed by Six et al. (1999), Fig. 4B demonstrates that C associated with microaggregates (both free and intraaggregate) is subject to physical protection through inclusion in macroaggregates and that the mechanisms of C stabilization in microaggregates are similar under the NT and TM treatments.

Residue Stabilization across Depths in the Tillage Management Treatment
During tillage, surface residues are incorporated throughout the tillage profile, and consequently tillage depth substantially influences their distribution and concentration (Allmaras et al., 1996; Etana et al., 1999). Thus, the amount of C stabilized with aggregate classes was strongly influenced by the initial amount of residue added in the TM treatment. Significantly more residue-derived C (data not shown, P < 0.05) was stabilized in the 15- to 30-cm increment of the TM treatment where more residues were added. We must also recognize that there was the potential for leaching of dissolved organic C into the deeper increments during the course of the field incubation, which would complicate our results. It seems unlikely, however, that a significant amount of C would be transported deeper in the profile through leaching in the semiarid dryland agricultural systems of eastern Colorado.

After normalization for the initial amount of residue added, there were no significant differences within any aggregate class between the three depth increments (Fig. 5 ). More total C was stabilized in the 0- to 30-cm depth, however, than the 0- to 15-cm depth on a whole-soil basis even after normalization (Table 1). The results of our aggregate distributions (data not shown) suggest that fresh residues may not be as important in aggregate dynamics in our semiarid dryland agroecosystem site; however, there may have been less disruption of the macroaggregates at the deeper depth from nontillage events such as dry–wet and freeze–thaw cycles. As shown above, a higher proportion of macroaggregates occurred below the typical plow depth (12–15 cm), suggesting greater potential for stabilization of C by physical protection provided by macroaggregates (Fig. 3). Decomposition could also be limited by soil moisture and decreased temperature deeper in the profile, which could also reduce the loss of SOM deeper in the profile.


Figure 5
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  		Fig. 5. Residue-derived C stabilization across the three depths (0–5, 5–15, and 15–30 cm) of the tillage management (TM) treatment at Sterling, CO. The macroaggregate (macros), microaggregate (micros), silt plus clay (S+C), coarse particulate organic matter (cPOM), microaggregates within macroaggregates (mM), and silt plus clay within macroaggregates (minM) fractions are expressed in terms of grams residue-derived C per kilogram soil per kilogram residue-derived C added. Error bars represent ± one standard error of the mean. Different letters represent significant differences (Bonferroni's adjustment, {alpha} = 0.05) between the three depths within each aggregate class. For example, there was not a comparison made between macroaggregates and microaggregates for any depth. Uppercase and lowercase letters correspond to the wet sieving and macroaggregate-derived fractions, respectively.

 
Efficiency of Residue Carbon Stabilization
Efficiency of residue-derived C stabilization is defined as the percentage of residue-derived C that is associated with soil aggregate fractions at the end of a given time frame (e.g., 1 yr in our study). Accordingly, residue-derived C stabilization was significantly (P < 0.05) more efficient under NT (14.5 ± 1.6%) than TM (10.2 ± 0.8%) management in the 0- to 15-cm depth, but efficiency did not differ significantly compared with TM in the 0- to 30-cm depth (Table 1). Thus, the efficiency of C stabilization in the tillage treatment increased by distributing residues deeper in the profile to a 30-cm depth compared with the typical plow increment (12–15 cm). When residues were distributed throughout a 30-cm depth, they were exposed to a greater volume of soil and had more potential for stabilization in soil aggregates. The simulated tillage event over 30 cm appeared to cover a depth where the C stabilization efficiency was roughly similar for the NT (14.5 ± 1.6%) and TM (12.2 ± 0.5%) treatments.


   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Under the proposed aggregate turnover model of Six et al. (1999), the addition of residues into the soil matrix plays an important role in new aggregate formation. Our findings from a single-year incubation in a semiarid dryland agriculture setting indicate that residue additions did not foster greater macroaggregate formation. The greatest stabilization of residue-derived C occurred, however, in the macroaggregate fraction (>250 µm) under NT management in the 0- to 5-cm depth increment (Fig. 4A), confirming the model presented by Six et al. (1999) on macroaggregate dynamics and the physical protection provided by soil aggregates under NT management. Accordingly, cPOM is more sensitive to tillage management that can disrupt macroaggregates and expose the physically protected cPOM to the soil environment and decomposition. Thus, limiting the disruption of aggregates by tillage down to 15 cm has a stronger influence than residue addition on aggregate formation and the physical protection of SOM during an annual cycle.

Alternatively, when residues were incorporated into the soil to a 30-cm depth, the relative efficiency of residue-derived C stabilization of the NT treatment (0–15 cm) was not significantly different from the tilled soil at the Sterling site. This comparison suggests that if fresh residues are incorporated below a tillage depth of 15 cm through annual tillage in a semiarid, dryland agriculture setting, the negative impact of aggregate disruption through tillage may be offset by slower decomposition of residues deeper in the profile, possibly from decreased temperature and moisture availability that may limit decomposition. Thus, an annual deep tillage event followed by no additional tillage passes could potentially serve as a strategy to sequester C during a 1-yr period in semiarid dryland cropping systems. In addition, the long-term dynamics of residue-derived C stabilization below the typical plow depth may have the potential for increased soil C sequestration beyond an annual cycle, which requires further investigation. No-till also has other benefits, however, such as decreased soil erosion and improved soil water conservation, which should also be considered when making a management decision.


   ACKNOWLEDGMENTS
 
We are grateful for the laboratory assistance provided by Dan Reuss, Karolien Denef, Catherine Stewart, Nikhil Shelke, Scott Greene, and Elena Yakimenko. We also wish to thank Dr. Gary Peterson for access to the plots at the Sustainable Dryland Agroecosystem Management Project located in Sterling, CO. This research was supported by the USDA/CSREES (Agreement no. 2001-38700-11092) through funding from the Consortium for Agricultural Soils Mitigation of Greenhouse Gases (CASMGS). Additional support provided by the USDOE under Grant DE-FG02-04ER63912 is gratefully acknowledged.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
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Received for publication December 1, 2006.


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




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