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Soil Science Society of America Journal 66:1965-1973 (2002)
© 2002 Soil Science Society of America

DIVISION S-6—SOIL & WATER MANAGEMENT & CONSERVATION

Aggregate-Protected Carbon in No-tillage and Conventional Tillage Agroecosystems Using Carbon-14 Labeled Plant Residue

Heleen Bossuyt*,a, Johan Sixb and Paul F. Hendrixa

a Institute of Ecology, Univ. of Georgia, Athens, GA, 30602
b Dep. of Agronomy and Range Science, Univ. of California, Davis, CA, 95616

* Corresponding author (hbossuyt{at}arches.uga.edu)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 METHODOLOGICAL ISSUES
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
No-tillage (NT) management can result in higher soil organic matter (SOM) levels than conventional tillage (CT) practices. The objective was to investigate the underlying mechanisms in which C is protected under NT management, using 14C-labeled plant residue as a tracer. Samples were collected from the Horseshoe Bend Research area in Athens, GA. Aggregate-size distribution, total C, and 14C were measured together with different pools of aggregate-associated C and 14C from 21-d laboratory incubations of intact and crushed macro and microaggregates. Compared with CT, NT practices resulted in higher total C and 14C in all aggregate-size classes of the 0- to 2.5- and 2.5- to 5-cm layers, except for 14C in the <53- and 250- to 2000-µm aggregate-size classes at the 2.5- to 5-cm layer. At the 5- to 15-cm depth, more 14C was found in the >2000-µm aggregate-size class under NT than CT. In contrast, more 14C was found in the 53- to 250-µm and <53-µm size classes under CT than NT. Unprotected C and 14C pools, microaggregate-protected and micro within macroaggregate-protected C and 14C pools were significantly higher for the 0- to 2.5- and 2.5- to 5-cm layers under NT than CT. Carbon-14 pools were generally higher in CT than in NT at the 5- to 15-cm depth, while total C did not differ between tillage treatments at this depth. The results indicate that (i) more young C (14C) is accumulated in the subsurface soil of CT than NT, but this C is not stabilized in the long term, and (ii) short- and long-term stabilization of C is higher in the soil surface layers under NT compared with CT. This C stabilization occurs mainly at the microaggregate level.

Abbreviations: CT, conventional tillage • NT, no-tillage • SOM, soil organic matter • WSA, water-stable aggregates


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 METHODOLOGICAL ISSUES
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
MAINTENANCE OF SOM is desirable in agroecosystems because of its beneficial effects on nutrient dynamics and soil structure. Soil organic matter improves soil structural stability, reduces surface crusting and compaction, and increases infiltration, percolation, and water holding capacity of the soil (Tisdale et al., 1993; Stevenson, 1994). The high surface area and chemical nature of SOM buffer pH changes and increase cation-exchange capacity (Stevenson, 1994; Sparks, 1995). Sequestration of C in soil is also becoming increasingly important because of the concern over global warming and rising levels of atmospheric CO2. Soil is estimated to be the largest terrestrial pool of C, containing 1500 Pg or twice as much as the atmosphere (Sundquist, 1993; Schlesinger, 1997).

Management practices, such as NT and reduced tillage, that promote maintenance and accumulation of soil C, are increasingly adopted by farmers because of the growing interest in conservation of SOM (Kern and Johnson, 1993; Burke et al., 1995). No-tillage management can increase C storage in soils, as well as improve physical, chemical, and biological soil characteristics (Paustian et al., 1997; Hendrix et al., 1998). Soil organic matter may be protected from microbial attack by adsorption to clay minerals (Oades, 1984; Ladd et al., 1985), by the formation of microaggregates (Edwards and Brenner, 1967; Gregorich et al., 1989; Besnard et al., 1996; Six et al., 2000b), by isolation in soil micropores (Adu and Oades, 1978; Foster, 1981), and by physical protection within stable macroaggregates (Elliott, 1986; Gupta and Germida, 1988).

Improvements of soil structure, determined by the level of aggregation, are thought to play an important role in reducing decompositional losses of SOM under NT. Soil aggregates are disrupted by tillage practices and this may lead to an enhanced aggregate turnover and increased decomposition of SOM (Six et al., 1998). The highly dispersible, kaolinitic clay-based soils of the southeastern USA are very susceptible to aggregate disruption, surface crusting, reduced infiltration, and erosion (Miller and Baharuddin, 1986). These processes can contribute to rapid losses of SOM and a decrease in productivity of agricultural soils in this region (Sanchez et al., 1989; Bruce et al., 1990).

Tisdall and Oades (1982) proposed a conceptual model of soil structure describing the binding of mineral particles into microaggregates (50–250 µm) and of microaggregates into macroaggregates (>250 µm). Free primary particles are mainly cemented together into microaggregates by persistent binding agents (e.g., clay-polyvalent metal-humified organic matter complexes), characterized as older, more humified, or recalcitrant SOM. The more temporary (e.g., roots and fungal hyphae) and transient (e.g., polysaccharides) agents are thought to bind microaggregates into macroaggregates and are generally considered to be relatively more labile or decomposable (van Veen and Paul, 1981; Elliot, 1986). Because of the nature of the binding agents involved, macroaggregates are less stable than microaggregates (Oades, 1984; Beare et al., 1994b) and consequently more susceptible to the disruptive forces induced by cultivation (Tisdall and Oades, 1980, 1982).

The conceptual model of Tisdall and Oades (1982) was later modified by Oades (1984) who suggested that microaggregates are predominantly formed within macroaggregates. This modification has formed the basis of several recent studies (Beare et al., 1994a; Gale et al., 2000; Six et al., 1999, 2000b). First, macroaggregates (250–2000 µm) are formed around fresh residue. If the macroaggregates are not disturbed (e.g., under NT), residue decomposes and fragments into finer organic matter that gradually becomes encrusted with clay particles and microbial products forming microaggregates within macroaggregates. When the macroaggregates become destabilized because of degradation of the binding agents, they fall apart and release the stable microaggregates. These microaggregates then form the building blocks for the formation of new macroaggregates, as suggested by Tisdall and Oades (1982).

Because of the relatively high background levels of soil C, it is usually not possible to detect changes in total C resulting from management practices in the short term (3 yr). In this study, we used 14C-labeled plant residue to trace C transformations in soils under CT vs. NT management. It allowed us to follow a one-time organic input of 14C into different soil fractions and layers of the soil profile.

While several studies have investigated the influence of management practices on SOM dynamics and C sequestration, the exact fate of newly added C and the level at which C is protected in NT soils compared with CT soils is still unclear. The objective of this study was to investigate the underlying mechanisms involved in incorporation and protection of C after plant residue application under different management practices. Our main hypothesis is based on the aggregate-turnover model described by Six et al. (2000b). Based on this model (described above), we hypothesized that microaggregates within macroaggregates may be important in physically protecting C. Since NT soils contain more stable microaggregates within macroaggregates, we hypothesized that more C would be protected in NT soils than in CT soils. In this study, we looked at the effect of tillage practices on (i) aggregate-size distribution; (ii) total and young (14C) C concentrations and; (iii) decomposition and stabilization of aggregate-associated C fractions 3 yr after application of 14C-labeled crop residue.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 METHODOLOGICAL ISSUES
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Site Description and Soils
Soils were sampled from a set of 16-yr-old CT [moldboard plowed (15-cm depth), disked (10-cm depth) and rotary-tilled (10-cm depth)] and NT (direct-drilled) plots at the Horseshoe Bend Research Area near Athens, GA. The site is located in the Piedmont of the southern Appalachian Mountains (33° 54'N lat., 83° 24'W long.). The soil is a Hiwassee series fine loamy, siliceous, thermic, Rhodic Kanhapludult (66% sand, 13% silt, 21% clay). Total C and 14C contents of the soils (measured in September 1999) are given in Table 1. Prior to tillage treatments, the plots had been in grass or forage since at least 1938 (Hendrix, 1997). Corn (Zea mays L.) or sorghum [Sorghum bicolor (L.) Moench] was no-till planted with an Allis Chalmers seed drill (Allis Chalmers, Milwaukee, WI) as a summer crop, and rye (Secale cereale L.) or crimson clover (Trifolium incarnatum L.) was broadcast as a winter cover crop. In November 1996, 20-cm wide acrylic sheets were inserted 15 cm deep to create 1 by 2 m plots within the larger plots. Carbon-14 (144 Mbq m-2) labeled, dry, corn leaf and stem material were incorporated and the soil turned and pulverized with a shovel to a depth of 15 cm in the CT plots and left on the surface of the NT plots without soil disturbance. General C data from adjacent long-term NT and CT plots suggest that tilling with a shovel was a good simulation of the CT treatment. A complete description of the labeling technique has been reported by Kisselle et al. (1999). Total C addition averaged 346 g m-2 assuming 45% C in the residue. Cropping and nonlabeled residue additions continued as in the larger plots. Crop biomass was mowed and incorporated in the CT plots and left on the surface of NT plots (summer and winter). No crop biomass (including grain and grain heads) was removed. Soil samples were taken from the three replicate NT and CT plots on 29 Sept. 1999. No residues were removed prior to soil sampling. Six replicate soil cores were collected randomly from each plot with a 6-cm diam. hammer-driven corer, with a plastic insert, to a depth of 15 cm. The intact samples were sectioned into 0- to 2.5-, 2.5- to 5-, and 5- to 15-cm depth increments and bulked by depth within plots. All soil samples were sieved (10 mm) in field moist state prior to wet sieving by gently breaking apart the soil along natural planes of weakness.


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  		Table 1. Total C and 14C contents of conventional tillage (CT) and no-tillage (NT) soils at the Horseshoe Bend Experimental Area.

 
Aggregate-Size Distributions
Aggregate-size distribution of field moist soils was determined by wet sieving, as described by Elliott (1986). A series of three sieves was used to obtain four aggregate-size fractions: (i) >2000 µm (large macroaggregates); (ii) 250 to 2000 µm (small macroaggregates); (iii) 53 to 250 µm (microaggregates); (iv) <53 µm (silt and clay fraction). Following wet sieving, the aggregate-size classes >53 µm were dried on the sieves in a dehumidifying chamber (10°C). Particles <53 µm were collected in a bucket, total volume was measured, and a subsample of a known volume was taken for analysis. Subsamples from each aggregate size class were ground to fine powder consistency with a 8000 SPEX CertiPrep Mixer/Mill (Spex CertiPrep Inc., Metuchen, NJ) and analyzed for total C and 14C content.

Carbon Analyses
Total organic C content of aggregate-size classes was measured on a Carlo Erba, NA 1500, CHN Combustion Analyzer (Carlo Erba, Milan, Italy). Carbon-14 was measured using a Harvey Oxidizer (Harvey OX500, R.J. Harvey Instruments Co., Hillsdale, NJ) and a Beckman LS 3801 (Beckman Instruments, Fullerton, CA) liquid scintillation counter. Carbon contents were expressed as amount of C in the aggregate-size class per kilogram of sandfree aggregate. Because of the difference in amount of sand between aggregate-size classes, Elliott et al. (1991) suggested that comparisons of C content across aggregate-size classes should be based on sand-corrected C data. The sand content of the aggregate-size class was determined by dispersing 5 g of the fraction in 20 mL of sodium hexametaphosphate and sieving through a 53-µm sieve. Sandfree C concentration was calculated as follows:

Incubations
Five sets of incubations were conducted for each cultivation treatment and for each depth: (i) macroaggregates (large and small mixed); (ii) crushed macroaggregates (large and small mixed) that is, ground until <250 µm; (iii) crushed macroaggregates (large and small mixed) that is, ground until <53 µm; (iv) microaggregates and; (v) crushed microaggregates that is, ground until <53 µm. Soil samples (25–30 g) to be incubated were weighed into plastic cups and deionized water was added to achieve 55% water-filled porosity. Plastic cups were incubated (30°C) in sealed jars containing alkali CO2 traps (1 M NaOH). The CO2 traps were changed on Days 3, 7, 14, and 21 and the respired C was measured by titration with standardized 0.25 M HCl after precipitation of carbonates with BaCl2. The respired 14C was measured by adding 0.5 mL NaOH from base traps to 10 mL EcoLite scintillation fluor (ICN Biomedicals, Costa Mesa, CA), maintaining samples in the dark for 7 d, and counting for 20 min on a Beckman LS 3801 liquid scintillation counter.

Calculations
The results of the aggregate incubations were used to define five aggregate-associated C pools: (i) unprotected-macroaggregate C, (ii) unprotected-microaggregate C, (iii) macroaggregate-protected C, (iv) microaggregate-protected C and; (v) micro within macroaggregate-protected C. These different pools were calculated as follows (pools were expressed in grams of C per kilogram sandfree aggregate and kilobecquerels of 14C per kilogram sandfree aggregate): Unprotected macroaggregate C = intact macroaggregate Cmin

Unprotected microaggregate C = intact microaggregate Cmin

Macroaggregate-protected C = <250-µm crushed macroaggregate Cmin - intact macroaggregate Cmin

Microaggregate-protected C = <53-µm crushed microaggregate Cmin - intact Microaggregate Cmin

Micro within macroaggregate-protected C = <53-µm crushed macroaggregate Cmin - macroaggregate-protected C - Unprotected macroaggregate C

[replace macroaggregate-protected C (see Eq. 3)]

= <53 µm crushed macroaggregate Cmin - (<250 µm crushed macroaggregate Cmin - intact macroaggregate Cmin) - intact macroaggregate Cmin

= <53 µm crushed macroaggregate Cmin - <250 µm crushed macroaggregate Cmin with Cmin the cumulative C mineralized after 21 d from the intact and crushed aggregate treatments.

Statistical Analysis
The data were analyzed, using the SAS statistical package for analysis of variance (ANOVA-PROC MIXED, SAS Institute, 1990). Tillage treatments and depths were considered fixed effects while replicate was considered a random effect (n = 3). Separation of means was tested with the DIFF option of the LSMEANS statement with a significance level of P < 0.05.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
METHODOLOGICAL ISSUES
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Water-Stable Aggregates
There was a significant impact of tillage practices on the distribution of water-stable aggregates (WSA) in the surface samples (0–2.5 and 2.5–5 cm) (Fig. 1) . Large macroaggregates (>2000 µm) made up the largest percentage (~50% on average) of the whole soil for NT samples and were on average 2.4 times greater than in CT samples. The smaller aggregate-size classes (250–2000, 53–250, and <53 µm) made up a greater proportion of the surface soil in CT than in NT. For the lowest depth (5–15 cm), only the <53-µm size class was significantly different between CT and NT and was higher in CT than in NT.



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  		Fig. 1. Aggregate-size distribution. Values followed by a different lowercase letter within aggregate-size class within depth between tillage treatments, are significantly different.

 
Carbon Concentrations
Total aggregate-associated C concentrations were significantly influenced by tillage, aggregate-size class, depth, and their interactions. In the surface layers (0–2.5 and 2.5–5 cm), total aggregate-associated C concentrations were significantly higher in NT than in CT for all aggregate-size classes (Fig. 2) . There were no significant differences between tillage treatments for total aggregate-associated C concentrations at the 5- to 15-cm depth. For all the NT aggregate size classes, the total aggregate-associated C content was higher in the surface layers than in the 5- to 15-cm layer. There was no significant effect of depth on the total aggregate-associated C concentrations in CT, except for the <53-µm size class where total aggregate-associated C content was higher in the 2.5- to 5-cm layer than in the other layers. In the surface layers of NT, total aggregate-associated C concentrations were significantly higher in the 53- to 250-µm aggregate-size class than in the other classes. The difference in total aggregate-associated C content between the different aggregate-size classes was much larger in the NT surface samples than in the CT and the lowest depth of NT.



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  		Fig. 2. Total aggregate-associated C concentrations. Values followed by a different lowercase letter within aggregate-size class within depth between tillage treatments, are significantly different. Values followed by a different uppercase letter within aggregate-size class within tillage treatments among depths are significantly different.

 
Unprotected and Aggregate-Protected Carbon Pools
The amount of C mineralized after 21 d is expressed in Table 2 as cumulative respired C (g C kg-1 sandfree aggregate). There were significant differences between CT and NT in the surface layers for the unprotected, microaggregate-protected, and micro within macroaggregate-protected C pools. The unprotected macroaggregate C pool was higher in NT than in CT for the 0- to 2.5-cm layer (1.5 times) and for the 2.5- to 5-cm layer (2.1 times). The unprotected microaggregate C pool was also higher in NT than in CT for the 0- to 2.5-cm layer (2.6 times) and for the 2.5- to 5-cm layer (2.7 times). Macroaggregate-protected C was not significantly different between CT and NT. The microaggregate-protected C pool was 2.9 times greater in NT than in CT in the 0- to 2.5-cm layer and 2.5 times in the 2.5- to 5-cm layer. The micro within macroaggregate-protected C pool was 4.5 times higher in NT than in CT in the 0- to 2.5-cm layer. Tillage practices did not have any effect on the C pools at the 5- to 15-cm layer. Within tillage treatments, there was a significant effect of depth. Within NT, all C pools were highest in the 0 to 2.5 cm and lowest in the 5 to 15 cm. Within CT, the unprotected and macroaggregate- protected C pools were higher in the 0- to 2.5-cm layer than in the 5- to 15-cm layer, while the microaggregate-protected and micro within macroaggregate-protected C pools were not significantly different across depths in CT.


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  		Table 2. Aggregate-associated C and 14C pools of conventional tillage (CT) and no-tillage (NT) soils at the Horseshoe Bend Experiment Area.

 
Carbon-14 Concentrations
Aggregate-associated 14C concentrations were significantly influenced by tillage practice, aggregate-size class and their interactions (Fig. 3) . Aggregate-associated 14C concentrations were significantly higher for all aggregate-size classes in NT than in CT in the 0- to 2.5-cm layer. For the 2.5- to 5-cm layer, only the aggregate-associated 14C in the large macroaggregate-size class (>2000 µm) and the microaggregate-size class (53–250 µm) were higher for NT than for CT. The other aggregate-size classes were not significantly influenced by tillage. At the 5- to 15-cm depth, the 53- to 250- and <53-µm aggregate-size class had a higher aggregate-associated 14C content in CT than in NT whereas the >2000-µm aggregate-size class had a higher aggregate-associated 14C content in NT than in CT. The aggregate-associated 14C content of all the aggregate-size classes was significantly lower in the 5- to 15-cm layer than in the 0- to 2.5-cm layer of NT. There was no significant effect of depth on the aggregate-associated 14C concentrations in the CT, except for the >2000-µm aggregate-size class where the aggregate-associated 14C content was lower in the 5- to 15-cm layer than in the other layers. In the surface layers of NT, aggregate-associated 14C concentrations were significantly higher in the 53- to 250-µm aggregate-size class than in the other classes. In the 5- to 15-cm layer of NT, there were no significant differences between aggregate-size classes. In CT, the differences in aggregate-associated 14C concentrations between the aggregate-size classes were much smaller than in NT surface samples. In this experiment, we did not make a total C mass balance. A total recovery of 14C is described in Kisselle et al. (2001).



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  		Fig. 3. Aggregate-associated 14C concentrations. Values followed by a different lowercase letter within aggregate-size class within depth between tillage treatments, are significantly different. Values followed by a different uppercase letter within aggregate-size class within tillage treatments among depths are significantly different.

 
Unprotected and Aggregate-Protected Carbon-14 Pools
The amount of 14C mineralized after 21 d is expressed in Table 2 as cumulative respired 14C (g 14C kg-1 sandfree aggregate). The unprotected, microaggregate-protected and micro within macroaggregate-protected 14C pools were significantly influenced by tillage practices, depth, and their interactions. The unprotected macroaggregate 14C pools were on average 5.8 times higher in NT than in CT in the 0- to 2.5- and 2.5- to 5-cm layers but did not differ between CT and NT in the 5- to 15-cm layer. The unprotected microaggregate 14C pool was 8 times higher in NT than in CT in the 2.5- to 5-cm layer. The macroaggregate-protected 14C pools were not significantly different between CT and NT in the surface layers. The microaggregate-protected 14C pool was 2.6 times higher in NT than in CT in the 0- to 2.5-cm layer, but no significant differences were found for the 2.5- to 5-cm layer. The micro within macroaggregate-protected 14C pool was 2.7 times higher in NT than in CT for the 0- to 2.5-cm layer. In the 5- to 15-cm layer, the macroaggregate-protected 14C pool and microaggregate-protected 14C pool were respectively 6.7 and 8.5 times higher in CT than in NT. Within tillage treatments, there were some significant depth effects. Within NT, the unprotected 14C pool was highest in the 2.5- to 5-cm layer and lowest in the 5- to 15-cm layer for both macroaggregates and microaggregates. Within CT, the unprotected macroaggregate 14C pool was significantly lower at the 0- to 2.5-cm layer than the other layers. The macroaggregate-protected and micro within macroaggregate-protected 14C pools did not differ significantly across depth in both CT and NT. The microaggregate-protected 14C pool was significantly higher in the 0- to 2.5-cm layer than in the two other depths in NT and no significant differences were detected across depths in CT for this pool.

Portion of Carbon Mineralized
Table 3 shows the amount of C mineralized as a percentage of the aggregate-associated C present before the incubation. There were only a few significant differences between CT and NT and these showed a higher percentage of C mineralized in CT samples than in NT samples. Overall, the amount of protected C mineralized is a rather low percentage of the C present. The amount of macroaggregate-protected C mineralized varied between 0.6 and 3.2%, the amount of microaggregate-protected C mineralized varied between 2.7 and 11% and the amount of micro within macroaggregate-protected C mineralized varied between 1.5 and 5.5%.


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  		Table 3. Portions of aggregate-associated C mineralized of conventional tillage (CT) and no-tillage (NT) soils at the Horseshoe Bend Experimental Area.

 

   METHODOLOGICAL ISSUES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 METHODOLOGICAL ISSUES
DISCUSSION
 CONCLUSIONS
 REFERENCES
 
It should be noted that crushing of aggregates also breaks up the plant material present within the aggregates. Since it has been suggested that C mineralization increases with decreasing residue particle size, it is possible that only part of the C mineralized is C previously protected physically in the aggregates, and the other part is due to the grinding of the plant material. We do not think that this is an important factor in our experiment. In studies with legume residues, less CO2 was evolved from soils amended with finely ground than with coarser material (Stickler and Frederick, 1959; Jensen, 1994). Sörensen et al. (1996) found no significant effect of grinding of the wheat (Triticum aestivum L.) leaf material on the release of 14CO2, but found a lower decomposition in soils where ground, compared with unground, subclover leaves were added. They suggested that grinding of plant residues might result in a greater contact of substrate and decomposer organisms with the soil matrix, resulting in a less extensive decomposition. Sims and Frederick (1970) found that particle size had the greatest effect on CO2 evolution during the first few days of incubation; after 16 d, soils amended with plant material in the size range of 0.25 to 2.37 mm evolved more CO2 than did the soils with 4.8-mm size plant materials. The soils with plant material <250 µm, however, evolved the least CO2. Bremer et al. (1991) and Angers and Recous (1997) only found a higher C mineralization after 20 d of incubation in soils with ground plant residues when the C/N ratio of the plant material (straw) was high (C/N = 270). When rye residue was added (C/N = 9), the decomposition was higher when residues were larger after 2 d of incubation. Vestergaard et al. (2001) investigated the influence of the size of maize leaves (Zea mays L.) on respiration and they found on average higher respiration rates in soils amended with larger pieces (4–5 mm). Comparing these different studies with our experiment, where soil C/N ~ 12, where aggregates were crushed <250 or <53 µm, and where 14C containing residue had been applied to the soil 3 yr previously, we believe that the C mineralized in the crushed aggregates was due to the breakdown of the aggregates and not to the grinding of the plant material.


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 METHODOLOGICAL ISSUES
 DISCUSSION
CONCLUSIONS
 REFERENCES
 
Results of this study indicate that there are significant differences in aggregate-size distribution and aggregate-associated C fractions between NT and CT soils. No-till soils have more large macroaggregates and more C in most aggregate-size classes in the surface layers. At the lower depth, however, there are no differences between NT and CT soils for the aggregate-size distribution and the total aggregate-associated C content. Several researchers found more macroaggregates in NT soils compared with CT soils (Carter, 1992; Beare et al., 1994a; Paustian et al., 1997; Six et al., 2000a). Macroaggregates are less stable than microaggregates (Elliott, 1986; Cambardella and Elliott, 1993), and therefore more susceptible to the disruptive forces of tillage. In the surface layer of CT soil, new soil is continuously exposed to drying and wetting, which causes the breakdown of larger, less stable aggregates. When these aggregates are disrupted, more organic substrates become available for microbial attack, resulting in an increase in SOM decomposition, and therefore a decrease in C content. Since residues are placed on the surface in NT soils and aggregates are less disturbed, higher C concentrations are found in these surface layers. Beare et al. (1994a), Dick et al. (1997) and Six et al. (1999) also found higher C concentrations in surface samples of NT soils than of CT soils. Similarly to our results, Beare et al. (1994a) and Six et al. (1998) found no significant differences in aggregate-size distribution or C content between CT and NT in deeper soil layers. Since the residues are buried in CT soils, there might be a flush of microbial activity at a lower depth, which causes the formation of aggregate binding agents, countering the disruption of large macroaggregates by plowing.

The 14C concentrations followed the same pattern as the total C for the surface layers in both CT and NT soils with generally more 14C in the NT aggregate-size classes than in CT. At the 5- to 15-cm depth, however, the microaggregates and clay and silt particles contain more 14C in the CT soils than in the NT soils. This indicates that in the short term, more new C is incorporated in the lower depths in CT soils because of plowing, but this C is not stabilized in the long term since there are no significant differences in total C content between CT and NT at this depth. Within tillage treatment, the differences between different depths were greater in NT samples than in CT samples. Balesdent et al. (1990) found more than 50% of new C in the first 4 cm of NT soils and only 20% below 25 cm, while the new C was homogeneously spread in the ploughed layers of the tilled soils.

Generally, there was more C in the microaggregate fraction in NT soils than in any other fraction. In this experiment, we worked with field-moist aggregates that are not slake-resistant. Consequently, the larger aggregates consist mainly of a loose structure that is very easily disrupted by tillage and that contains less C than the smaller fractions. Since these structures are disrupted under CT, the difference in C content between aggregate-size classes is much smaller than under NT. Accordingly, Puget et al. (1995) found that intermediate fractions (0.5–0.05 mm) contained most C when the aggregates were dry-sieved (not slake-resistant).

Unprotected C and 14C pools were higher in NT than in CT for the 0- to 2.5-cm and 2.5- to 5-cm layer. This demonstrates that, when aggregates were isolated and incubated, NT aggregates contained more labile, readily mineralizable C than CT aggregates. Elliott (1986) and Gupta and Germida (1988) reported higher C mineralization in intact aggregates from native sod compared with cultivated soils. No-tillage aggregates in the field hold more labile C that is easily mineralized once these aggregates are isolated in the laboratory. Since the aggregates are regularly disrupted under CT soils, this labile C is decomposed very fast and less of this labile C is still present when the aggregates are isolated and incubated.

Microaggregate-protected and micro within macroaggregate-protected C and 14C pools were higher in NT than in CT surface layers, while macroaggregate-protected C and 14C pools did not differ between tillage practices. Other researchers also found that significant amounts of SOM are physically protected in microaggregated structures (Gregorich et al., 1989; Balesdent et al., 2000). This suggests that the protection of SOM in NT soils occurs at the microaggregate level, rather than at the macroaggregate level. However, other studies (Elliott, 1986; Beare et al., 1994b and Balesdent et al., 2000) reported an increase in mineralization when breaking macroaggregates (>250 µm) into microaggregates (<250 µm), with a weak or insignificant increase in tilled soils, while it was larger in untilled soils. This indicates that SOM protection might occur at the macroaggregate level in some NT soils.

Although we suggest that the microaggregates offer most of the SOM protection, the stabilization of macroaggregates is important for this protection to occur. When the macroaggregates are not disrupted (e.g., in NT soils), residue that forms the center of a new macroaggregate decomposes and fragments inside of the macroaggregate into finer organic matter which gradually becomes encrusted with clay particles and microbial products, forming microaggregates within macroaggregates (Oades, 1984). This organic matter is then stabilized and protected against further microbial decay. When macroaggregates are disrupted under CT, the organic matter is released and never has the time to fragment and encrust with clay particles and microbial products, resulting in a much smaller amount of microaggregates within macroaggregates (Six et al., 2000b).

Table 3 shows the amount of C mineralized as a percentage of the total aggregate-associated C present before the incubation. It shows only a few significant differences between NT and CT soils where C pools in CT are greater than in NT. Since NT soils generally contained more C, the proportion of the C mineralized during the incubation will usually not be greater than in CT samples where less C is present. The microaggregate- and micro within macroaggregate-protected C pools are on average still <5% of the total C pool (Table 3), indicating that most of the protected C is associated with silt and clay particles.

At the 5- to 15-cm depth, there were no significant differences for unprotected and protected C pools between CT and NT, whereas macroaggregate-protected and microaggregate-protected 14C pools were higher in CT than in NT, indicating again that more young C is protected under CT soils at this depth, but this C is not stabilized in the long term.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 METHODOLOGICAL ISSUES
 DISCUSSION
 CONCLUSIONS
REFERENCES
 
Based on this study we can draw the following conclusions. The impact of tillage practices on aggregate-size distribution, C content, and organic matter protection is most pronounced in the upper layers of the soil. In these layers, NT soils contain more large macroaggregates and more total and young C. Organic matter is more protected at the surface under NT than under CT and this protection occurs more at the microaggregate level than at the macroaggregate level. At lower depths, more young C is accumulated in CT soils than in NT soils, but this C is not stabilized in the long term.


   ACKNOWLEDGMENTS
 
Thanks to Betty Weise and Kimber Collins for field and laboratory assistance. Thanks to the Institute of Ecology, University of Georgia, Athens, GA. This research was supported by grants from the National Science Foundation (DEB 9527957, DEB 9626770, and IBN 9987996).

Received for publication October 30, 2001.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 METHODOLOGICAL ISSUES
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 




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