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a Natural Resource Ecology Lab., Colorado State Univ., Fort Collins, CO 80523
b Dep. of Agronomy and Range Science, Univ. of California, Davis, CA 95616
c Lab. for Soil and Water Management, Katholieke Universiteit Leuven, Kasteelpark Arenberg 20, 3001 Heverlee, Belgium
d Dep. of Soil and Crop Sciences and Natural Resource Ecology Lab., Colorado State Univ., Fort Collins, CO 80523
* Corresponding author (karolien{at}nrel.colostate.edu)
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONREFERENCES |
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Abbreviations: CEC, cation exchange capacity • CT, conventional tillage • inter-mM-POM, particulate organic matter located outside microaggregates within macroaggregates • intra-mM-POM, particulate organic matter located inside microaggregates within macroaggregates • iPOM, intra-macroaggregate particulate organic matter • MAP, mean annual precipitation • MAT, mean annual temperature • mineral-mM, the silt plus clay fraction associated with the microaggregates within macroaggregates • mM, microaggregates contained within macroaggregates • NT, no-tillage • POM, particulate organic matter • SOC, soil organic carbon • SOM, soil organic matter
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONREFERENCES |
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Several studies have elucidated the relationship between aggregate and associated SOM dynamics (Elliott, 1986; Jastrow, 1996; Six et al., 1998, 2000b). Tisdall and Oades (1982) presented a conceptual hierarchical model for aggregate formation and stabilization in temperate soils. Their model suggests that three different classes (persistent, transient, and temporary) of organic matter are associated with three different soil physical units (silt and clay, microaggregates <250 µm, and macroaggregates >250 µm). This conceptual model has been employed and further elaborated in several studies to explain C sequestration under NT compared with CT (e.g., Beare et al., 1994a, 1994b; Six et al., 1998). Cultivation generally results in reduced stability and amount of macroaggregates but does not affect microaggregate stability (Tisdall and Oades, 1982). Therefore, the SOM that binds microaggregates into macroaggregates has been suggested to be the primary source of organic matter lost upon cultivation (Elliott, 1986). However, Six et al. (1999) found that the greater C stabilization with NT relative to CT is only partly explained by a greater amount of macroaggregates. They suggested that a reduced rate of macroaggregate turnover under NT increases the formation of microaggregates in which C is stabilized and sequestered in the long term. In a temperate soil, characterized by 2:1 clay minerals, Six et al. (2000b) found that the amount of microaggregates protected inside macroaggregates was two times greater with NT compared with CT. Similar differences were observed for the POM protected inside these microaggregates.
However, the mechanisms of microaggregate formation and associated C dynamics have received less attention in more weathered soils where the specific clay mineralogy may influence the mechanisms controlling macro- and microaggregate formation and C stabilization. These soils are characterized by mineral particles with variable charge (1:1 clay minerals and oxides). The positive role of oxides in aggregation is well known and is attributed to electrostatic interactions between positive charges associated with oxides and negative charges of clay minerals (Kemper and Koch, 1966; El-Swaify and Emerson, 1975; Pinheiro-Dick and Schwertmann, 1996). According to Duiker et al. (2003), poorly crystalline Fe oxides are more effective than crystalline Fe oxides in stabilizing soil aggregates. In addition, several authors have demonstrated the high flocculation capacity of 1:1 clay minerals due to the presence of both positive and negative charges, coexisting at field pH (El-Swaify, 1980; Dixon, 1989). Consequently, in soils dominated by 1:1 clay minerals and oxides, aggregate formation is less dependent on SOM as the primary binding agent compared with soils dominated by 2:1 clay minerals (Tisdall and Oades, 1982; Six et al., 2000a; Denef et al., 2002). However, in a previous study (Denef et al., 2002), we observed the greatest response in aggregate formation to organic matter input in an Alfisol with a mixed mineralogy (vermiculite, kaolinite, and oxides) compared with an Oxisol, dominated by kaolinite and oxides, and a 2:1 clay-dominated Mollisol. This was attributed to the high binding capacity of vermiculites with oxides, kaolinites, and SOM, in addition to electrostatic interactions between oxides and kaolinites that are typical for more weathered soils.
The aim of this study was to investigate the effect of CT and NT practices on the stabilization of C in microaggregates formed within macroaggregates, and how much the difference in microaggregate-associated C contributes to the difference in total SOC between NT and CT systems. This was evaluated in three soils characterized by different clay mineralogy to investigate if the effect of tillage on C sequestration by microaggregates within macroaggregates is similar across mineralogical types.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONREFERENCES |
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At Sidney, the experiment was established in 1969 on native grassland (short grass prairie). The experimental design was a randomized complete block with three field replicates. Three tillage treatments (CT, NT, and stubble mulch) in addition to the control native grassland were randomly assigned to 8.5 by 45.7 m plots within each of the replicates. The cropped treatments consisted of non-fertilized winter wheat [Triticum aestivum (L.)] for 1 yr followed by a fallow. We sampled during the fallow phase of the rotation. For the CT treatment, a moldboard plow was used, followed by several field cultivator and rodweeder operations. Under NT, herbicides were used for weed control.
At Lexington, a bluegrass [Poa pratensis (L.)] pasture occupied the site for at least 50 yr before the start of the experiment (1970). Treatments were continuous corn [Zea mays (L.)] at four N fertilizer levels and two tillage (CT and NT) systems. The experimental design was a split block with four replications. Each 11 by 48.8 m block was split horizontally for randomized N fertilizer treatments and vertically for randomized tillage treatment. Corn was cultivated every year with rye [Secale cereale (L.)] as a winter cover crop. We sampled 5.5 by 12.2 m subplots from both tillage treatments but only one of the fertilizer levels (85 kg N ha–1). The CT treatment consisted of moldboard plowing followed by several disk harrowings.
The experimental site at Passo Fundo was established in 1986 on a pasture recently converted from native forest vegetation. Before the establishment of the experiment, the area was covered by deep-rooted grasses for more than 5 yr. Treatments consisted of three crop rotations and two tillage systems (CT and NT) in a randomized complete block (split plot) design with three replications; tillage treatments were in main plots and rotations in subplots of 4 by 10 m. Conventional tillage consisted of disc plow, followed by light disc harrowings. The crop rotations were characterized by two growing seasons per year. Only the 3-yr crop rotation was sampled and it consisted of winter wheat–soybean [Glycine max (L.)]–vetch [Vicia sativa (L.)]–maize–oat [Avena sativa (L.)]–soybean. Nitrogen, P, and K were applied to maize (120 kg N ha–1, 56 kg P2O5 ha–1, and 56 kg K2O ha–1). With soybean, only P and K were applied (56 kg P2O5 ha–1 and 56 kg K2O ha–1). Oat and vetch served as green manure and were not fertilized.
At Sidney (NE) and Lexington (KY), eight soil samples were taken with a 5.5-cm diam. steel core to a depth of 20 cm from NT and CT plots along a transect in the middle of the plot to avoid edge effects. The litter layer was removed and the soil cores were divided into two depth increments: 0 to 5 and 5 to 20 cm. At Passo Fundo (Brazil), soil compaction was severe when core sampling was attempted. Therefore, two soil samples were taken per plot, each from the side of one trench of approximately 50 by 50 cm square, from which a 10 by 10 cm monolith was taken at two depths (0–5 and 5–20 cm). The litter layer (where present) was removed. Once in the laboratory, the field moist soil samples were passed through an 8-mm sieve by gently breaking apart the soil. The sieved soil samples were composited per plot and per depth, then air-dried, and stored at room temperature.
Soil Characteristics
The Sidney soil is a temperate loamy Duroc soil (fine-silty, mixed, superactive, mesic, Pachic Haplustolls) dominated by 2:1 clay minerals (illite and chlorite) (Six et al., 1999) with a cation-exchange capacity (CEC) of approximately 53.9 cmolc kg–1 clay (Denef et al., 2002). The Lexington soil is a moderately weathered, silt to silt-loam Maury soil (fine, mixed, semiactive, mesic, Typic Paleudalfs) and characterized by a mixed clay mineralogy dominated by 1:1 clay minerals (kaolinite) and 2:1 clay minerals (vermiculite), and by high concentrations of amorphous, non-crystalline Fe and Al oxides (Six et al., 1999; Denef et al., 2002). The Lexington soil was also characterized by a high CEC of approximately 82.9 cmolc kg–1 clay, which is attributed to the presence of vermiculites (Alexiades and Jackson, 1965). The soil from Passo Fundo is a highly weathered clay soil, classified as a very fine, kaolinitic, isothermic, Typic Hapludox in the USDA classification (Soil Survey Staff, 1999) and a Latossolo Vermelho Escuro Distrófico típico in the Brazilian classification (Embrapa, 1999) with a CEC of approximately 16.2 cmolc kg–1 clay (Denef et al., 2002). This weathered soil is dominated by 1:1 clay minerals (kaolinites) and has the highest amount of crystalline Fe and Al sesquioxides (Denef et al., 2002). Throughout the text, the temperate soil from Sidney, the moderately weathered soil from Lexington, and the highly weathered soil from Passo Fundo will be referred to as the 2:1 soil, the mixed soil, and the 1:1 soil, respectively. Table 1 summarizes the general soil and site characteristics. Further detailed descriptions of the sites and experiments are reported by Lyon et al. (1997) for Sidney, Frye and Blevins (1997) for Lexington, and Santos et al. (1995) for Passo Fundo.
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Microaggregate Isolation
Microaggregates protected within macroaggregates (>250 µm) were mechanically isolated using a method described by Six et al. (2000b). For the Sidney soil, microaggregate isolations were only done on small macroaggregates from both 0- to 5- and 5- to 20-cm depths because not enough large macroaggregates were present in both soil layers. For the soils from Lexington and Passo Fundo, we isolated microaggregates from both large and small macroaggregates in the 0- to 5- and 5- to 20-cm layers. Briefly, macroaggregate subsamples of approximately 8 g were immersed in deionized water on top of a 250-µm mesh screen inside a cylinder and reciprocally shaken (120 rpm) with 50 glass beads (diam. 4 mm) until the complete disruption of all macroaggregates (varying from 5 to 15 min). For the Passo Fundo soil, the macroaggregates were first submerged in deionized water for 4 h to enhance subsequent macroaggregate breakup. Minimal breakup of microaggregates isolated from the macroaggregates was ensured by continuous water flow flushing the <250-µm sized material immediately to a 53-µm sieve (Six et al., 2000b). Water-stable microaggregates were isolated from this <250-µm sized material by regular wet-sieving (see above). The fraction retained on the 250-µm mesh consisted of coarse POM and 250- to 2000-µm sized sand (no material >2000 µm was found in the Lexington and Passo Fundo soils). For the isolations done on the small macroaggregates, the 250- to 2000-µm sized sand is of the same size as the macroaggregates and therefore not considered part of the aggregates. When calculating the proportion of microaggregates found within macroaggregates (mM), a correction for this sand was applied (Six et al., 2000b).
Intra- and Intermicroaggregate Particulate Organic Matter
From the isolated microaggregate fraction, fine POM (53–250 µm) not occluded inside microaggregates (inter-mM-POM) was physically separated from the microaggregates by density flotation in 1.85 g cm–3 sodium polytungstate based on Six et al. (1998). After floating off the inter-mM-POM, the fine POM occluded inside the microaggregates (intra-mM-POM) was isolated from the heavy fraction (i.e., microaggregates plus sand) by shaking the heavy fraction in water with 12 glass beads (diam. 4 mm) for 18 h. After shaking, the dispersed fraction was transferred onto a 53-µm sieve to isolate intra-mM-POM and sand. This sand is of the same size as the microaggregates and therefore not considered part of the aggregates. The amount of mM was corrected for sand content according to Six et al. (2000b):
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Carbon Analyses
Total C analyses were done on: (i) total soil, (ii) microaggregate fractions isolated from macroaggregates, and (iii) inter- and intra-mM-POM fractions. Total C analyses were done on a CHN analyzer (model LECO CHN-1000, Leco Corp., St. Joseph, MI), except for the C concentration of the inter- and intra-mM-POM fractions. These concentrations were measured on a CN analyzer (model Carlo Erba NA 1500, Carlo Erba, Milan, Italy) because of the smaller size of these fractions.
To determine the concentration of C (g kg–1 macroaggregates) associated with the microaggregates themselves (mM-C), inter-mM-POM-C was subtracted from the total C content of the total microaggregate fraction isolated from the macroaggregates:
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This mM-C is a combination of intra-mM-POM-C as well as C associated with the mineral fraction of the microaggregates (mineral-mM-C) (g kg–1 macroaggregates), which can be calculated by difference:
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Microaggregate-C concentrations were expressed on a total macroaggregate (large plus small) basis, using weighted averages, to allow comparisons of microaggregate-C concentrations among the three soils (for the Sidney soil, all microaggregates were derived from small macroaggregates due to too few of the >2000-µm fraction). When comparing microaggregate-associated C concentrations among the three soils, expressed on a macroaggregate basis, we corrected for the total sand content of the macroaggregates, because of textural differences between the three soils and the fact that there is little or no binding of organic matter with sand particles (Elliott et al., 1991). Sand-free C concentrations (g kg–1 sand-free macroaggregates) were calculated as follows:
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The sand fraction was calculated from the weight of the coarse (250- to 2000-µm) fraction retained on the 250-µm mesh in the microaggregate isolator, combined with the weight of the fine (53–250 µm) fraction retained on the 53-µm sieve after dispersion of the heavy fraction during the intra-mM-POM isolation. Although both fractions consisted of sand plus POM, the weight of the POM was considered negligible compared with the weight of the sand.
To evaluate the importance of the microaggregate fractions in protecting soil C under NT relative to CT management, we calculated how much of the difference in total soil C was explained by the difference in intra-mM-POM-C and mineral-mM-C concentrations between NT and CT. All C concentrations were expressed as g C m–2 soil for a specific depth, by multiplying the SOC content (g kg–1 soil) with bulk density (g cm–3) and depth (cm). The conversion from the C concentration of the different mM-fractions to g kg–1 soil was based on weight data for (i) the mM-C fractions as a proportion of the microaggregates within macroaggregates (data not shown), (ii) the microaggregates as a proportion of the macroaggregates (Fig. 1) , and (iii) the macroaggregate fraction, obtained from the wet-sieving, as a proportion of the total soil weight (data not shown). Bulk density was measured on replicate whole soil samples for each treatment and depth (Table 1).
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Statistical Analyses
The experiments at Sidney (NE) and Passo Fundo (Brazil) consist of a randomized block design with tillage treatment in three replications. The experiment at Lexington (KY) consists of a split block design with tillage treatment in four replications. The data were analyzed using the SAS statistical package for analysis of variance (ANOVA-PROC MIXED) (SAS Institute, 1990) with tillage treatment and soil type considered fixed effects and replicates as random effects. Differences between means were tested with the DIFF option of the LSMEANS statement with a significance level of P < 0.05, except if noted differently.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONREFERENCES |
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Within each tillage treatment and depth, the proportion of microaggregates was different among the three soils in the following order: mixed > 1:1 > 2:1, except for similar proportions of microaggregates in the large macroaggregate fraction of the 0- to 5-cm depth layer of the mixed and 1:1 soil under CT, and of the small macroaggregate fraction in the 2:1 and 1:1 soil under NT (Fig. 1).
Microaggregate-Associated Carbon Fractions
In the 2:1 soil, no differences were observed between NT and CT for the inter-mM-POM-C for the 0- to 5-cm depth (Fig. 2)
. In the 5- to 20-cm depth, inter-mM-POM-C was greater in CT compared with NT (Fig. 2). Intra-mM-POM-C was greater in NT vs. CT for the 0- to 5-cm depth, but the opposite was noticed for the 5- to 20-cm depth. Mineral-mM-C contributed more than 75% of the total mM-C and was approximately two-fold greater in NT compared with CT for both 0- to 5-cm and 5- to 20-cm depths.
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In the 1:1 soil, both the intra-mM-POM-C (P = 0.06) and the mineral-mM-C (P < 0.05) were greater in NT compared with CT for the 0- to 5-cm depth (Fig. 2). No differences were found between NT and CT for the 5- to 20-cm depth, except for the inter-mM-POM-C, which was slightly greater under NT compared with CT. At both depths, more than 85% of the total mM-C was associated with the mineral fraction of the microaggregates.
Within the NT and CT treatment, the 2:1 soil had the greatest concentration of inter-mM-POM-C in both soil layers. No differences in inter-mM-POM-C concentrations were found between the mixed and 1:1 soil. Among all three soils, intra-mM-POM-C concentrations were smallest in the 1:1 soil in both NT and CT treatments.
Contribution of Microaggregate-Protected Carbon Differences to Total Soil Carbon Differences
Surface (0–5 cm) Layer
In the 0- to 5-cm layer, total SOC was greater in NT compared with CT in all three soils (Table 2). The greatest difference was observed in the mixed soil (Table 3). In all three soils, mineral-mM-C, intra-mM-POM-C, and consequently total mM-C were greater in NT compared with CT (Table 2). The greatest differences in these mM-associated C fractions between NT and CT were also observed in the mixed soil (Table 3). In both the mixed and 1:1 soil, approximately 50% of the total SOC under NT was associated with the microaggregates (mM-C), whereas only 14% of the total SOC under NT was associated with microaggregates in the 2:1 soil (Table 2). The percentage of the total SOC difference explained by intra-mM-POM-C was <15% in all three soils (Table 3). A greater percentage of the difference in total SOC between NT and CT was explained by the mineral-mM-C, but varied among the three soils and in the following order: mixed > 1:1 > 2:1. As a result, the difference in total mM-C between NT and CT represented 87% (mixed), 59% (1:1), and 33% (2:1) of the difference in total SOC between NT and CT in the surface 5 cm (Table 3).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONREFERENCES |
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Microaggregates and Microaggregate-Associated Carbon
Previous studies have reported less macroaggregation and less 53- to 250-µm sized intra-macroaggregate POM (fine iPOM) in CT relative to NT systems for the same three soils used in this study (Six et al., 1999; Blauwet and Leyssens, 2002; J. Six, unpublished data, 2003). This suggested that in all three soil types, intensive tillage (CT) caused a greater disruption of macroaggregates and inhibited the formation and/or persistence of fine iPOM within the macroaggregates. Based on results from temperate soils under long-term NT and CT management, Six et al. (1998)( 1999, 2000b) suggested that slower macroaggregate turnover in the absence of tillage disturbance results in greater formation of microaggregates within macroaggregates and stabilization of SOC within microaggregates. In our study, NT resulted in greater concentrations of intra-mM-POM-C and mineral-mM-C compared with CT in all three soils. This is consistent with the results presented by Six et al. (1998)(1999; 2000b) for temperate soils and indicates that the link between macroaggregate turnover, microaggregate-C sequestration and tillage is expressed across drastically different soil types and environments.
Several authors have discussed the importance of POM in the formation of microaggregates within macroaggregates and, conversely, the importance of microaggregates for long-term C sequestration through the protection and stabilization of POM (e.g., Jastrow and Miller, 1998; Six et al., 1998; Gale et al., 2000; Puget et al., 2000). For the three soils in this study, <25% of the microaggregate-C (mM-C) was associated with the intra-mM-POM fraction, whereas more than 75% was found in the mineral-mM fraction (Fig. 2). Similarly, Jastrow (1996) and Six et al. (1999) found that the majority of C in macroaggregates was associated with the mineral fraction. These results suggest that the primary role of POM is to serve as nucleation sites for micro- and macroaggregate formation, whereas long-term C sequestration within micro-and macroaggregates is mainly as mineral-associated C. The mineral-associated C is formed during decomposition of POM and stabilized through SOM bindings with the clay minerals.
Similar to the findings by Six et al. (2000b), increased mM-C in NT vs. CT in the 2:1 soil was accompanied by greater proportions of microaggregates within macroaggregates. However, the difference in the proportions of microaggregates between NT and CT was less in the mixed than in the 2:1 soil, and no difference was observed in the 1:1 soil. These observations suggest that the concept of inhibited formation of microaggregates due to enhanced macroaggregate turnover by tillage (Six et al., 2000b) does not hold true for all soil types. Similarly, Bayer et al. (2002) found that the amount of silt-sized microaggregates (<53 µm) in a 1:1 clay-dominated soil was unaffected by tillage treatment. It is possible that the lack of difference in proportion of microaggregates between NT and CT in our 1:1 soil is due to the younger age of the NT-CT experimental plots in the tropical 1:1 soil (established in 1986) compared with the 2:1 soil (1969). However, age does not appear to be the only factor responsible for the different response of the proportion of microaggregates to tillage among the different soils; the difference in the proportions of microaggregates between NT and CT was much smaller in the mixed compared with the 2:1 soil, while the NT-CT experiments were established around the same time in these two soils. We suggest that the formation of microaggregates within macroaggregates in the mixed and 1:1 soil is less affected by SOM losses upon tillage compared with the 2:1 soil due to the greater role of mineral-mineral binding in aggregation in these oxide-rich soils (Oades and Waters, 1991; Six et al.; 2000a; Denef et al., 2002).
Interestingly, the mixed soil had the greatest proportion of microaggregates within macroaggregates among the three soils (2:1 < 1:1 < mixed). Many studies have reported positive correlations between aggregate stability and clay content (Kemper and Koch, 1966; Boix-Fayos et al., 2001). The soils used in this study differed in clay content (mixed < 2:1 < 1:1) as well as silt plus clay content (2:1 < 1:1 < mixed), suggesting that microaggregate stability is rather linked to silt plus clay content than clay content only. The mixed soil was also characterized by the greatest microaggregate-associated C levels (per kilogram of sand-free macroaggregates) under NT. Therefore, the high degree of microaggregation in the mixed soil might also partly be attributed to the strong bindings between 2:1 clays, and 2:1 and 1:1 clays by SOM in addition to the high electrostatic binding capacity of the 2:1 clay minerals (in particular vermiculites), 1:1 clay minerals and oxides present in this soil. Vermiculites are characterized by a large CEC (Alexiades and Jackson, 1965) (Table 1), which could cause strong interactions with 1:1 clay minerals, oxides, and SOM in this soil.
Contribution of Microaggregate-Protected Carbon Differences to Total Soil Carbon Differences
To evaluate the contribution of the microaggregates to sequestration of SOC under NT management, we compared microaggregate-associated C and total SOC concentrations in the three soils under NT and CT management. In both the surface and the entire plow layer, the microaggregates stabilized within macroaggregates significantly contributed to the difference in total SOC between NT and CT in all three soils (Table 3 and 5). Although only 14% of the total SOC in the 2:1 soil was associated with the microaggregates, the difference in mM-C explained 91% of the total increase in SOC with NT vs. CT in the entire plow depth. Greater proportions (50%) of total SOC were found in the microaggregates of the mixed and 1:1 surface soil under NT, which was probably attributed to the higher degree of macroaggregation (data not shown) and the greater proportion of microaggregates within macroaggregates in the mixed and 1:1 soil compared with the 2:1 soil. However, similar to the 2:1 soil, the difference in mM-C also explained more than 90% of the difference in total SOC between NT and CT in the entire plow layer of the mixed and the 1:1 soil. The contribution of the difference in mM-C to the difference in total SOC between NT and CT was more than 80% accounted for by the contribution of mineral-mM-C (Table 3 and 5). Therefore, the greater C stabilization under NT compared with CT management is mainly attributed to the mineral fraction of the microaggregates within macroaggregates.
These results demonstrated that we successfully isolated a C fraction that almost entirely explained the difference in total SOC between NT and CT systems for three widely varying soil types and environments. This greatly suggests the potential of the microaggregate-associated C fraction in serving as a highly accurate diagnostic fraction for changes in total SOC in response to changes in tillage management. Further investigation of the generality of this potential diagnostic fraction would be of great significance for further elucidating mechanisms of SOC stabilization and for providing more accurate and mechanistic models to predict changes in SOC in agricultural soils.
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CONCLUSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONREFERENCES |
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Furthermore, we demonstrated that microaggregate-associated C, isolated from water-stable macroaggregates, accounted for more than 90% of the difference in total SOC between NT and CT across all soil types when the entire plow depth was considered. The contribution of the difference in microaggregate-associated C to the difference in total SOC between NT and CT was mainly accounted for by the difference in C associated with the mineral fraction of the microaggregates.
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ACKNOWLEDGMENTS |
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Received for publication September 16, 2003.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONREFERENCES |
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 X. Hao and A. N. Kravchenko Management Practice Effects on Surface Soil Total Carbon: Differences along a Textural Gradient Agron. J., January 1, 2007; 99(1): 18 - 26. [Abstract] [Full Text] [PDF]
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H. Blanco-Canqui, R. Lal, L. B. Owens, W. M. Post, and R. C. Izaurralde Mechanical Properties and Organic Carbon of Soil Aggregates in the Northern Appalachians Soil Sci. Soc. Am. J., August 4, 2005; 69(5): 1472 - 1481. [Abstract] [Full Text] [PDF]
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A. Y. Y. Kong, J. Six, D. C. Bryant, R. F. Denison, and C. van Kessel The Relationship between Carbon Input, Aggregation, and Soil Organic Carbon Stabilization in Sustainable Cropping Systems Soil Sci. Soc. Am. J., June 2, 2005; 69(4): 1078 - 1085. [Abstract] [Full Text] [PDF]
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