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DIVISION S-6-SOIL & WATER MANAGEMENT & CONSERVATION

Soil Structure and Organic Matter

I. Distribution of Aggregate-Size Classes and Aggregate-Associated Carbon

Natural Resource Ecology Lab., Colorado State Univ., Fort Collins, CO 80523 USA

johan{at}nrel.colostate.edu

	ABSTRACT

TOP ABSTRACT INTRODUCTION Materials and methods Results Discussion Conclusions REFERENCES

 
Cultivation reduces soil C content and changes the distribution and stability of soil aggregates. We investigated the effect of cultivation intensity on aggregate distribution and aggregate C in three soils dominated by 2:1 clay mineralogy and one soil characterized by a mixed (2:1 and 1:1) mineralogy. Each site had native vegetation (NV), no-tillage (NT), and conventional tillage (CT) treatments. Slaked (i.e., air-dried and fast-rewetted) and capillary rewetted soils were separated into four aggregate-size classes (<53, 53–250, 250–2000, and >2000 µm) by wet sieving. In rewetted soils, the proportion of macroaggregates accounted for 85% of the dry soil weight and was similar across management treatments. In contrast, aggregate distribution from slaked soils increasingly shifted toward more microaggregates and fewer macroaggregates with increasing cultivation intensity. In soils dominated by 2:1 clay mineralogy, the C content of macroaggregates was 1.65 times greater compared to microaggregates. These observations support an aggregate hierarchy in which microaggregates are bound together into macroaggregates by organic binding agents in 2:1 clay-dominated soils. In the soil with mixed mineralogy, aggregate C did not increase with increasing aggregate size. At all sites, rewetted macro- and microaggregate C and slaked microaggregate C differed in the order NV > NT > CT. In contrast, slaked macroaggregate C concentration was similar across management treatments, except in the soil with mixed clay mineralogy. We conclude that increasing cultivation intensity leads to a loss of C-rich macroaggregates and an increase of C-depleted microaggregates in soils that express aggregate hierarchy.

Abbreviations: CT, conventional tillage • IPOM, intra-aggregate particulate organic matter • LF, light fraction • NV, native vegetation • NT, no-tillage • POM, particulate organic matter • SOM, soil organic matter

	INTRODUCTION

TOP ABSTRACT INTRODUCTION Materials and methods Results Discussion Conclusions REFERENCES

 
TISDALL AND OADES (1982) presented a conceptual model for aggregate hierarchy that described how primary mineral particles are bound together with bacterial, fungal, and plant debris into microaggregates. These microaggregates, in turn, are bound together into macroaggregates by transient binding agents (i.e., microbial- and plant-derived polysaccharides) and temporary binding agents (roots and fungal hyphae). Three consequences of this aggregate hierarchy are: (i) a gradual breakdown of macroaggregates into microaggregates before they dissociate into primary particles, as increasing dispersive energy is applied to soil (Oades and Waters, 1991), (ii) an increase in C concentration with increasing aggregate-size class because large aggregate-size classes are composed of small aggregate-size classes plus organic binding agents (Elliott, 1986), and (iii) younger and more labile organic matter is contained in macroaggregates than in microaggregates (Elliott, 1986; Puget et al., 1995; Jastrow et al., 1996).

Oades and Waters (1991) tested the aggregate hierarchy theory in different soils by applying a range of treatments to disaggregate soils. They concluded that aggregate hierarchy existed in two Alfisols and a Mollisol because organic materials were the major binding agents for aggregate formation and stabilization in these soils. In contrast, they found that an Oxisol did not express any hierarchical aggregate structure, probably because oxides, rather than organic materials, were the dominant stabilizing agents. Elliott (1986) observed more organic matter associated with macroaggregates than with microaggregates in a temperate grassland soil. He also found that organic matter associated with macroaggregates was more labile than organic matter associated with microaggregates. Jastrow et al. (1996) reported greater amounts of recently incorporated organic matter as aggregates became larger, supporting the idea that microaggregates are bound together by young organic matter into larger macroaggregates.

Aggregate hierarchy theory has been used by many authors to explain the correlation between a reduction in aggregation and loss of soil organic matter (SOM) with cultivation (Elliott, 1986; Cambardella and Elliott, 1993; Beare et al., 1994). A breakdown of macroaggregates results in a release of labile SOM (Elliott, 1986) and its increased availability for microbial decomposition. The increased microbial activity depletes SOM, which eventually leads to lower microbial biomass and activity and consequently a lower production of microbial-derived binding agents and a loss of aggregation (Jastrow, 1996; Six et al., 1998).

Reduced aggregation (and subsequent lower levels of SOM) in conventional tillage (CT) vs. no-tillage (NT) (Paustian et al., 1999) is a result of several indirect effects on aggregation. Tillage brings subsurface soil to the surface where it is then exposed to wet–dry and freeze–thaw cycles and subjected to raindrop impact (Beare et al., 1994; Paustian et al., 1997), thereby increasing the susceptibility of aggregates to disruption (Willis, 1955; Hadas, 1990; Edwards, 1991). Plowing changes the soil conditions (e.g., temperature, moisture, and aeration) and increases the decomposition rates of litter (Rovira and Greacen, 1957; Cambardella and Elliott, 1993). In NT, residues accumulate at the surface where the litter decomposition rate is slowed due to drier conditions and reduced contact between soil microorganisms and litter (Salinas-Garcia et al., 1997). Finally, the proportion of the microbial biomass composed of total fungi (Frey et al., 1999) and mycorrhizal fungi (O'Halloran et al., 1986) is generally higher in NT compared to CT and it has been observed that fungi (especially mycorrhizal) contribute to macroaggregate formation and stabilization (Tisdall and Oades, 1982).

The objectives of this study were to (i) test the validity of the aggregate hierarchy theory over a range of soils and (ii) study the affect of increased cultivation intensity on aggregation and aggregate-associated C.

	Materials and methods

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Sampling
Soils from four long-term agricultural field experiments (Table 1) were sampled at two depths (0–5 cm and 5–20 cm) in November 1995. The four sites are located in Sidney, NE (41°14'N, 103°00'W), Wooster, OH (40°48'N, 82°00'W), W. K. Kellogg Biological Station (KBS), MI (42°24'N, 85°24'W), and Lexington, KY (38°07'N, 84°29'W). All four sites have NV, NT, and CT treatments with either three or four replicates. At Sidney, all management treatments were established directly from the native shortgrass prairie. At KBS, NT and CT plots were under long-term cultivation before establishment, whereas the adjacent NV plot was in grass vegetation on a former woodlot that had never been cultivated. At Wooster, the NV plot was in nearby forest, but the NT and CT plots were established in a 6-yr-old grass meadow that was cultivated for many years prior to establishment. At Lexington, the experimental plots were initially under long-term cultivation, but for 50 yr prior to treatment establishment were in bluegrass pasture. The Lexington soil is characterized by a mixed-clay mineralogy dominated by kaolinite (1:1 layer type) and vermiculite (2:1 layer type). In addition, a 2 to 16 times higher concentration of amorphous and poorly crystalline Fe- and Al-oxides was reported for the Lexington soil compared to the Sidney, Wooster, and KBS soils (Six et al., 1999b). The Sidney, KBS, and Wooster soils have only 2:1 minerals, predominantly illite and chlorite (Six et al., 1999b).

The method used for aggregate-size separation was adapted from Elliott (1986). Briefly a 100-g subsample (air-dried or capillary-rewetted) was submerged for 5 min on a 2000-µm sieve. Aggregates were separated by moving the sieve (by hand) up and down 3 cm with 50 repetitions during 2 min. The >2000-µm aggregates were collected and sieving was repeated for the <2000-µm fraction with the next smaller-sized sieve. This procedure was repeated for every sieve size (250 and 53 µm). All aggregate fractions were oven-dried (50°C) and weighed. Sand content (>53 µm) of the aggregates was determined on a subsample of aggregates that were dispersed with sodium hexametaphosphate (5 g L^-1).

Free Light Fraction and Mineral-Associated Fraction Analysis
The free light fraction (POM outside of the aggregates, or free LF) and mineral-associated organic matter fraction were isolated from the three largest aggregate-size classes according to Six et al. (1998). Briefly, free LF was isolated by density flotation in 1.85 g cm^-3 sodium polytungstate. The free LF probably includes both LF outside of aggregates and some LF released from the aggregates upon submersion in sodium polytungstate. However, the dispersion of aggregates in sodium polytungstate is minimal and therefore the released fraction was only a small proportion of the free LF. Sodium polytungstate was recycled according to Six et al. (1999c) to avoid cross- contamination of C and N between samples. After isolation of free LF, aggregates were dispersed in 5 g L^-1 sodium hexametaphosphate by shaking for 18 h on a reciprocal shaker. Intra-aggregate (within aggregate) particulate organic matter (IPOM) was isolated by sieving. Aggregate-associated C and mineral-associated C were calculated by difference

(1)

(2)

where total aggregate C is total C measured in aggregates prior to free LF flotation. Aggregate-associated C and mineral-associated C were only determined for the slaked microaggregates (53–250 µm) and small macroaggregates (250–2000 µm). The yield of large macroaggregates (>2000 µm) after slaking was often too small to be analyzed. For the <53-µm fraction, the LF yield was too small for C analysis and by definition (Cambardella and Elliott, 1992) there is no IPOM in particles <53 µm.

Carbon, Nitrogen, and Isotope Analyses
Isotope and organic C and N analyses were performed according to Six et al. (1998). The natural abundance ¹³C methodology for SOM studies was only done at the Sidney site. The other locations did not have a single shift in the dominance of plant species with different metabolic pathways (C₃ vs. C₄) or archived soil samples from the beginning of the experiment. At Sidney, the delta ¹³C values () were used to calculate the proportion of wheat-derived C (f) in each organic matter fraction:

(3)

where _t = ¹³C at time t, _w = ¹³C of wheat straw (crop), ₀ = ¹³C of original grassland-derived SOM, and f = fraction of wheat-derived C in the soil. The proportions of wheat-derived C vs. grassland-derived C (1 - f) provide a measure of the relative age of the organic matter in the different size fractions. The proportions of crop-derived C are only presented for the 0- to 5-cm layer because changes in the ¹³C signature with depth confound interpretations at the 5- to 20-cm depth. In addition, differences in C concentrations were mainly observed in the 0- to 5-cm layer.

Statistical Analyses
The experimental field design was a randomized complete block design for all sites. However, the NV were not replicated within the experiments at Wooster, KBS, and Lexington and therefore not included in the statistical analysis. Analysis of variance (ANOVA-GLM, SAS Institute, 1990) was performed with multiple comparisons within a depth. Tillage treatment was the main factor in the model, with aggregate size and replicate as secondary factors. Separation of means was tested using Tukey's honestly significant difference at a level of P < 0.05.

	Results

TOP ABSTRACT INTRODUCTION Materials and methods Results Discussion Conclusions REFERENCES

 
Whole Soil Characteristics
Total organic C and N (0–20 cm) generally decreased (but not always significantly) in the order NV > NT > CT (Table 2) . At all sites, significant differences in total organic C and N between treatments were observed in the 0- to 5-cm depth, except at KBS where NT and CT were not significantly different. Organic C and N levels were on average 38% lower in CT compared to NT in the 0- to 5-cm depth (Table 2). In contrast, there were no significant differences observed in total C and N in the 5- to 20-cm depth at any site. Bulk density was not significantly affected by tillage at any of the sites (Table 2).

View larger version (37K): [in this window] [in a new window]   Fig. 1 Slaked and rewetted aggregate-size distributions in the surface layer (0–5 cm) of four long-term agricultural experiment sites (SID = Sidney, NE; WO = Wooster, OH; KBS = Kellogg Biological Station, MI; LX = Lexington, KY) with three management treatments (NV = native vegetation; NT = no-tillage; CT = conventional tillage). Bars are standard deviations

Aggregate Carbon Concentrations
It is frequently observed that the major differences in organic matter content between NT and CT soils are in the upper few centimeters of soil (Doran, 1987; Dick et al., 1997). Similarly, we found few differences in aggregate C among treatments at the 5- to 20-cm depth (data not shown). The only exception was KBS, where the NV treatment had substantially higher aggregate C concentrations at 5 to 20 cm compared to NT and CT (data not shown). This trend may be due to the long-term cultivation of NT and CT plots before establishment of the experiment. While not reported, trends across aggregate-size classes within treatments were the same in the subsurface layer and surface layer. Therefore, further comparisons are made only for the 0- to 5-cm layer.

In general, sand-free C concentrations of all rewetted aggregate-size classes differed in the order NV > NT > CT (Fig. 2) . At KBS, NT and CT did not differ significantly in this respect, which may again be a result of the young age of the experiment. The apparent large differences between rewetted (and slaked) aggregate C concentrations in NV versus NT and CT at KBS is probably also a result of the long-term cultivation of the NT and CT plots prior to establishment of the experiment. In contrast to the other sites, rewetted aggregate C concentrations at Wooster (the only site with forest vegetation) were not different between NV and NT, except for the microaggregates. This suggests that forest vegetation is not as beneficial as grassland vegetation for the accumulation of aggregate C.

View larger version (34K): [in this window] [in a new window]   Fig. 2 Slaked and rewetted aggregate C concentrations in the surface layer (0–5 cm) of four long-term agricultural experiment sites (SID = Sidney, NE; WO = Wooster, OH; KBS = Kellogg Biological Station, MI; LX = Lexington, KY) with three management treatments (NV = native vegetation; NT = no-tillage; CT = conventional tillage). All C concentrations are corrected for sand content. Bars are standard deviations

	Discussion

TOP ABSTRACT INTRODUCTION Materials and methods Results Discussion Conclusions REFERENCES

 
In soils where SOM is the major binding agent an aggregate hierarchy has been observed (Tisdall and Oades, 1982; Oades and Waters, 1991). SOM is expected to be the primary binding agent in 2:1 clay-dominated soils because polyvalent-organic matter complexes form bridges between the negatively charged clay platelets. In contrast, SOM is not the only binding agent in oxide- and 1:1 clay-dominated soils. Electrostatic attractions occur between and among oxides and kaolinite platelets due to simultaneous existence of positive and negative charges at field pH (Schofield and Samson, 1954; El-Swaify, 1980). Thus in those soils aggregate formation is partly induced by electrostatic interactions and aggregate hierarchy should be less pronounced (Oades and Waters, 1991).

The increased aggregate- and mineral-associated C content of small macroaggregates vs. microaggregates (within treatment) at Sidney, Wooster, and KBS (Table 3) indicates that both IPOM C and mineral-associated C are incorporated during formation of macroaggregates. This also suggests that IPOM C is a major C source for microbial activity and thereby induces the binding of clay- and silt-sized particles and microaggregates into macroaggregates (Jastrow, 1996; Six et al., 1998, 1999a) in these 2:1 clay-dominated soils. In addition, the similarity of aggregate-associated C concentrations of slaked macroaggregates across management treatments indicates the stability of slaked macroaggregates is correlated to their C content (Elliott, 1986; Cambardella and Elliott, 1993). The stability of microaggregates, in contrast, does not seem to be correlated to C content, because there is a difference in slaked microaggregate C content among treatments at all sites (Table 3). Perhaps the physical characteristics of microaggregates such as lower porosity and higher bulk density (Oades and Waters, 1991) are the main factors that confer resistance to slaking rather than their C content.

The comparison of rewetted and slaked aggregate distribution and the aggregate-associated C concentrations provides information on the degree of aggregate hierarchy exhibited by the different soils. The aggregate hierarchy theory seems applicable to the 2:1 clay-dominated soils at Sidney, Wooster, and KBS because of (i) small increases in silt and clay particles, but large increases in microaggregates, upon disruption of the macroaggregate (Fig. 1) (Elliott, 1986; Oades and Waters, 1991), (ii) small differences in silt and clay proportion between NT and CT (Fig. 1) (Elliott, 1986), and (iii) increased aggregate-associated C concentrations with increasing aggregate sizes in slaked soils (Table 3) (Elliott, 1986). Additional support for the aggregate hierarchy at Sidney is provided by the ¹³C natural abundance data (Table 4). The increase in proportions of young C with aggregate size indicates that microaggregates are bound together into larger macroaggregates by crop-derived C (Puget et al., 1995; Jastrow et al., 1996).

Elliott (1986) used the aggregate hierarchy theory as a basis to explain reduced SOM level in a stubble mulch agroecosystem compared to native sod. We apply this theory to explain the decreasing SOM content in the order NV > NT > CT at Sidney, Wooster, and KBS (soils that express aggregate hierarchy). However, NV at KBS had much higher slaked aggregate C contents than NT and CT, probably a result of the difference in history of the NV vs. the NT and CT plots; therefore the NV treatment is ignored in the discussion below.

Increasing cultivation intensity led to a loss of C-rich macroaggregates and an increase of C-depleted microaggregates. There were no consistent significant differences in the proportions of rewetted macroaggregates (>250-µm fractions) among management treatments (Fig. 1), but the rewetted large and small macroaggregate C concentrations differed in the order NV > NT > CT (Fig. 2). In contrast, the proportions of slaked macroaggregates differed in the order NV > NT > CT (Fig. 1), but the slaked macroaggregate C concentrations were similar across management treatments (Fig. 2 and Table 3). These observations suggest that the C lost with increasing cultivation intensity is responsible for the higher proportions of stable macroaggregates (i.e., slaked macroaggregates) in the order NV > NT > CT; it is the C of binding agents that binds individual microaggregates into stable macroaggregates (Tisdall and Oades, 1982; Elliott, 1986). In our study, increasing cultivation intensity increased the proportion of slaked microaggregates, which were depleted in C compared to macroaggregates and increasingly depleted in C with increasing cultivation intensity (Fig. 2). Therefore we conclude that increasing cultivation intensity leads to a loss of C-rich macroaggregates and an increase of C-depleted microaggregates in soils that express aggregate hierarchy. This shift results in a loss of total organic C. The C lost was that which binds microaggregates into macroaggregates. These observations made at Sidney, Wooster, and KBS extend Elliott's results from stubble mulch agroecosystems to NT and CT agroecosystems characterized by 2:1 clay-dominated soils.

The aggregate hierarchy theory seems to be less applicable to the Lexington soil (mixed mineralogy) because within management treatments (i) similar total aggregate C, aggregate-associated C, and mineral-associated C concentrations were observed across aggregate-size classes (Table 3), (ii) the proportion of silt- and clay-sized particles increased the most from the rewetted to slaked aggregate distribution at Lexington (Fig. 1), and (iii) large macroaggregates broke up into silt and clay particles and microaggregates with increasing cultivation intensity, whereas the proportion of small macroaggregates was about the same.

Beare et al. (1994) also observed only small differences in aggregate distribution between NT and CT in a kaolinitic soil in Georgia. The largest difference between NT and CT was in the proportions of large macroaggregates, which primarily fell apart into silt- and clay-sized particles. The proportions of small macroaggregates were similar across tillage regimes (Beare et al., 1994). As previously mentioned, Feller et al. (1996) and Elliott et al. (1991) did not observe increased C concentrations with increasing aggregate size in 1:1 clay-dominated soils. The less-pronounced aggregate hierarchy in the Lexington soil is probably a result of the presence of Fe- and Al-oxides and kaolinite (1:1 clay) which contribute to soil stability through electrostatic interactions (Oades and Waters, 1991). We conclude that the Lexington soil does not express as much aggregate hierarchy as the 2:1 clay-dominated soils because of the presence of oxides and 1:1 clays.

In capillary-wetted aggregates from a kaolinitic soil, Beare et al. (1994) observed a higher C content in microaggregates than in macroaggregates. We observed the same trend of increasing C content from large macroaggregates to microaggregates in rewetted soils at Lexington. However, at the other sites no significant differences in C content between macroaggregates and microaggregates in rewetted soils within a management treatment were observed. Other authors also found no differences in misted or rewetted aggregate C contents within a treatment (Elliott, 1986; Cambardella and Elliott, 1993). The higher C content observed in nonslaked microaggregates may therefore be specific for soils with 1:1 clay minerals.

	Conclusions

TOP ABSTRACT INTRODUCTION Materials and methods Results Discussion Conclusions REFERENCES

 
Aggregate hierarchy was observed in soils from Sidney, KBS, and Wooster, all dominated by 2:1 clay mineralogies. The Lexington soil expressed less aggregate hierarchy, which may be due to the presence of oxides and low-activity clays (kaolinite). There was a clear relationship between loss of soil structure and loss of SOM in the soils that expressed aggregate hierarchy. Increasing cultivation intensity induced a loss of C-rich macroaggregates and a gain of C-depleted microaggregates, resulting in an overall loss of SOM C.

	ACKNOWLEDGMENTS

 
Thanks are extended to Scott Pavey and Matt Nemeth for their many hours of sieving, weighing, and C and N analysis. Dan Reuss's help during the laboratory work is greatly appreciated. We acknowledge the assistance of Drew Lyon (Univ. of Nebraska, Panhandle Research and Extension Center, Scottsbluff) at the Sidney site, Edmund Perfect and Robert Blevins (Univ. of Kentucky, Lexington) at the Lexington site, H.P. Collins and G.P. Robertson (Univ. of Michigan, W.K. Kellogg Biological Station, Hickory Corners) at the Kellogg site, and W.A. Dick (Ohio State Univ., Wooster) at the Wooster site. Comments on the manuscript by three anonymous reviewers and the associate editor are acknowledged. This research was supported by grant DEB-9419854 from the National Science Foundation.

Received for publication December 21, 1998.

	REFERENCES

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