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DIVISION S-3-SOIL BIOLOGY & BIOCHEMISTRY

Surface Residue– and Root-derived Carbon in Stable and Unstable Aggregates

^a USDA-ARS National Soil Tilth Lab., 2150 Pammel Dr., Ames, IA 50011 USA
^b Dep. of Statistics, Iowa St. Univ., Ames, IA 50011 USA

cindyc{at}nstl.gov

	ABSTRACT

TOP ABSTRACT INTRODUCTION Materials and methods NOTES Results and discussion Conclusions REFERENCES

 
Stable macroaggregates are enriched in new C relative to unstable macroaggregates, but the origin and form of this new C is not known. Under simulated no-till (NT) conditions, we used a ¹⁴C label to monitor changes in the concentration of new surface residue– and root-derived C in aggregates of different size and stability during a 1-yr incubation. Two water pretreatments (capillary-wetted and slaked) were applied to the soil samples collected during the incubation. The samples were then wet sieved to obtain five aggregate size classes. Densiometric separations were used to isolate free and released particulate organic matter (frPOM) and intraaggregate POM (iPOM). Root-derived ¹⁴C was distributed differently in the soil compared to surface residue–derived ¹⁴C. A comparison of the two water pretreatments indicated that root-derived aggregate-¹⁴C and iPOM-¹⁴C concentrations were significantly higher in stable (slaking-resistant) small macroaggregates (250–2000 µm) relative to those in the capillary-wetted pretreatment. In contrast, there were no significant differences in the amount of surface residue–derived aggregate-¹⁴C or iPOM-¹⁴C in small macroaggregates (250–2000 µm) between the two pretreatments. We conclude that in relatively undisturbed systems like no-till, new root-derived iPOM-C is more important than surface residue–derived C in the stabilization of small macroaggregates (250–2000 µm).

Abbreviations: frPOM, free and released particulate organic matter • iPOM, intraaggregate particulate organic matter • LSD, least significant difference • POM, particulate organic matter

	INTRODUCTION

TOP ABSTRACT INTRODUCTION Materials and methods NOTES Results and discussion Conclusions REFERENCES

 
AGGREGATE STRUCTURE INFLUENCES MANY SOIL PROPERTIES including aeration, water infiltration, drainage, bulk density, and resistance to erosion (Allison, 1973). A decrease in aggregate stability can have detrimental effects on the physical properties of soil and may lead to a reduction in crop production (Lynch and Bragg, 1985). An understanding of the factors that determine aggregate stability may help efforts to develop management practices that promote the formation of stable aggregate soil structure.

The cultivation of grassland soils results in a loss of soil organic matter and a deterioration in aggregate stability (Elliott, 1986; Gupta and Germida, 1988; Cambardella and Elliott, 1993). Cambardella and Elliott (1993) suggested that the decrease in aggregate stability in cultivated soils was related to a loss of POM in the soil. Puget et al. (1995) and Angers and Giroux (1996) reported that stable macroaggregates were enriched in newly deposited C relative to less stable macroaggregates and microaggregates, but the physical and chemical nature of this new C was not determined.

Golchin et al. (1994) isolated two POM fractions that were defined by their position in the soil matrix. Free POM (frPOM) was located between the soil aggregates and, because of its position in the soil, was not likely to contribute to the stability of aggregates. The second POM fraction, intraaggregate POM (iPOM), was occluded within aggregates and closely associated with mineral particles. In a followup study, Golchin et al. (1995) reported a strong correlation ( ) between iPOM C and the stability of 1 to 2 mm aggregates isolated from paired, non-tilled, and cultivated soils from five different sites. These observations support the hypothesis that iPOM C is contributing to aggregate stability, however, we could find no reports in the literature that directly compared the concentration of new, iPOM C in stable and unstable aggregates of the same size.

Sieving methods and soil wetting pretreatments prior to sieving can have profound effects on the distribution of C among aggregate size fractions. Suddenly wetting a dry soil causes considerable disruption (slaking) of the soil structure as a result of internal pressure that builds up when air is trapped within the pore spaces of macroaggregates. Only highly stable macroaggregates are able to withstand these forces. In contrast, slowly wetting (e.g., vapor or capillary wetting) a soil prior to wet sieving allows the air to escape with minimal disruption of existing aggregates in the soil. The macroaggregate size classes obtained by the latter approach contain relatively unstable macroaggregates in addition to those that are highly stable. Overall, the stability of macroaggregates that survive capillary wetting is less than the stability of macroaggregates that survive slaking (Kemper and Rosenau, 1986; Cambardella and Elliott, 1993). A comparison of the characteristics of aggregates obtained from these two pretreatments may help in understanding the factors that influence aggregate stability.

In this paper, we report the results from a simulated no-till experiment that was designed to investigate the relative contributions of surface residue and in situ roots to soil organic matter. A combination of a ¹⁴C label, periodic sampling, and physical and chemical fractionation methods allowed us to monitor progressive changes with time in aggregation and the distribution of new, surface residue– or root-derived C. A comparison of the two water pretreatments indicated that root-derived aggregate-¹⁴C and iPOM-¹⁴C concentrations were significantly higher in stable (slaking-resistant) small macroaggregates (25–2000 µm) relative to those in the capillary-wetted pretreatment. The specific objective of this study was to compare concentrations of new C in stable (slaking-resistant) macroaggregates with new C concentrations in macroaggregates isolated after capillary-wetting. We hypothesized that stable macroaggregates have higher concentrations of new POM C relative to less stable macroaggregates.

	Materials and methods

TOP ABSTRACT INTRODUCTION Materials and methods NOTES Results and discussion Conclusions REFERENCES

 
Experimental Setup
The experiment was designed to simulate no-till conditions and consisted of two treatment combinations: (1) ¹⁴C-labeled oat (Avena sativa cv. Ogle) leaves placed on the soil surface of pots containing unlabeled roots and soil (labeled surface residue treatment) and (2) unlabeled oat leaves on the soil surface of pots containing in situ, ¹⁴C-labeled roots and soil [labeled (roots + soil) treatment]. All of the pots were arranged in a completely randomized design in one growth chamber at 25°C. A detailed description of the experimental set-up and ¹⁴C labeling procedure was reported previously (Gale and Cambardella, 2000, this issue).

Soil Sampling
Three pots from each treatment were destructively sampled on Days 0, 90, 180, 270, and 360. After the surface residue was removed, the plastic pots were cut away, and the soil and roots were gently separated. Roots that were longer than 2 cm were removed by hand and rinsed with water, although most roots were clean when separated from the soil. The moist soil remaining in each pot was passed through an 8-mm sieve, air dried, and stored at room temperature.

Aggregate Separations
The air-dried 8-mm sieved soil from each pot was systematically mixed to ensure representative subsampling. Four 100-g subsamples of soil from each pot were removed for wet sieving and placed separately on filter paper (150-mm diam.; Whatman 1)¹ in plastic petri dishes (140-mm diam.). Two subsamples were capillary wetted to 280 g H₂O kg^-1 by slowly adding water to the edges of the filter paper and allowing it to absorb into the soil. The other two subsamples were left to air dry. Samples from these treatments will be referred to as capillary-wetted and slaked, respectively. After the water pretreatments were applied, all four subsamples were placed in a refrigerator at 4°C overnight.

The four subsamples were wet sieved to obtain five aggregate size fractions (µm diameter): (i) >2000 (ii) 250 to 2000, (iii) 53 to 250, (iv) 20 to 53, and (v) <20. Soils were submerged in water on the 2000-µm sieve for 5 min before sieving. Soils were then sieved under water by moving the sieve 3 cm vertically 50 times during a period of 2 min, being careful to break the surface of the water with each stroke. The material retained on the sieve was backwashed into an aluminum pan. Soil plus water that passed through the sieve was poured on to the next finer sieve and the process repeated. The number of vertical movements was reduced to 35 times and 10 times for the 53- and 20-µm sieves, respectively. The soil slurry that passed through the 20-µm sieve was captured in a receiving pan. All size fractions were dried at 70°C. After drying, the duplicate samples from each pot were combined by size class and prewetting method. Subsamples from each of these treatment groupings were ground on a roller mill to pass a 250-µm sieve and stored at room temperature.

Fractionation of Organic Matter
The densiometric fractionation sequence used for the separation of frPOM and iPOM is given in Fig. 1 . By definition, POM is retained on a sieve with 53-µm openings (Cambardella and Elliott, 1992); therefore, this fractionation sequence was only applied to subsamples from the three largest aggregate size classes.

View larger version (18K): [in this window] [in a new window]   Fig. 1 Physical fractionation sequence

Each sample was rinsed from the nylon filter into a 225-mL wide-mouth jar using 55 mL sodium polytungstate (Geoliquids Inc., Prospect Heights, IL) adjusted to a density of 1.89 g cm^-3. A preliminary test showed that the water contained in the aggregates would lower the density of the polytungstate to 1.85 g cm^-3. The samples were allowed to separate overnight at room temperature. On the following day, frPOM was aspirated from the surface of the liquid.

Separation of Intraaggregate Particulate Organic Matter
After removal of frPOM, we aspirated off as much of the polytungstate as possible and washed the aggregates into 250-mL polypropylene bottles with 175 mL of water. The samples were centrifuged at 900 x g for 10 min and the liquid was aspirated off. The soil pellet was rinsed into a 120 mL polypropylene bottle with 100 mL of 5 g L^-1 sodium hexametaphosphate and shaken for 18 h on a reciprocal shaker (3.7-cm stroke length, 120 strokes min^-1). The dispersed soil samples were passed through a 53-µm sieve and rinsed thoroughly with water. The material retained on the sieve was backwashed onto a 20-µm nylon filter and a vacuum was applied to remove excess water. Then, the material was rinsed with sodium polytungstate (1.85 g cm^-3) into 100 mL beakers and the volume adjusted to 50 mL. The samples were allowed to separate overnight, and we then aspirated the iPOM floating on the surface of the liquid. The frPOM and iPOM fractions were separately washed with 300 mL of water on a 20-mm nylon filter, transferred to aluminum weighing pans and dried at 50°C. After drying, both fractions were ground to a fine powder in a ball mill and stored at room temperature.

Carbon and Nitrogen Determination
The amount of ¹⁴C in each fraction was measured by combusting subsamples in a Harvey Biological Oxidizer, model OX500 (R.J. Harvey Instrument Corp., Hillsdale, NJ). The ¹⁴C released during oxidation was trapped in Harvey's ¹⁴C cocktail and counted on a 1900 TR Liquid Scintillation Analyzer (Packard Instrument Co., Downers Grove, IL).

In order to make comparisons across aggregate size classes, C concentrations must first be corrected for sand content (Elliott et al., 1991). Microscopic examination of nonground, dispersed soil samples showed a large quantity of silt- to sand-size (20–53 µm) mineral particles in this loess-derived soil. To determine the amount of mineral particles in each size class, we dispersed subsamples of each aggregate size class >20 µm with sodium hexametaphosphate as previously described and then passed the samples through a 20-µm sieve. The material that was retained on the sieve was dried, combusted for 24 h at 430°C to remove the organic matter, and weighed. All reported ¹⁴C concentrations are corrected for >20-µm mineral particles.

Statistical Analysis
Our experiment examined two treatment factors and was analyzed as a split plot. The whole plot factor was water pretreatment (capillary-wetted or slaked) prior to sieving. The aggregate size classes isolated for each water pretreatment were treated as subplots. Thus a total of 10 treatment combinations were measured on five dates (Day 0, 90, 180 270, and 360).

We used two statistical analyses corresponding to the major objectives of the study. First, changes in each variable across time (days) were characterized for each combination of water pretreatment and size class by using regression models that included linear and quadratic components. Second, to characterize differences between stable and unstable aggregates, we compared the mean for each aggregate size class in the slaked pretreatment with the corresponding mean in the capillary-wetted treatment by calculating values for the LSD (Steel and Torrie, 1997). We considered differences significant at .

	Results and discussion

TOP ABSTRACT INTRODUCTION Materials and methods NOTES Results and discussion Conclusions REFERENCES

 
Aggregate Size Distribution
The amount of soil in each size class changed significantly with time for both pretreatments (Table 1) . In the capillary-wetted treatment, the combined weight of the two macroaggregate size classes (>250 µm) increased by 13% during the first 180 d and then declined during the remaining portion of the incubation. We observed the opposite trend in each of the three smaller size classes. In the slaked treatment, the amount of material in the large macroaggregates (>2000 µm) was near zero on all sample dates. Small macroaggregates (250–2000 µm) and large microaggregates (53–250 µm) increased from Days 0 to 180 and then declined. The amount of material in the two smallest aggregate size classes followed the opposite trend. Based on the amount of material retained in the size classes >53 µm in both pretreatments, these data indicate that aggregate stability increased until Day 180 and then declined.

Concentration of Carbon in Aggregates
Labeled Surface Residue Treatment
Surface residue–derived aggregate-¹⁴C concentrations increased rapidly in all aggregate size classes between Days 0 and 90 (Table 2) . In both the capillary-wetted and slaked pretreatments, macroaggregate-¹⁴C (>250-µm) concentrations tended to reach a maximum about Day 180 and then declined. The ¹⁴C concentrations in large and small microaggregates (53–250 and 20–53 µm) generally increased throughout the incubation. In both pretreatments, the concentration of surface residue–derived ¹⁴C tended to be higher in large microaggregates (53–250 µm) compared to the other aggregate size classes for all sample dates. Small microaggregates (20–53 µm) generally had the second highest ¹⁴C concentration.

View this table: [in this window] [in a new window]   Table 2 Changes with time in the concentration of surface residue–derived 14C within aggregate size fractions (after removal of frPOM) under two differnt prewetting treatments. Values are normalized to a sand-free basis

The concentration of iPOM-¹⁴C followed a pattern similar to our observations for surface residue–derived aggregate-¹⁴C (Table 3) . In both the capillary-wetted and slaked pretreatments, iPOM-¹⁴C concentrations of macroaggregates (>250 µm) generally increased until Day 180 and then declined. In large microaggregates (53–250 µm), iPOM-¹⁴C increased significantly throughout the incubation. The concentration of iPOM-¹⁴C was 4.8 to 6.2 times higher in large microaggregates (53–250 µm) compared to the other aggregate size classes in the capillary-wetted treatment. In the slaked treatment, the concentration of iPOM-¹⁴C also tended to be higher in the large microaggregates (53–250 µm) compared with small macroaggregates (250–2000 µm). The iPOM-¹⁴C concentration of small macroaggregates (250–2000 µm) was not significantly different in the slaked compared to capillary-wetted treatment. This suggests that surface residue–derived iPOM C was not a primary factor in the formation and stabilization of small macroaggregates.

View this table: [in this window] [in a new window]   Table 3 Changes with time in the 14C concentration of surface residue–derived iPOM within aggregate size fractions under two prewetting treatments. Values are normalized to a sand-free basis

Labeled (roots + soil) Treatment
Water pretreatment prior to sieving strongly influenced the concentration of root-derived, aggregate-¹⁴C in different size classes (Table 4) . In the capillary-wetted treatment, ¹⁴C concentrations were higher in large microaggregates (53–250 µm) compared to other aggregate size classes, whereas large macroaggregates (>2000 µm) had the lowest ¹⁴C concentration. In contrast, aggregate-¹⁴C concentrations tended to decline with aggregate size in the slaked treatment.

View this table: [in this window] [in a new window]   Table 4 Changes with time in the concentration of root-derived 14C within aggregate size fractions (after removal of frPOM) under two different prewetting treatments. Values are normalized to a sand-free basis

The concentration of root-derived ¹⁴C in most aggregate size fractions changed significantly with time (Table 4). For example, there was a significant decrease in the concentration of root-derived aggregate-¹⁴C in small macroaggregates (250–2000 µm) in both pretreatments. In contrast, the concentration of root-derived ¹⁴C in large microaggregates (53–250 µm) increased significantly with time. These results agree with Angers et al. (1997) who observed that, when labeled wheat (Triticum aestivum L.)straw was incorporated into the soil, the concentration of newly added C in macroaggregates was initially high and then declined, but the concentration of new C in microaggregates increased with time.

Root-derived iPOM-¹⁴C concentrations followed patterns similar to those which we previously described for aggregate-¹⁴C (Table 5) . In both treatments, the concentration of iPOM-¹⁴C decreased significantly with time in small macroaggregates (250–2000 µm), but increased with time in large microaggregates (53–250 µm). In the capillary-wetted treatment, iPOM-¹⁴C concentrations were higher in large microaggregates (53–250 µm) compared to macroaggregates (>250 µm) on all sample dates except Day 0. In the slaked treatment, small macroaggregates (250– 2000 µm) had higher iPOM-¹⁴C concentrations than large microaggregates (53–250 µm).

View this table: [in this window] [in a new window]   Table 5 Changes with time in the 14C concentration of rootderived, iPOM within aggregate size fractions under two prewetting treatments. Values are normalized to a sand free basis

	Conclusions

TOP ABSTRACT INTRODUCTION Materials and methods NOTES Results and discussion Conclusions REFERENCES

 
Our study shows clear differences in the distribution of surface residue– and root-derived C among aggregates of different size and stability under simulated no-till conditions. Root-derived aggregate-¹⁴C and iPOM-¹⁴C concentrations were significantly higher in stable compared to relatively less stable macroaggregates, however, this was not true for surface residue–derived C. For our experimental system, we conclude that root-derived C has an important role in the stabilization of small macroaggregates, whereas new C inputs from surface residue do not contribute significantly to macroaggregate stability under simulated no-till. These data emphasize the potential importance of plant roots and root exudates to the formation of stable macroaggregates in relatively undisturbed systems like no-till. Future research work will address the interaction of plant roots and soil aggregation in more detail.

	NOTES

TOP ABSTRACT INTRODUCTION Materials and methods NOTES Results and discussion Conclusions REFERENCES

 
¹ Reference to trade names and companies is made for information purposes only and does not imply endorsement by the USDA.

Received for publication June 30, 1998.

	REFERENCES

TOP ABSTRACT INTRODUCTION Materials and methods NOTES Results and discussion Conclusions REFERENCES

 

• Allison F.E. Soil organic matter and its role in crop production. New York: Elsevier Publ. Co, 1973.
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• Angers D.A., Recous S., Aita C. Fate of carbon and nitrogen in water-stable aggregates during decomposition of ¹³C¹⁵N-labelled wheat straw in situ. Eur. J. Soil Sci. 1997;48:295-300.
• Cambardella C.A., Elliott E.T. Particulate soil organic matter changes across a grassland sequence. Soil Sci. Soc. Am. J. 1992;56:777-783.[Abstract/Free Full Text]
• Cambardella C.A., Elliott E.T. Carbon and nitrogen distribution in aggregates from cultivated and native grassland soils. Soil Sci. Soc. Am. J. 1993;57:1071-1076.[Abstract/Free Full Text]
• Elliott E.T. Aggregate structure and carbon, nitrogen, and phosphorus in native and cultivated soils. Soil Sci. Soc. Am. J. 1986;50:627-633.
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• Gale W.J., Cambardella C.A. Carbon dynamics of surface residue– and root-derived organic matter under simulated no-till. Soil Sci. Soc. Am. J. 2000;64:190-195 (this issue).[Abstract/Free Full Text]
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• Gupta V.V.S.R., Germida J.J. Distribution of microbial biomass and its activity in different soil aggregate size classes as affected by cultivation. Soil Biol. Biochem. 1988;20:777-786.
• Harris R.F., Chesters G., Allen O.N. Dynamics of soil aggregation. Adv. Agron. 1966;18:107-169.
• Kemper W.D., Rosenau R.C. Aggregate stability and size distribution. In: Klute A., ed. Methods of soil analysis, Part 1, 2nd ed Madison, WI: ASA, 1986:425-442 Agron. Monogr. 9..
• Lynch J.M., Bragg E. Microorganisms and soil aggregate stability. Adv. Soil Sci. 1985;2:133-171.
• Puget P., Chenu C., Balesdent J. Total and young organic matter distributions in aggregates of silty cultivated soils. Eur. J. Soil Sci. 1995;46:449-459.
• Steel R.G.D., Torrie J.H. Principles and procedures of statistics, 3rd ed New York: McGraw-Hill, 1997.

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This Article Abstract Figures Only Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in ISI Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via ISI Web of Science (29) Citing Articles via Google Scholar Google Scholar Articles by Gale, W.J. Articles by Bailey, T.B. Search for Related Content PubMed Articles by Gale, W.J. Articles by Bailey, T.B. Agricola Articles by Gale, W.J. Articles by Bailey, T.B.