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Published online 28 September 2007
Published in Soil Sci Soc Am J 71:1779-1787 (2007)
DOI: 10.2136/sssaj2006.0340
© 2007 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
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FOREST, RANGE & WILDLAND SOILS

Aggregation and Aggregate Carbon in a Forested Southeastern Coastal Plain Spodosol

Deoyani V. Sarkhota, N. B. Comerforda,*, Eric J. Jokelab, James B. Reeves, IIIc and Willie G. Harrisd

a Dep. of Soil and Water Science Univ. of Florida P.O. Box 110290 Gainesville, FL 32611-0290
b School of Forest Resources and Conservation Univ. of Florida P.O. Box 110410 Gainesville, FL 32611-0410
c USDA-ARS, Environmental Management and Byproducts Utilization Lab. Bldg. 306, Rm. 101 BARC-East Beltsville, MD 20705
d Dep. of Soil and Water Science Univ. of Florida P.O. Box 110290 Gainesville, FL 32611-0290

* Corresponding author (nbc{at}ufl.edu).


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Physical protection of C by aggregates and their response to forest management are important components of soil C management. This study was conducted to examine the morphology and strength of aggregates, to quantify C held by aggregates, and to study the effects of forest management intensity on aggregation. Surface horizon soil (0–5- and 5–10-cm depths) was collected from a 4-yr-old loblolly pine (Pinus taeda L.) plantation in North Florida under two contrasting management regimes (intensive vs. operational fertilization and chemical weed control, called high- and low-intensity treatments, respectively). Samples were dry sieved into four size classes: 2000 to 250, 250 to 150, 150 to 53, and <53 µm. Soil aggregates of varying morphology and strength were observed in the three sand size fractions. Aggregate strength, as measured by sonication, varied with size fraction and ranged from approximately 17 J mL–1 for the least stable macroaggregates in the 2000- to 250-µm fraction to 113 J mL–1 for the most stable microaggregates in the 150- to 53-µm fraction. Aggregate organic matter (AOM) was an important C pool in these soils, accounting for nearly half of the total soil organic matter. The high-intensity management treatment had lower AOM in the 2000- to 250-µm fraction, probably due to lower biomass input of understory roots caused by chemical understory control. Modification of the sonication technique proved useful for studying different aspects of aggregation and gave indications of an aggregate hierarchy even in these extremely sandy soils.

Abbreviations: AOM, aggregate organic matter • DRIFTS, diffuse reflectance infrared Fourier-transform spectroscopy • POM, particulate organic matter • SOC, soil organic carbon


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Soil organic C (SOC) management requires an understanding of the processes by which SOC is sequestered. Secondary forests in the southeastern United States, covering an area of 1.4 million km2, accumulated C at a rate >70 million Mg yr–1 (Delcourt and Harris, 1980), identifying them as important regional C sinks. Richter et al. (1995) reported that a 34-yr-old loblolly pine (Pinus taeda L.) plantation in South Carolina sequestered C at a rate of 5.16 Mg ha–1 yr–1. Many of these plantations are underlain by sandy Spodosols (Adegbidi et al., 2002), which represent the dominant soil order in Florida, covering 27% of the state (Stone et al., 1993). Many Spodosols are exceptionally sandy, with <5% silt plus clay and <10 cmolc kg–1 of cation exchange capacity (Carlisle et al., 1981, 1988, 1989; Sodak et al., 1990).

Forest fertilization and chemical weed control are two management inputs that have increased the productivity of southern pine stands in these landscapes (Albaugh et al., 1998; Jokela et al., 2004) and altered N and P dynamics and cycling (Polglase et al., 1992a,b,c,d; Grierson et al., 1998, 1999; Gurlevik et al., 2004; Meason et al., 2004; Sanchez, 2004). Fertilization has not promoted an increase in SOC (Harding and Jokela, 1994; Shan et al., 2001), however, while chemical weed control has reduced the SOC content (Shan et al., 2001; Echeverria et al., 2004), presumably by reducing detrital inputs of understory plants. The effects of these practices on SOC within aggregates and the development of aggregates have yet to be considered.

Soil organic C can be protected from decomposition through four mechanisms: sorption onto clay (chemical protection), incorporation into aggregates (physical protection), translocation and storage in the subsoil, particularly the Bh horizon, and biochemical transformation into products that are resistant to microbial attack (biochemical protection) (Stone et al., 1993; Six et al., 2002; Blanco-Canqui and Lal, 2004; Jiménez and Lal, 2006). The soil structure of Florida's Spodosols is described as weak crumb to granular or single grain (Carlisle et al., 1981, 1988, 1989; Sodak et al., 1990), suggesting poor soil aggregation. In these soils, the potential for chemical protection of soil C is limited by the low clay content. The low cation content also limits aggregate formation through clay–polyvalent cation–organic matter complexes (Edwards and Bremner, 1967). Aggregates may form under the influence of microbially synthesized products, fungal hyphae, and roots (Tisdall and Oades, 1982; Blanco-Canqui and Lal, 2004); however, little information is available on this topic. Given these factors, the interest in aggregation in sandy Spodosols has been low, as evidenced by the few studies addressing the topic.

In Russian Spodosols, aggregation responded to agricultural management. Water-stable aggregation and total SOC content increased after 2 yr under the influence of a grass–clover mixture, but decreased when followed by spring wheat (Triticum aestivum L.; Buchkina and Balashov, 2001). In Florida, soil aggregation influenced P extractability. Higher water-extractable P and heavy metals, along with slower rates of release, were found in the 500- to 250- and 250- to 125-µm aggregates than the smaller size fractions (Zhang et al., 2003). Sandy Spodosols in Florida represent a unique soil condition and it was deemed necessary to better understand aggregation under these conditions.

The purpose of this study was to develop a better awareness of aggregation and its relation to SOC in a representative forested Spodosol of northern Florida. The first specific objective was to observe the type of aggregation present in these soils. We hypothesized that aggregation, albeit weak, was present in the <2-mm fraction in these extremely sandy soils because of the high input from root turnover and aboveground litter. The second objective was to determine the strength of aggregates in the <2-mm fraction, and quantify the amount of aggregate C. The hypotheses related to this objective were (i) that aggregate strength, as measured by an aggregate's resistance to dispersion, would increase with decreasing aggregate size, and (ii) that particulate C would be the dominant pool of SOC. The first hypothesis grew from the concept that smaller particles have greater surface area available for binding; hence, the aggregate's strength and stability would be greater. The greater stability of microaggregates compared with macroaggregates has been reported by others (Edwards and Bremner, 1967; Tisdall and Oades, 1982). The second hypothesis recognized that SOC could be found as either particulate organic matter (POM) or aggregate organic matter (AOM). Given the weak structure often described for Coastal Plain Spodosols, the dominance of POM was expected.

The third objective of this research was to provide preliminary information on the short-term influence of two contrasting management intensities on the amount and distribution of AOM and POM. The hypothesis for this objective was that more intensive management (more fertilization and chemical weed control) would equate to decreases in aggregation. It was expected that reduced root turnover of understory plants, resulting from sustained chemical control, would cause reduced aggregation and short-term reductions in soil C and aggregation. Reports on the effects of fertilization on aggregation vary, but the reduction of soil C due to chemical weed control was reported by Shan et al. (2001) and Echeverria et al. (2004).


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Experimental Site
A 4-yr-old loblolly pine plantation in North Florida (30°24'N, 82°33'W) was the study site. This study was installed in a recently harvested slash pine (Pinus elliottii Engelm.) plantation that was managed for pulpwood. The site is currently managed by the Forest Biology Research Cooperative at the University of Florida as part of the Pine Productivity Interactions Experimental Study (Roth et al., 2007). This long-term study aims at understanding the genotype x environment interactions in full-sib loblolly and slash pine families. The climate is warm, humid subtropical, with 1394-mm average annual rainfall, 27°C average annual maximum temperature, and 13°C average annual minimum temperature. Approximately 70% of total mean annual rainfall occurs between the months of March and September (National Climatic Data Center, 2002). In the months of March, June, September, and December, the maximum temperatures are 24.5, 32.9, 31.7, and 20.2°C; the corresponding minimum temperatures are 9.6, 20.3, 20, and 6.1°C, respectively.

The soils are classified in the U.S. taxonomic system as belonging to the Leon series (sandy, siliceous, thermic Aeric Alaquods), and are characterized by low silt plus clay (<5%) and low cation exchange capacity (<10 cmolc kg–1). The Leon series consists of poorly to very poorly drained, sandy soils with a Bh horizon beginning within 76 cm of the soil surface. The water table is as high as 15 to 45 cm below the soil surface for 1 to 4 mo during most years. The SOC content in the entire soil profile was not recorded for this particular location; however, a study of Florida Spodosols (Stone et al., 1993) reported that the SOC density in the soil profile (1-m depth, mean ± SD) was 10.4 ± 0.8 g kg–1 and the SOC concentrations in the A, E, Bh, and B'h horizons were 19 ± 20, 1.9 ± 2.1, 17 ± 12, and 8.5 ± 5.0 g kg–1, respectively.

The trees were planted in January 2000 in four replicate blocks using a randomized complete block, split-plot design. Each treatment plot (excluding the border trees) was 0.016 ha for the close-spacing treatment and 0.036 ha for the wide-spacing treatment. There were eight rows in each treatment plot. The total area of the study was approximately 10 ha. Before planting, the entire study was double bedded and treated with imazapyr (2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-3-pyridinecarboxylic acid, 1.02 L ha–1) and triclopyr ([(3,5,6-trichloro-2-pyridinyl)oxy]acetic acid, 7.02 L ha–1) to remove the understory vegetation and to provide a competition-free environment. The woody vegetation in the understory included sawtooth palmetto [Serenoa repens (W. Bartram) Small], wax myrtle (Myrica cerifera L.), runner oak (Quercus pumila Walter), blueberry (Vaccinium spp.), gallberry [Ilex glabra (L.) A. Gray], and St. John's-wort (Hypericum fasciculatum Lam.), while the herbaceous species included bluestem grasses (Andropogon spp.), panicum grasses (Panicum spp.), sedges (Carex and Cyperus spp.), and dogfennel [Eupatorium capillifolium (Lam.) Small].

The experimental design was a 2 x 2 x 8 factorial, which included two planting densities (close spacing at 1.22 by 2.75 m and wide spacing at 2.75 by 2.75 m with tree densities of 2990 vs. 1334 trees ha–1), two management regimes (high and low inputs of fertilization and understory control), and eight loblolly pine full-sib families (six elite growing families, a mix of these elite families, and one poor growing family). The family designations were chosen a priori based on their aboveground growth performance in long-term genetic experiments. The low-management regime approximated current operational management practices and included a one-time fertilizer application and understory competition control treatment at the time of planting. The fertilization of the low-management regime included 45 kg ha–1 N and 51 kg ha–1 P applied as diammonium phosphate and the understory competition control was achieved using imazapyr and triclopyr as described above. The high-management regime included sustained understory competition control using herbicides and annual fertilization with a complete fertilizer that included most micronutrients. The nutrients added during the study period included 368 kg ha–1 N and 128 kg ha–1 P applied as urea and diammonium phosphate. Other essential nutrients were 121 kg ha–1 K, 45 kg ha–1 Mg, 45 kg ha–1 Ca, 35 kg ha–1 S, 0.89 kg ha–1 B, 3 kg ha–1 Zn, 2 kg ha–1 Mn, 16 kg ha–1 Fe, 4 kg ha–1 Cu, and 0.01 kg ha–1 Mo. Directed applications of imazapyr (0.28 L ha–1) and sulfometuron (2-[[[[(4,6-dimethyl-2-pyrimidinyl)amino]carbonyl]amino]sulfonyl]benzoic acid, 0.14 L ha–1) were used as needed to provide sustained understory competition control for the first 2 yr. Afterward, the ground cover was kept below 30% through age 3 yr. The entire study was treated when necessary with insecticides diflubenzuron (N-[[(4-chlorophenyl)amino]carbonyl]-2,6-difluorobenzamide, 292 mL ha–1), permethrin [(3-phenoxyphenyl)methyl 3-(2,2-dichloroethenyl)-2,2-dimethylcyclopropanecarboxylate, 439 mL ha–1], or tebufenozide [3,5-dimethylbenzoic acid 1-(1,1-dimethylethyl)-2-(4-ethylbenzoyl)hydrazide, 585 mL ha–1] for tip moth (Rhyacionia spp.) control during the first growing season.

Out of these treatments, two treatment combinations representing the maximum differences in biomass production were selected to evaluate the capacity for short-term SOC changes, with the idea that the differences in input would be reflected in differences in the SOC pools. The high-intensity treatment included the most productive family under the high-management regime, while the low-intensity treatment was represented by the poorest performing family under the low-management regime. The closest spacing (1.22 by 2.75 m, tree density 2900 trees ha–1) was chosen to maximize the biomass input. At age 5 yr, the best and poorest growing families differed by 21% in stem volume and 11% in aboveground biomass (Roth et al., 2007).

Soil samples were collected in September 2003 from the A horizon at depth increments of 0 to 5 and 5 to 10 cm. One composite sample representing four individual samples was collected from each of the two depths in the high- and low-intensity treatment plots in three replicate blocks. The four individual soil samples were randomly collected from alternate interbed rows. Soil was collected by removing a monolith with a shovel and then separating the soil depths with a ruler. Although subsurface horizons are known to store significant amounts of SOC in these soils (Stone et al., 1993), this study focused on the surface horizons only. The plantation was 4 yr old at the time of sampling and the surface soil was expected to exhibit treatment effects most clearly due to the differences in litter fall and fine roots added to the soil.

Laboratory Methods
Sieving
Soil samples obtained from the field were first air dried and passed through a 2-mm sieve. They were then dry sieved on a horizontal mechanical shaker for 5 min at 75 rpm, using 100 g of air-dried soil and a stack of three 20-cm (8-inch)-diameter brass sieves of sizes 250, 150, and 53 µm. This duration was used to achieve size fractionation with minimum disruption of aggregates. For the soil used in this study, there was little difference in the size distribution after 5 and 20 min of shaking (unpublished data). Three aggregate size fractions including a macroaggregate fraction 2000 to 250 µm, two microaggregate fractions 250 to 150 and 150 to 53 µm, and a silt plus clay size fraction <53 µm were obtained. The >2000-µm fraction was ground and analyzed for C and N, but was not used for aggregate studies since it was composed only of large units of POM.

Light and Electron Microscopy
The first part of the investigation was a microscopic examination of aggregates in the dry-sieved size fractions to address the first objective. Aggregate samples were examined and photographed using a dissecting light microscope (Carl Zeiss 475003–9902, Carl Zeiss Inc., Thornwood, NY) with mounted digital camera (Sony MVC FD90). A scanning electron microscope (JEOL JSM 6400, JEOL, Tokyo) equipped with an energy-dispersive x-ray fluorescence elemental microanalysis system was used for obtaining images and Si dot maps within aggregates. Samples were prepared for scanning electron microscopy by mounting on C stubs and coating with C. The Si dot maps show the spatial distribution of Si within the aggregate. A concentration of dots in the figure represents a quartz sand grain.

Quantifying Aggregate Carbon by Sonication
Sonication was used to address the second objective, that is, to examine aggregate strength and to determine the quantity of C contained in stable aggregates. It has been used for aggregate disruption by others (North, 1976; Christensen, 1992; Cambardella and Elliott, 1993; Six et al., 2001; Swanston et al., 2005) because, unlike chemical dispersion techniques, it avoids chemical modification of the organic matter. It also allows measurement of aggregate strength on an energy basis, which allows quantitative comparison of samples.

This analysis was performed on the three sand-sized fractions (>53 µm) in a water–soil system. The sonic energy inputs to the size fractions ranged from 0 to 27,000 J and were achieved with a Sonic Dismembrator (Model 500, Fisher Scientific, Hampton, NH) by using a range of amplitude (20–60%) and time (1–7 min) combinations. The energy output of the sonicator was calculated by internal software using voltmeter readings every 10 s (Fisher Scientific, personal communication, 2006). The energy output thus calculated was replicable (coefficient of variation <10%). The pulse method (60 s on and 30 s off) was used to avoid an excessive rise in temperature.

The actual energy output was calibrated calorimetrically by using a Dewar vessel as described by Schmidt et al. (1999) and a correction factor was developed to convert the instrument energy output into actual energy input to the soil–solution system. Instead of using the peak power output values (watts) for calibration, however, we used the total energy output values (joules) for each individual run, since the peak power values were found to change considerably for a given amplitude setting. Schmidt et al. (1999) compared different instruments and found that the actual power supplied ranged from 29 to 85% of the power output given by the instrument. The value calculated in the current study was 70% for the Sonic Dismembrator Model 500 (R2 = 0.99). The dispersion energy was calculated as E = 0.7E'/V, using the energy output given by the instrument for each run (E') and the suspension volume (V), calculated by using the soil weight for each run and a particle density of 2.65 g cm–3 for these quartz-dominated soils (Harris and Carlisle, 1987).

Each size fraction was sonicated at incremental energy levels until complete aggregate breakdown was achieved. This was accomplished by using one subsample for each energy level. Microscopic observation and release of soil organic matter (SOM) was used to ensure that all the aggregates were disrupted. This analysis was repeated on the three sand-size fractions of 12 soil samples representing three replications of both management intensities and soil depths. For each sample, sonication was applied at 9 to 11 energy levels. For each subsample described above, 2 g of soil was weighed into a 250-mL beaker, to which 100 mL of distilled water was added. The suspension was sonicated at the desired energy level. The depth of immersion of the sonicator probe was kept constant at 10 mm, as this variable is known to influence the degree of disruption (North, 1976). The suspension was then passed through the same-sized sieve used to obtain the size fraction (e.g., 250-µm sieve for the 2000- to 250-µm fraction). The SOM remaining on the sieve and the SOM passing through the sieve were measured by loss-on-ignition.

Organic matter passing through the sieve after sonication was termed AOM, as it contained finer organic matter held inside the aggregates, which was released after aggregate disruption. It was expressed as a percentage of the total organic matter in each sample. Organic matter remaining on the sieve after sonication was termed POM, which, after complete aggregate dispersion, contained SOM of the same particle size. Microscopic observation of the samples after sonication at the highest energy level exhibited intact POM [Fig. 1(9) ] and thus, while the breakdown of POM by sonication was possible, although not previously observed, it was assumed to be negligible. Energy input was plotted against AOM to obtain the response curve for each sample. The AOM removed after addition of water without any sonic energy input (0 J) represented the organic matter associated with water-dispersible aggregates.


Figure 1
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  		Fig. 1. Light microscope and scanning electron microscope and microprobe observations of soil aggregation from 0- to 5- and 5- to 10-cm depths of an Aeric Alaquod under a 4-yr-old loblolly pine plantation in North Florida. Figures 1(1) to 1(4) are irregularly shaped aggregates showing fungal hyphae, organic debris, and mineral matter enmeshed together. The upper row shows scanning electron microscope (SEM) images. Magnification: 1(1) = 87x, 1(2) = 55x, 1(3) = 295x, 1(4) = 217x. The lower row shows SEM probe analysis of Si. The Si dot maps show the spatial distribution of Si within the aggregate. A concentration of dots in the figure represents a quartz sand grain. Figures 1(5) to 1(7) show light microscope images of spherical aggregates, which are encrustations of mineral matter on organic debris, combined with fungal hyphae and fine roots. Figures 1(8) and 1(9) are light microscope images of the 250- to 150-µm fraction. Figure 1(8) shows the soil after dry sieving, while Fig. 1(9) is the same soil after sonication, with clean sand grains and particulate organic matter but no aggregates.

 
Diffuse Reflection Infrared Fourier-Transform Spectroscopy Analysis
The chemical nature of AOM and POM was investigated using diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS). The DRIFTS analysis presented in this study was exploratory and it was performed on the 250- to 150-µm fraction samples of two block replicates representing the low management intensity and the 5- to 10-cm depth. Samples were scanned before and after ashing (at 550°C for 6 h) in the mid-infrared on a Digilab FTS-7000 Fourier-transform mid-infrared spectrometer (Varian Instruments, Walnut Creek, CA). Samples were scanned from 4000 to 400 cm–1 at 4 cm–1 resolution using a KBr beamsplitter and deuterated triglycine sulfate (DTGS) detector and a Pike Autodiff autosampler (Pike Technologies, Madison, WI) using ground, non-KBr diluted samples. Spectral subtraction of ashed samples from unashed samples was performed to accentuate the differences in organic matter characterization using GRAMS/AI software, Version 7.02 (Thermo Galactic, Salem, NH).

Statistical Analysis
The PROC MIXED procedure (SAS Institute, 2001) was used to contrast the effects of management intensity and depth based on a completely randomized design with energy level (i.e., each unique combination of amplitude and time), size fraction, management intensity, and soil depth as fixed effects and block as a random effect. The differences were considered significant if P < 0.05 was met. If there was a significant energy level x management intensity x size fraction interaction, further statistical analyses were run separately for each size fraction. A multiple post-hoc comparison procedure with the Tukey–Cramer adjustment was used.


   RESULTS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Aggregate Morphology
Microscopic observation identified aggregation that could be qualitatively categorized into two groups. Irregular-shaped aggregates [Fig. 1(1)–1(4)] had mineral matter, organic debris, and fungal hyphae or fine roots enmeshed together. Spherical aggregates exhibited mineral matter encrusted on organic debris or plant remains with [Fig. 1(5)] or without [Fig. 1(6)] fungal hyphae or fine roots. The internal structure of the aggregates, as shown by the Si dot maps [Fig. 1(1)–1(4)], indicated mineral matter embedded in the organic matter. Images suggested a role of fungal hyphae and fine roots in aggregate formation, either through mechanical meshing [Fig. 1(1)–1(5)] or through encrustation of mineral matter on plant remains [Fig. 1(7)].

Quantifying Soil Organic Carbon in Aggregates
On average, 45% of the total SOC (Table 1 ) was contained in the soil aggregates. As the energy input to the soil sample increased, aggregates were destroyed, increasing the amount of AOM removed from that size class of soil material (Fig. 2 ). Eventually a plateau was reached, indicating that all the organic matter that could be removed from the aggregates had been removed (Fig. 3 ). This finding was supported by microscopic observations [Fig. 1(8) and 1(9)], which indicated that only POM was present when the plateau was reached. The energy level at which this plateau was reached exceeded the strength of the most stable aggregates in that size fraction. The dispersion energy required for the complete breakdown of aggregates (Fig. 3) increased in the order 250 to 150 µm (59 J mL–1) < 2000 to 250 µm (92 J mL–1) < 150 to 53 µm (113 J mL–1).


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  		Table 1. Amount of organic C held in soil aggregates from the 0- to 5- and 5- to 10-cm depths of an Aeric Alaquod under a 4-yr-old loblolly pine plantation in North Florida.

 

Figure 2
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  		Fig. 2. Effect of sonication energy input on the removal of aggregate organic matter (AOM, % of total OM in size fraction) of the 150 to 53-µm fraction from 0 to 5 and 5 to 10-cm depths of an Aeric Alaquod under a 4-yr-old loblolly pine plantation in north Florida. The error bars represent the range of values (n = 12 samples), while the box represents interquartile range (upper quartile = 75th percentile, lower quartile = 25th percentile). The plus sign in the box represents the mean and the line in the box represents the median.

 

Figure 3
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  		Fig. 3. Removal of aggregate organic matter (AOM, as a percentage of total organic matter in the size fraction) with increasing dispersion energy for the various soil size fractions. Soils are from 0- to 5- and 5- to 10-cm depths of an Aeric Alaquod under a 4-yr-old loblolly pine plantation in North Florida. The vertical lines indicate the steps in the continuity of organic matter removed from aggregates for the 2000- to 250- and 250- to 150-µm fractions. The error bars represent the standard error (n = 12).

 
The shapes of the response curves were different (Fig. 3) among the three size fractions, which indicated diversity in the nature of the aggregates. The aggregates in the 250- to 150-µm fraction were the least stable among the three fractions, losing the highest proportion of their organic matter at all energy levels. The 150- to 53-µm fraction had the most stable aggregates (Fig. 3, Table 2 ). The 2000- to 250- and 250- to150-µm fractions (Fig. 3) exhibited a stepwise removal of organic matter, with steps at 17 and 41 J mL–1 (Fig. 3). The 150- to 53-µm fraction (Fig. 2 and 3) exhibited a continuous spectrum of organic matter removal with increasing energy input. The variability of organic matter release from aggregates, as shown by the coefficients of variation (Table 2), decreased with decreasing fraction size, while within a fraction, the variability decreased with increasing energy output. The highest variability was observed at 0 J (i.e., in the water-dispersible aggregates). All three size fractions were susceptible to organic matter removal by wetting, with values ranging from 5 to 17% (Table 2).


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  		Table 2. Dispersion energy and the amount of organic matter removed from each soil size class (2000–250, 250–150, and 150–53 µm) as aggregate organic matter (AOM) for each energy level. Aggregates are from the 0- to 5- and 5- to 10-cm depths of an Aeric Alaquod under a 4-yr-old loblolly pine plantation in North Florida.

 
The exploratory DRIFTS spectra separated the POM and AOM fractions (Fig. 4 ) in the 250- to 150-µm fraction. The AOM fraction exhibited higher quantities of polysaccharides (1160 cm–1), aromatic compounds (1500 cm–1), esters (1730 cm–1), and amides (1650 cm–1) than the POM fraction. The negative peak at 1350 cm–1 on the POM spectra was due to distortions caused by the spectral subtraction and can be ignored. The peaks at 2880 and 2950 cm–1 were due to aliphatic-CH groups. Peak heights indicated higher content of these materials in the AOM component.


Figure 4
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  		Fig. 4. Diffuse reflectance infrared Fourier-transform spectra showing characteristics of particulate organic matter (POM) and aggregate organic matter (AOM) of the 250- to 150-µm fraction. The data show samples of two block replicates from the 5- to 10-cm depth of an Aeric Alaquod under a 4-yr-old loblolly pine plantation under low-intensity management in North Florida. The AOM spectra show peaks of: (a) polysaccharides, (b) aromatics, (c) esters, (d) amides, and (e) C–H bonds.

 
Effect of Management Intensity
The statistical analysis showed that the effects of energy input and size fractions were significant (P < 0.01). There was also a significant energy level x management intensity x size fraction interaction (P = 0.02). The effect of management intensity on AOM was statistically significant in the 2000- to 250-µm fraction (Table 3 ), with an energy level x management intensity interaction (Fig. 5 ). At lower energy levels, treatment differences were small. Beyond 41 J mL–1, however, the low-intensity treatment had higher AOM. The energy level x management intensity interaction was also significant for the 250- to 150-µm fraction, but the absolute difference between the management intensities was <4%. The 150- to 53-µm fraction was not affected by the intensity of management (Table 3).


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  		Table 3. Effect of forest management intensity and soil depth on aggregate organic matter in the 2000- to 250-, 250- to 150-, and 150- to 53-µm fractions. Soils were from the 0- to 5- and 5- to 10-cm depths of an Aeric Alaquod under a 4-yr-old loblolly pine plantation in North Florida. Since there was a significant fraction x management intensity x energy level interaction (P = 0.02), the effects are shown for each individual fraction.

 

Figure 5
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  		Fig. 5. Effect of forest management intensity on the amount of aggregate organic matter (AOM, as a percentage of total organic matter in the size fraction) for the 2000- to 250-µm size fraction. Soils are from the 0- to 5- and 5 to 10-cm depths of an Aeric Alaquod under a 4-yr-old loblolly pine plantation in North Florida. The error bars represent the standard error (n = 6).

 

   DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
CONCLUSIONS
 REFERENCES
 
Aggregate Morphology
The microscopic observations confirmed the presence of aggregates and a range of aggregate forms (Fig. 1) in the three sand size fractions. The images further implicated the role of fungal hyphae and fine roots in aggregation [Fig. 1(1)–1(5)], while the DRIFTS spectra of the 250- to 150-µm fraction suggested polysaccharides as an aggregate binding agent (Fig. 4). The biological agents of aggregation are important in sandy soils (Oades, 1993), while the abiotic mechanisms of aggregation are most important in soils with clay contents >15% (Horn, 1990). The binding action of polysaccharides, secreted by fungi or bacteria, has also been reported by others (Tisdall, 1994; Caesar-Tonthat, 2002; Blanco-Canqui and Lal, 2004). Our observations warrant additional studies on the significance of roots, fungi, and polysaccharides for aggregation and physical protection of SOC in these sandy soils.

Aggregate Stability and Organic Matter Content
The dispersion energy levels reported in this study (17–113 J mL–1) were comparable to the values reported by other researchers; however, the values of AOM were difficult to compare because fractionation schemes used by researchers varied widely. For example, the dispersion energy levels were similar to those for macroaggregates in Mollisols (22 J mL–1; Cambardella and Elliott, 1994), but significantly lower than those reported for Ultisols (200 J mL–1; Swanston et al., 2005) and Oxisols (>825 J mL–1; Roscoe et al., 2000), as would be expected. This indicates that Spodosol aggregates are worthy of more scientific scrutiny, yet the wide range of stability values among soils also indicates that more detailed investigations of the controlling mechanisms of aggregate stability are necessary to determine the extent of physical protection offered by each set of aggregates.

Aggregates, through physical occlusion, protect organic matter from destructive agents such as physical breakdown by tillage, removal of finer particles by erosion, or decomposition by soil organisms (Foster, 1988; Carter, 1992; Barthes and Roose, 2002; Blanco-Canqui and Lal, 2004; Malhi et al., 2006). Not all aggregates offer protection from all of these agents. The extent of protection depends on the size of pores within the aggregates and the strength of the aggregates, which in turn depends on the binding agents and the size of the primary particles in the aggregate. For example, macroaggregates are reported to be sensitive to management influences while microaggregates are reported to offer long-term protection to SOC (Six et al., 2004). The aggregate strength, as measured by sonication, should indicate the extent of mechanical protection (e.g., from breakdown by tillage), but the extent of protection from soil microbes is not certain.

The hypothesis (second objective) suggesting an inverse relationship between aggregate size and stability was rejected because the 250- to150-µm fraction was less stable than either the 2000- to 250- or 150- to 53-µm fractions (Fig. 3). The higher aggregate strength of the 2000- to 250-µm fraction than the 250- to 150-µm fraction, which was counter to our hypothesis, is probably a function of a microaggregate–macroaggregate hierarchical structure (see below; Oades and Waters, 1991), although further study is needed to substantiate the functioning mechanism. The hypothesis suggesting the dominance of POM was only partially supported since this fraction accounted for just over half of the total organic matter.

Aggregate organic matter is an important pool of soil C; however, POM may be more important than AOM in determining the short-term turnover of essential nutrients, as many researchers have reported the responsiveness of POM (also called the free or occluded light fraction or free low-density organic matter; Romkens et al., 1999; Swanston et al., 2005; Gregorich et al., 2006; Liao et al., 2006).

Effect of Management Intensity
The AOM in the 2000- to 250-µm size fraction was reduced by intensive management (Fig. 5). Therefore, the hypothesis of a short-term reduction in aggregation by intensive management was accepted. The effect of management intensity on aggregate C could be partly attributed to changes in fine root biomass, since intensive management, especially chemical control of understory plants, has been reported to decrease the fine root biomass and length (Escamilla et al., 1991; Shan et al., 2001). It is unclear, however, whether these differences will be sustained over time. The higher levels of productivity and C inputs reported under intensive management (Dalla-Tea and Jokela, 1991; Jokela and Martin, 2000; Will et al., 2002; Martin and Jokela, 2004; Samuelson et al., 2004) could result in longer term opportunities for higher aggregation.

Methodological Considerations
The response of the 2000- to 250-µm size fraction to management intensity illustrated the sensitivity of this method for detecting soil C changes in as few as 4 yr after treatment. It also used operationally defined C fractions that could be related to meaningful C pools. The two pools separated by this procedure within the 250- to 150-µm size fraction, POM and AOM, showed a marked difference in their chemical composition as indicated by the DRIFTS spectra. The AOM showed a higher content of polysaccharides, phenols, esters, and amides, of which polysaccharides have already been shown to function as binding agents (Tisdall, 1994). The higher aromatic C content also suggested the presence of more biochemically inert organic matter. Higher amounts of esters and amides, on the other hand, suggested that this fraction would be susceptible to decomposition if the aggregates were destroyed, since esters and amides are decomposable C forms. For example, Gallo et al. (2005) reported up to 51% reduction in esters (carbonyl peak area in Fourier-transform infrared spectra) in litter from a sugar maple (Acer saccharum Marshall ssp. saccharum)–basswood (Tilia americana L. var. americana) forest in Michigan in just 2 yr, while Smidt et al. (2002) reported a significant reduction in the amide peak during sewage sludge decomposition in as little as 23 wk. The differences in chemical composition indicate that this method may be useful in separating soil C into more functional pools and lead to better conceptualization of the cycling of SOC when used in conjunction with chemical characterization studies.

North (1976) used a similar method of aggregate strength measurement, which has been reproduced by others (Schmidt et al., 1999; Roscoe et al., 2000); however, North (1976) used the amount of clay removed as an indicator of aggregate destruction, which made it unsuitable for the highly sandy soils examined in this study. The extremely low clay content in these soils hinders the accurate measurement of clay removed by incremental sonication energy input. Clay can also become saturated with organic matter (Hassink et al., 1997), particularly in soils with low clay content and, therefore, it is not necessarily an appropriate measure for protection of C in these soils. The use of organic matter removed after sonication, as reported in this study, eliminates this difficulty. This method also allows simultaneous measurement of aggregate C. Since this method can separate the aggregate and nonaggregate C pools with minimal chemical alteration, it can also be used to study different aspects such as age or mineralizability of the aggregates of varying stability.

Aggregate Structure in Coastal Plain Spodosols—Additional Considerations
Structure in Spodosols has been described as weak (Carlisle et al., 1981, 1988, 1989; Sodak et al., 1990). Consequently, aggregation in these soils has received little, if any, attention. Oades and Waters (1991) studied the patterns of aggregate breakdown in Mollisols, Alfisols, and Oxisols. In Oxisols, the aggregates broke down to release primary particles, whereas in Mollisols and Alfisols, the researchers reported a hierarchical structure. The larger, weaker aggregates broke down to release smaller, stronger aggregates before breaking down into primary particles. Results from the present study indicated that the surface horizon of sandy Spodosols also had an aggregate hierarchy, as exhibited by the stepwise breakdown of aggregates in the 2000- to 250- and 250- to 150-µm fractions (Fig. 3). In the 2000- to 250-µm fraction, the least stable aggregates were disrupted at energies ≤17 J mL–1. At energy levels >17 J mL–1, a plateau in the percentage of organic matter indicated that there was no further aggregate destruction until 26 J mL–1. We interpret this energy level as the threshold for aggregates of second order, which were destroyed between 26 and 41 J mL–1. A second plateau was observed between 41 and 52 J mL–1. This was the threshold for the third order of aggregates, which started to break down at 52 J mL–1 and the removal went on until 92 J mL–1 when all the aggregates in this fraction were destroyed. In the 2000- to 250-µm fraction, the steps were well defined, indicating a well-developed structure. In the 250- to 150-µm fraction, although the first two steps were observed at the same energy levels, the steps were less distinct and all the aggregates were destroyed at 59 J mL–1, indicating poorly developed structure.

The 150- to 53-µm fraction exhibited a continuous spectrum, suggesting that this fraction was simply a continuum of aggregates of different stabilities. This behavior was similar to the pattern of clay removal reported by North (1976) and Schmidt et al. (1999), although no studies have reported the pattern of organic matter removal with sonication. In these soils, it appeared to be unique for this fraction and indicated the need for further attention, especially since this C pool remained unaffected by management intensity.

Edwards and Bremner (1967) defined microaggregates as the water-stable aggregates bound by strong clay–polyvalent metal–organic matter complexes. These reseachers used 250 µm as the separation point between macro- and microaggregates. Tisdall and Oades (1982) also used 250 µm as the separation point between micro- and macroaggregates. They described microaggregates as those stabilized by organo–mineral complexes and resistant to disruption by wetting, cultivation, or other disturbances. In contrast, macroaggregates were described as those stabilized by roots and fungal hyphae, having varying stability depending on management and other factors. Apparently, the separation point at 250 µm was chosen based on the increase in aggregate strength and difference in binding agents. The similar behavior of the 2000- to 250- and 250- to150-µm fractions, as well as the low stability and presence of roots and fungal hyphae in the 250- to 150-µm fraction, suggests that 150 µm is a more appropriate separation point for these soils. The 150- to 53-µm fraction, although stable and unaffected by management intensity, was also susceptible to removal by wetting. Therefore, size alone is insufficient to define aggregate types in these sandy-textured soils. A quantitative measure of aggregate strength, such as the energy required to achieve complete aggregate disruption, can be used to improve the definition. It should be noted that sonication was used to measure mechanical strength. Chemical modifications may break aggregates before the threshold for mechanical failure is reached. These factors, in addition to the limited potential for clay or cation binding, indicate the necessity of a different approach for studying aggregation in sandy Spodosols.


   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
REFERENCES
 
This study found that aggregates form in forested, sandy Spodosols and have a hierarchal structure in the large soil size fractions. The use of organic matter release instead of clay release after aggregate breakdown by sonication was useful for studying aggregate properties. It allowed simultaneous measurement of aggregate strength and the amount of aggregate C. Aggregate C was an important pool in these soils and the intensive management regime used to enhance pine plantation productivity reduced this C pool significantly in the short term. Results from this study highlight the necessity for innovative approaches for studying aggregation in sandy soils and the need to assess the long-term, management-related changes in these C pools.


   ACKNOWLEDGMENTS
 
We wish to thank the Forest Industry members of the Forest Biology Research Cooperative, School of Forest Resources and Conservation, University of Florida, for their monetary support for this research with particular thanks to International Paper Co. for the use of the study on their lands. We also wish to acknowledge the Major Analytical Instrumentation Center, Department of Materials Science and Engineering, University of Florida, for the use of the scanning electron microscope.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher.

Received for publication September 29, 2006.


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





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