C. A. Cambardella's research while affiliated with Colorado State University and other places

The amount of organic matter present in soil and the rate of soil organic matter (SOM) turnover are influenced by agricultural management practices. Because SOM is composed of a series of fractions, management practices will also influence the distribution of organic C and N among SOM pools. Our study examined SOM fractions that are occluded within the aggregate structure. Aggregates were disrupted by sonication and the disrupted soil suspensions were passed through a series of sieves to isolate size fractions. Densiometric separations were carried out on the size fractions, creating size-density fractions. Fine-silt-size particles having a density of 2.07 to 2.22 g/cm3 isolated from inside macroaggregates contained the highest percentage of total soil C and N for all cultivation treatments and, because of its properties, will be referred to as the enriched labile fraction (ELF). As cultivation intensity was reduced, the amount of N in the ELF increased from 110 mg N/kg in the bare fallow treatment to 405 mg N/kg in the no-till treatment. About 5% of the N in the ELF was mineralized during a 28-d laboratory incubation, averaged across treatments. The proportion of N mineralized from the ELF (4.7%) was significantly higher than from intact macroaggregates (2.1%), which suggests this fraction may be protected from decomposition within the aggregate structure. We postulate that the ELF is a byproduct of microbial activity and that it contributes to binding microaggregates into macroaggregates in cultivated grassland soils.

This study examined the effects of bare fallow, stubble mulch fallow, and no-till fallow management on aggregate size distribution and organic C and N contents. Mineral-associated organic matter was isolated by dispersing in sodium hexametaphosphate and removing the sand and particulate organic matter (POM) by passing through a 53-μm sieve. A large proportion of the total soil dry weight (50-60%) was isolated in the 250-2000 μm aggregate size class. The native grassland soil was more stable than the cultivated soils when slaked, and the no-till was more stable than the stubble mulch and bare fallow soil when slaked. The data reported relates the loss of structural stability from cultivation to losses of organic C and N from the POM fraction. -from Authors

Turnover of soil organic matter (SOM) is coupled to the cycling of nutrients in soil through the activity of soil microorganisms. Biological availability of organic substrate in soil is related to the chemical quality of the organic material and to its degree of physical protection. SOM fractions can provide information on the turnover of organic matter (OM), provided the fractions can be related to functional or structural components in soil. Ultrasonication is commonly used to disrupt the soil structure prior to physical fractionation according to particle size, but may cause redistribution of OM among size fractions. The presence of mineral particles in size fractions can complicate estimations of OM turnover time within the fractions. Densiometric separation allows one to physically separate OM found within a specific size class from the heavier-density mineral particles. Nutrient contents and mineralization potential were determined for discrete size/density OM fractions isolated from within the macroaggregate structure of cultivated grassland soils. Eighteen percent of the total soil C and 25% of the total soil N in no-till soil was associated with fine-silt size particles having a density of 2.07-2.21 g/cm3 isolated from inside macroaggregates (enriched labile fraction or ELF). The amount of C and N sequestered in the ELF fraction decreased as the intensity of tillage increased. The specific rate of mineralization (μg net mineral N/μg total N in the fraction) for macroaggregate-derived ELF was not different for the three tillage treatments but was greater than for intact macroaggregates. The methods described here have improved our ability to quantitatively estimate SOM fractions, which in turn has increased our understanding of SOM dynamics in cultivated grassland systems.

A simple method is described for the dispersion of soil to isolate a particulate organic-matter (POM) fraction that may represent an important soil organic matter (SOM) pool in grassland soils. Carbon contents were compared among three tillage treatments and an undisturbed grassland at Sidney, NE. Twenty years of bare-fallow, stubble-mulch, and no-till management reduced the C content of POM. The mineral associated organic-matter fraction showed no reduction in C content in the bare-fallow treatment, but increased in the no-till and stubble-mulch treatments. It is suggested that the POM fraction closely matches the characteristics of a SOM pool variously described as slow, decomposable, or stabilized organic matter. -from Authors

The distribution of organic matter within physical fractions of the soil can be assessed by disruption of the soil structure, followed by the separation of physical fractions based on particle size or density. Disruption of the soil structure can be accomplished by physical or chemical methods, or some combination of the two. The most commonly used methods of physical disruption are shaking and sonication. Shaking is the more gentle method with the advantage of being able to obtain a wide range of disrupting energies relatively easily. Sonication can impart more energy to the soil in a shorter period of time. The greatest potential problem associated with the use of sonication is the redistribution of organic matter among size/density fractions. Chemical extraction methods are commonly used prior to disruption of soil for particle size analysis. Some chemical dispersants can selectively solubilize organic matter. This specificity can be used to determine the kinds and amounts of organic matter that bind particles into aggregates. Three methods of physical separation of soil have been used, sieving, sedimentation and densitometry. Sieving separates soil particles based strictly on size and is used primarily for aggregate separations of non-disrupted soil samples. Sedimentation separates particles based on an equivalent spherical diameter, which may vary in size, shape and density. It is most often used in conjunction with a disruption pretreatment to obtain fine fractions. Densitometry separates particles based on the weight per unit volume, independent of size and shape, and is used to separate lighter from heavier fractions. It is possible to combine any or all of these separation methods in order to isolate, for instance, organic matter from a particular size fraction that has a specific density.

The destruction of soil aggregate structure in the field can lead to increased rates of erosion and decreased soil fertility. Methods developed to quantify aggregate stability have evolved around the application of disruptive forces that are comparable with those observed in the field, such as erosion, slaking and tillage. The indexes proposed here quantitatively summarize the degree of disruption with respect to a reference level based on the maximum disruption level possible for a given soil. The calculation takes into account the particle size distribution associated with the maximum level of disruption (texture) of the soil sample. The aggregation index can be calculated from the disruption index by difference. These indexes allow comparisons to be made between soil samples taken from several locations that have different initial aggregate distributions and textures. The characterization of an aggregate distribution in a single number (index) allows for easier statistical comparison of results from a wide range of studies. The indexes could potentially be used to characterize the change in distribution of organic matter after disruption associated with a given aggregate size distribution.