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The influence of three cropping treatments on the quantitative and qualitative composition of the carbohydrate fraction of a silt loam soil and its water-stable aggregates was assessed. Three cropping treatments were considered: bromegrass (Bromus inermisLeyss.) grown continuously for 15 yr (B15), grain-corn (Zea maysL.) grown continuously using conventional tillage for 15 yr (C15), and grain-corn grown continuously for 13 yr followed by 2 yr of bromegrass (B13C2). Six water-stable aggregate size fractions ranging from 8.00–4.70 to 0.25–0.10 mm in diameter were separated from the soil by wet sieving. The carbohydrate fraction was extracted by acid hydrolysis, reduced, acetylated to alditol acetates and analyzed by temperature-programmed capillary gas chromatography. Total carbohydrate content was not significantly influenced by cropping treatment but was observed to increase as aggregate size decreased irrespective of cropping treatment. The carbohydrate content in the different aggregate size fractions was positively correlated with both organic carbon and clay contents. With the exception of xylose and mannose, the monomeric sugars behaved similarly to total carbohydrates. Based on the magnitude of the ratio of the content of mannose to that of arabinose plus xylose, the contribution of plant carbohydrates to the total carbohydrate fraction increased as aggregate size decreased in the B15treatment, but decreased in the C13B2and C15treatments. No significant correlations existed between either the total carbohydrate content or the content of individual sugars, and the stability of aggregates in the soils from the different cropping treatments. Key words: Wet aggregate stability, soil organic matter, soil structure, crop rotation, corn, bromegrass

Changes in cropping practices can alter soil structural characteristics. The purpose of this study was to assess the extent and rate at which bromegrass (Bromus inermisLeyss) and continuous grain-corn (Zea maysL.) influence soil aggregation and various soil binding agents. Five cropping treatments ranging from 15 yr of continuous grain-corn to 15 yr of continuous bromegrass production were investigated. Soil aggregation and the nature of soil binding agents were determined by measuring the water-stable aggregate size distribution subsequent to the application of six chemical pretreatments — control, distilled water, 0.08 M sodium azide, 0.08 M sodium chloride, 0.08 M sodium periodate, and 0.02 M sodium pyrophosphate. A simple power function was used to describe the relationship between the cumulative percentage by weight of water-stable aggregates and aggregate diameter. The exponent in the power function provided a measure of the influence of cropping treatment and chemical pretreatment across the whole range of aggregate sizes. The size and stability of aggregates increased with increasing domination of bromegrass in the cropping history. Materials resistant to soaking in water contributed to the increased stability under bromegrass. Materials sensitive to periodate and pyrophosphate anions contributed to the stabilization of aggregates from all cropping treatments. However, these materials did not appear to account for the differences in structural stability observed across cropping treatments. The rate of structural deterioration induced by grain-corn production under conventional tillage appeared to be faster than the rate of structural enhancement promoted by bromegrass production. Key words: Polysaccharides, periodates, pyrophosphate, corn, bromegrass, aggregate stability

Information on rates of change of the structure of different soils under different cropping systems is vital to research in soil conservation. Much data on soil structure cannot be used to provide information on rates of change because of an inability to relate measurements at a given time to cropping history. A conceptual model is presented which facilitates quantification of cropping history in relation to soil structure. The model relates structural form, structural stability, stabilizing materials, biological factors and forces which may be exerted on the soil. Soil and crop management practices influence the nature and magnitude of forces involved, biological factors, and the level of stabilizing materials. The model is applied, for illustrative purposes, to five different cropping treatments involving the production of corn and bromegrass grown for different lengths of time over a 15–yr period. Functions to describe the accumulation or loss of stabilizing material under the two different cropping systems are assumed and functions describing the relations between calculated levels of stabilizing materials and structural stability, and between structural stability and structural form are then presented. The model provides a framework for expressing the impact of soil and crop management practices on soil structure and includes variables which could be used for assessing the "quality" of soil under current conditions. It also provides a method for characterizing the susceptibility of soils to structural change arising from soil and crop management practices. Extensive research is required to evaluate the model, however potential applications justify the effort which will be required. Key words: Tensile strength, wet aggregate stability, compaction index, corn, bromegrass, soil conservation