INFLUENCE OF CROPPING HISTORY AND CHEMICAL TREATMENTS ON THE WATER-STABLE AGGREGATION OF A SILT LOAM SOIL

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Abstract
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 inermis Leyss) and continuous grain-corn (Zea mays L.) 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