Sissoko, F. 1997. Enhancement of soil aggregation by the combined influences of soil wetting and drying and root-microbial associations. M.Sc. Thesis, Michigan State University, East Lansing, Michigan, USA.
Plant root modifications of soil aggregates in the rhizosphere have been reported frequently. However, there are conflicting reports of soil aggregate stabilization by multiple cycles of root exudation and soil water removal. Stimulation of soil microorganisms by root exudates was studied in soils collected to depths of 20 cm in conventionally tilled (CT), no-tilled (NT), and native grassland (NG) at the Kellogg Biological Station in the southwestern Michigan. This study presents a method for measuring the combined affects of root infusions of a mixture of 7 carbohydrates, 4 organic acids, and 12 amino acids compounds and root extractions of soil water with multiple soil wetting and drying cycles. Polyvinyl chloride (PVC) cylinders, 10 cm diameter x 12 cm length, prepared by drilling a longitudinal row of 11 holes at 1 cm intervals, were used as containers for soil samples. In this study, Rhizos soil solution samplers (SSS) have been used to simulate plant roots. Soil microbial biomass and mean weight diameter were estimated at the end of the wetting and drying cycles. Root exudate compounds increased soil aggregate stability in macroaggregate fractions by 5 times compared to the control after nine wetting and drying cycles. Additions of root exudate compounds increased microbial activities of soils adjacent to artificial roots for all management practices. Aggregate stabilities of soils exposed to long-term tillage management, responded more to additions of C and N compounds than did soil aggregates from native grasslands.
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