Effects of Dryland Continuous Spring Wheat Tillage Management On the Microbial Ecology of Soil Aggregates.

We evaluated the effects of no tillage (NT) and conventional tillage (CT) on bacterial communities predominant in soil microaggregates (0.25-0.05 mm) and on saprophytic basidiomycete fungi in various aggregate-size classes (4.75-2.00, 2.00-0.25, and 0.25-0.05 mm) at 0-20 cm depth of a Williams loam (fine-loamy, mixed, super active, Typic Argiborolls) from 2005 to 2007 in eastern Montana, USA. At 0-5 cm depth, NT increases aggregate proportion in 4.75-2.00 mm class as well as aggregate stability compared to CT treatment. The predominant bacteria were isolated from microaggregates and were tested for their ability to form aggregates using a soil sedimentation assay. Fatty acid methyl ester (FAME) profiling was used for taxonomic identification and DNA sequencing was used to confirm the identity of the aggregating species. Aggregate proportion was greater in NT than in CT in the 4.75-2.00 mm size class in 2007 at 0-5 cm but was not different between tillage within a year at 5-20 cm. Similarly, aggregate stability, as measured by mean-weight diameter (MWD) of aggregates, was greater in NT than in CT in 2007 at 0-5 cm. Enzyme-linked immunosorbent assay (ELISA), used to quantify the amount of basidiomycete fungi in aggregates, was higher in NT than in CT in the 4.75-2.00 mm size class in 2007 and higher in NT than in CT in all size classes and years at 0-5 cm but was not different between tillage, years, and aggregate-size classes at 5-20 cm. The proportion of predominant aggregating bacterial isolates in microaggregates was higher in NT than in CT at 0-5 cm but was not different between tillage at 5-20 cm. No-tillage favored dryland soil aggregation and stability compared with conventional tillage probably by reducing soil disturbance and residue incorporation and by enriching the proportion of soil microbial communities responsible for aggregation, especially at the surface soil.