Impact of Cover Crops on Soil Microbial Populations and Mycorrhizal Diversity in Dryland Cotton.

Cotton is grown on over five million hectares in the U.S., largely using conventional tillage practices with no cover crops and limited rotations. Although there is increasing interest in using reduced tillage approaches and cover crops due to potential soil health benefits, relatively little is known about the resulting impacts on soil microorganisms in modern cotton production systems. A multi-year, field study was conducted to evaluate the impact of cover crops on various soil microbial populations in dryland cotton. A randomized-complete block design with four replicates was used with cotton planted in June and harvested in November. Treatments included conventional tillage, no-till, and no-till with a variety of different cover crops. Prior to planting in year 2, soil samples (0-7.5 cm depth) were collected and characterized using phospholipid fatty acid analysis (PLFA). Mycorrhizal colonization of cotton was determined at multiple points during the growing season via microscopic examination of cotton roots stained with trypan blue. Additionally, individual root fragment were isolated from cotton roots, DNA extracted and used to identify mycorrhiza species by the ribosomal RNA gene, sequencing. The PLFA results showed little difference in microbial biomass levels between conventional tillage and no-till samples; however, inclusion of a cover crop increased microbial biomass up to 2-fold. Similarly, at mid-season (August) the use of cover crops tended to increase mycorrhizal colonization of cotton, being lowest in the conventional tillage plots (65%), slightly higher with no-till (75%), and increasingly higher with no-till and cover crops: mixed species (85%), wheat (88%), hairy vetch (95%), Austrian winter pea (97%), and crimson clover (98%). However, by the end of the growing season (October), the differences had largely disappeared with conventional tillage plots having 82% colonization and the no-till and various cover crops ranging from 79 to 93% colonization. Principal Coordinate analyses of sequencing results showed