Ozal, Onder (2018-05). Impact of Winter Cover Crops and Tillage on Soil Microbial Populations and Mycorrhizal Colonization in Dryland Cotton. Master's Thesis.
Cotton producers are interested in adopting conservation tillage for potential soil health benefits such as increasing soil microbial biomass. However, little is known about the resulting impacts of cover crops and tillage practices on soil microorganisms, especially plant beneficial microbes such as AMF abundance and interactions with cotton plants under dryland conditions. The objectives of this study were to evaluate soil microbial biomass and soil parameters in different tillage systems and cover crop rotations with dryland cotton, and to evaluate root colonization and AMF diversity in these treatments. A multi-year field study was conducted with a randomized-complete block design, with four replicates. 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 for microbial biomass using phospholipid fatty acid analysis (PLFA). Mycorrhizal colonization of cotton roots was determined at multiple time points during the growing season. Individual root fragments were isolated from cotton roots, DNA extracted and used to identify mycorrhizal community structure in cotton roots by ribosomal RNA gene sequencing. The PLFA results showed little difference in microbial biomass levels between conventional tillage and no-till samples Inclusion of a cover crop increased microbial biomass by up to 2-fold. In August, the use of cover crops increased percentage of mycorrhizal colonization of cotton, as higher root colonization was observed in hairy vetch, Austrian winter field pea and crimson clover. Root colonization was lowest in conventionally tilled plots. In October, the differences in root-colonization among the treatments had largely disappeared. Principal coordinate analyses of relative abundance of AMF species (operational taxonomic units) indicated that different cover crop treatments influenced AMF community structure in cotton roots. In August, AMF species colonizing cotton roots were similar in most treatments. The AMF community structure appeared distinct between conventional tillage and cover crop treatments by October. The results indicated that cover crops demonstrated positive legacy effects by increasing soil microbial biomass and AMF colonization of cotton roots, especially at the initial growth stages of cotton. These impacts could translate to higher drought tolerance and productivity under dryland conditions.