Long-Term Impacts of Tillage, Cover Crops, and Nitrogen Rates on Microbial Community Dynamics and Soil Quality Parameters under Continuous Cotton Production in West Tennessee

Microbial communities play a central role in nutrient cycling and soil quality in agro-ecosystems. This research focused on a comparative analysis of the microbial community structure and activity of soils on long-term (31 years) continuous cotton- Gossypium hirsutum L., production in West Tennessee under conservation agricultural (CA) and conventional tillage practices that included: Nitrogen (N) fertilizer rates (N-rates) (0, 34, 67 and 101 kg N per ha); Cover crops (Hairy vetch-Vicia villosa and winter wheat- Triticum aestivum, and a No Cover control); and Tillage (Till and No-till). It was expected that microbial diversity, activity and soil quality would be greater under CA practices relative to conventional tillage.

The microbial community structure profiled using Fatty Acid Methyl Ester extractions (FAME) revealed FAME indicators for Gram positive bacteria, actinomycetes and mycorrhiza fungi to be significantly greater (p < 0.05) in the No-till treatments relative to Till. In contrast, the saprophytic fungi indicators were significantly greater (p < 0.05) in the Till treatments resulting in significantly greater fungi to bacteria FAME ratio under Till than No-till. N-rate had a significant effect on the relative abundance of the mycorrhiza biomarker which decreased with increasing N-rate. Results from high-throughput 16S rRNA gene sequencing analysis revealed microbial diversity in soils under 101 N-rates to be significantly (p < 0.05) less diverse than the 34 and 67 N-rates. However, tillage and cover crop did not significantly influence bacterial diversity.

Soil quality properties revealed significantly greater (p < 0.05) total carbon and N in the combination of No-till treatments having cover crops, with the No-till treatments also having significantly greater extractable nutrients (phosphorous, potassium, and calcium, and enzymatic activity (beta-glucosidase, beta-glucosaminidase, and phosphodiesterase) indicating an improvement in soil quality and fertility.

This study reveals that CA practices involving No-till and cover crops promote conditions that support an increase in the abundance and activity of soil microbial communities, in turn leading to greater soil nutrient cycling capacity and soil quality. This long-term assessment was able to provide an overview of the benefits of C sequestration with these CA practices for low biomass crops like cotton under a monoculture production.