Populations of Ammonia Oxidizers and Arbuscular Mycorrhizal Fungi in a Long-Term Field Trial Comparing Tillage, Cover Crops and Crop Rotations.

Poor nutrient-use efficiency in agricultural soils remains a major environmental and human health concern. Best Management Practices (BMPs) such as cover crop use and crop rotation can enhance nutrient-use efficiency, but their long-term ecological effects are not well understood. This study assessed the effects of 30 year-old tillage, crop rotation, and cover crop systems on soil health using soil microbial communities as a proxy. Specifically, microbial biomass, nitrifiers; ammonia oxidizing bacteria (AOB) and ammonia oxidizing archaea (AOA), and arbuscular mycorrhizal fungi (AMF) were evaluated. Long-term tillage practices were hypothesized to have reduced soil microbial biomass and alter AOB, AOA, and AMF communities relative to no-till soils. Implementations of crop rotations and cover crops (BMPs) were predicted to mitigate these effects. This study analyzed eight different cropping systems under tillage and no-tillage; continuous corn (CC), soybean-soybean-corn-corn (SSCC), soybean-winter wheat-corn-corn (SWCC), and soybean-winter wheat (with red clover cover crop)-corn-corn (SW-RC-CC). Soil (400 g) was sampled before and after tillage and fertilization events. Surface 0-5 cm samples were collected for all treatments, additionally soil was sampled to 30 cm depth in the CC and SW-RC-CC plots. Nucleic acids were preserved in the field by placing soil immediately into Lifeguard Soil Preservation Solution (MoBio, Carlsbad, CA, USA), and DNA and RNA were extracted within 30 days. The microbial diversity of active (RNA) and total (DNA) AOB, AOA, and AMF populations was determined using PCR-denaturing gradient gel electrophoresis, and populations of AOB and AOA were quantified with quantitative-PCR. Microbial biomass C and N were determined with a chloroform-fumigation extraction. Greater recorded yields in the BMP plots suggest higher quantities of plant available nutrients. This research will determine if this coincides with larger and more diverse microbial communities associated with nitrogen and carbon cycling.