Occasional tillage of no -till systems to improve carbon sequestration, and soil physical and microbial properties

Stratification of soil chemical, physical and microbial properties results from continuous, long-term no-till (NT) management. The top 5 cm of soil is typically improved for plant growth, but unimproved at deeper soil depths. This stratification may slow or limit long-term soil organic carbon (SOC) accumulation, increase runoff phosphorus (P) concentrations and losses, and may eventually constrain crop performance. Occasional tillage of NT systems refers to a one-time tillage operation, conducted once in 12 or more years to redistribute SOC and nutrients, and improve overall soil physical properties. It was hypothesized that occasional tillage can further improve agronomic and environmental performance of NT systems. Two experiments in long-term NT fields were conducted under rainfed corn or sorghum rotated with soybean in eastern Nebraska. Tillage treatments included: continuous no-till, tandem disk, 10-cm wide twisted shank-chisel, moldboard plow and mini-moldboard plow. The four studies presented in this dissertation addressed soil physical, chemical and micro-biological properties to (1) identify which tillage implements most effectively redistribute the soil properties, and (2) evaluate if occasional tillage causes short-term detrimental effects to the soil. Moldboard plowing was most effective in redistribution of soil properties. Labile organic matter pools and total SOC were lower in the 0- to 5-cm layer, and an enrichment in soil organic matter was achieved in the 5- to 20-cm layer with moldboard plow tillage. Total SOC and nitrogen in the profile were not reduced by tillage. In a simulated rainfall study, dissolved phosphorus concentration in runoff was reduced with moldboard plow tillage. The soil microbial community was affected by tillage, and its return towards the NT baseline in the years following tillage was quicker in the 0-5 cm depth than in deeper soil. Moldboard plowing caused a loss in mycorrhizal fungi, which did not recover after 2 years post-tillage