Soil Organic Carbon Dynamics in the Pendleton Long-Term Experiments.

Long-term experiments (LTEs) serve as valuable resources for understanding the impacts of agricultural management practices on soil quality and crop production. Studies to determine the influence of crop rotation, tillage, and nutrient management practices on soil organic carbon (SOC) dynamics in Pendleton LTEs were conducted during 2014-2015. The first study evaluated the effects of tillage and N fertility management practices on microbial biomass, potentially mineralizable C, 24-hr soil respiration, and soil metabolic quotient (qCO₂) and used selected treatments from a tillage fertility long-term experiment. The second study evaluated the effect of temperature on soil C and N mineralization on soil samples collected from selected treatments of the crop residue, tillage fertility, and wheat-pea LTEs. Undisturbed grassland soil samples were used as a reference for these comparisons. Soil microbial biomass C content as well as 24-hr soil respiration was significantly greater under disk than under sweep and moldboard plow tillage in 0-10 cm depth. Plow tillage had the largest microbial biomass and 24-hr soil respiration in 10-20 cm depth. Soil microbial qCO₂ was influenced by tillage systems, crop rotations, as well as N application rates across all soil depths. Warming accelerated SOC mineralization. Reduced- and no-tillage systems can improve SOC sequestration at surface soil through their effects on soil mineralizable and microbial biomass C. Reducing fallow by using perennial grasses or crop rotation and N addition can complement the effects of reduced-tillage management to improve SOC accrual in winter wheat-summer fallow systems in the Pacific Northwest.