Temporal Variability of Soil C in Semi-Arid Soil on the Texas High Plains.

Soil health management (SHM) practices have been reported to increase soil organic (SOC) levels in agricultural soils due to stabilization of soil organic matter (SOM); however, increases of SOC in semi-arid cropping systems have been inconsistent. This project examined the temporal variability of SOC and potassium permanganate oxidizable carbon (POX-C) in a 19-year conservation tillage demonstration in Lamesa, TX. Treatments included 1) continuous cotton—conventional tillage (CT); 2) no-tillage—rye cover (NTR); and 3) no-tillage—mixed cover including hairy vetch, rye, winter pea, and radish (NTM). Soil samples were collected monthly from 0-15 cm soil depth beginning in July 2016 and were analyzed for SOC and POXC. Soil Organic C and POX-C varied greatly throughout the experiment but followed similar trends in the NTM and NTR treatments. Levels of SOC and POX-C increased significantly from April 2016 to October 2016 in all treatments before decreasing prior to cotton harvest. The levels decreased steadily with the CT treatment and fluctuated with the no-till treatments after planting the cover crops. This fluctuation in SOC and POX-C is likely the result of rhizodeposition and management practices. Overall, SOC and POX-C were closely related (R²=0.75) with POX-C accounting for approximately 6% of SOC which is considerably greater than has been previously reported. This research suggests SOC and POX-C levels are much more variable over the short-term than once believed and careful consideration should be used when evaluating soil C stocks regarding sampling times.