Redistribution and potential loss of soil carbon at watershed after redcedar encroachment in a mesic grassland

Background/Question/Methods

Rapid change in landscape cover associated with eastern redcedar (Juniperus virginiana) encroachment in the southern and central Great Plains plays an important role in regional carbon balance. However, complicated land use history, heterogeneity in soil properties and sparse tree canopy cover intrinsic to this process made it a formidable challenge to evaluate carbon stability such as change in soil carbon density and total soil carbon stock as a result of this vegetation cover change. Soil carbon change after a shallow rooted juniper encroaches into a mesic grassland with a relatively deep soil remains a key knowledge gap. We chose six small but adjacent watersheds with similar land use history at central Oklahoma. Currently, three of them are heavily encroached by J. virginiana and the other three remain primarily grass cover. A total of 120 random points were sampled for soil carbon in soil depth: 0 – 2 cm; 2 – 10 cm; 10 – 30 cm.  Soil carbon density for individual watershed was calculated two different ways - using each point as a random point to compare encroached and grassland sites or integration of soil type, depth and associated carbon density for individual watersheds to compare carbon density among grassland and encroached watersheds.

Results/Conclusions

Preliminary results showed that soil carbon densities were affected by vegetation cover (p =0.006), soil type (p<0.0001), soil depth (p<0.0001) and the interaction of vegetation and soil depth (p =0.03). Soil carbon density for the depth between 2 – 10 cm was significantly lower in encroached watersheds than in the grassland watersheds (n = 60, p = 0.0009) while there was no difference for 0-2 cm and 10 – 30 cm soil depths. For encroached watersheds, soil carbon density for locations between tree canopies was significantly lower than that under tree canopies in the upper 30 cm (p<0.05) indicating a redistribution of soil carbon within watersheds in response to J. virginiana encroachment.  Integration of soil type, depth and associated carbon density for individual watershed suggest that the average soil carbon density for encroached watershed with about 70% J. virginiana cover is slightly but consistently lower than grassland watersheds.