PS 40-109 - Eastern redcedar (Juniperus virginiana) encroachment into grasslands may increase soil organic carbon stocks

Abstract: Approximately one-third of the global terrestrial carbon is stored belowground. In particular, grassland ecosystems store large quantities of carbon belowground via soil organic matter. Woody plant encroachment is threatening grassland ecosystems worldwide, and it is uncertain how the increasing dominance of woody species globally will affect soil organic carbon (SOC) stocks Due to fire exclusion, eastern redcedar (Juniperus virginiana), is rapidly expanding into native grasslands across the Southern Great Plains of North America. The transition from grassland to eastern redcedar woodland has been shown to increase aboveground carbon storage; However, the impacts of this transition on belowground carbon stocks remain unclear. We hypothesized that grasslands would have greater SOC stocks relative to encroached eastern redcedar woodlands. To test our hypothesis, we sampled soils at nine sites across Oklahoma’s wet to dry precipitation gradient. Soil cores were extracted from two depth intervals, (0-10 cm) and (10-30 cm) under eastern redcedars and adjacent grasslands to determine soil bulk density and SOC content. Results did not support our hypothesis, as more SOC accumulated under eastern redcedar trees (199.7 t ha-1) than in grasslands (193.6 t ha-1) (p < 0.01), in the upper 10 cm of soil. SOC stocks at lower soil depths (10-30 cm) were not significantly different between the two vegetation cover types (p = 0.83). The precipitation gradient had no effect on SOC stocks at either soil depth. Our findings show that eastern redcedar encroachment results in greater SOC storage in upper soil layers. Thus, despite other negative consequences of grassland to forest transitions, soil organic carbon storage may increase with the encroachment of some woody species.