Effects of eastern redcedar (Juniperus virginiana) encroachment on soil organic carbon and soil microbial communities in grasslands along a precipitation gradient

Grassland ecosystems represent an important global carbon sink and store approximately one third of the global terrestrial carbon stock belowground in soil organic matter. Woody plant encroachment is an imminent threat to grasslands worldwide and may represent a potentially significant redistribution of carbon between terrestrial pools. Juniperus virginiana, hereafter eastern redcedar, has proliferated into grasslands across the Southern Great Plains due to fire exclusion. Transition from grassland to eastern redcedar woodland increases aboveground carbon stock but impacts on belowground carbon dynamics remain uncertain. Soil organic carbon dynamics are strongly influenced by plant and soil microbial interactions, and eastern redcedar encroachment may disrupt important plant-microbial associations that facilitation soil carbon cycling in grasslands. To investigate the effects of eastern redcedar encroachment on SOC stock and related soil microbial communities, we sampled nine soils and roots at nine sites across Oklahoma’s east to west precipitation gradient. We expected that eastern redcedar soils would have greater SOC stocks compared to grassland soils. Soil cores were extracted from two depth increments, (0-10 cm) and (10-30 cm), respectively, to determine bulk density and SOC content. We also assessed relative abundances of soil microbial functional groups, including intra- and extraradical arbuscular mycorrhizal (AM) fungi, gram-positive and gram-negative bacteria, and saprophytic fungi associated with eastern redcedar and adjacent grasslands. SOC stocks were greater beneath eastern redcedar trees (19.2 Mg C ha-⁻¹ as compared to grasslands (15.9 Mg C ha⁻¹) in upper soil depths (0-10 cm) and there was no difference in lower soil depth. Intraradical AM fungal colonization levels were lower in eastern redcedar roots (24.2%) than in the roots of grassland species (30.6%) (p value < 0.001), yet AM fungal biomass was greater in eastern redcedar soils (7.34 nmol g⁻¹ soil) compared to grassland soils (4.33 nmol g⁻¹ soil) in upper soil depths (0-10 cm) (p-value = 0.009). Saprophytic fungal biomass, gram positive and negative bacterial biomass, and total microbial biomass were also greater beneath eastern redcedar than grasslands in upper soil depths.