Woody plants are increasing in many grassland and savanna ecosystems around the world. As a case in point, the Edwards Plateau of Texas, USA, is a vast region (93 000 km2) in which rapid woody encroachment appears to be occurring. The native vegetation (prior to the Anglo-European settlement 150 – 200 yr ago) and the biogeochemical consequences of woody encroachment in this region, however, are poorly understood. To assess these matters we measured plant and soil δ13C, soil organic C and soil N content from grasslands and two important woody patch types (mature Quercus virginiana clusters and Juniperus ashei woodlands) in this region. Soil δ13C values showed that relative productivity of C3 species has increased in grassland and both woody habitats in recent times. δ13C of SOC in grasslands and Q. virginiana clusters increased with depth from the litter layer to 30 cm (grasslands = − 21 to − 13‰; Q. virginiana clusters = − 27 to − 17‰) and were significantly different between habitats at all depths, indicating that Q. virginiana has been a long-term component of the landscape. In J. ashei woodlands, soil δ13C values (at 20 – 30 cm depth) near the woodland edge (−13‰) converged with those of an adjacent grassland (−13‰) while those from the woodland interior (−15‰) remained distinct, indicating that the woodland has been present for many years but has recently expanded. Concentrations and densities of SOC and total N were generally greater in woody patches than in grasslands. However, differences in the amount of SOC and N stored beneath the two woody patch types indicates that C and N sequestration potentials are species dependent.Nomenclature: Jones et al. (1997).Abbreviations: SOC = soil organic carbon; SOM = soil organic matter.