Woody encroachment by Juniperus virginiana into Great Plains grasslands allowed us to answer: Does changing the type of plant input to soils alter soil organic carbon (SOC) distribution or soil carbon (C) storage? The answer is critical because woody encroachment may alter C cycling over millions of hectares in the Great Plains and Midwest. We predicted that (1) forest SOC would become concentrated in shallow soil layers compared to SOC distribution in grassland, (2) woody expansion would increase soil C storage, and (3) forest C would be apparent in the larger soil organic matter fractions. Using δ13C signatures of SOC, 1/5 of the C from 0 to 25 cm in juniper forest soils was derived from C3 juniper trees. Forest C3 input occurred primarily in shallow surface layers: Forest soils developed over former C4 prairie contained 42% C3-SOC from 0 to 2.5 cm depth, and decreased to 6% at 25 cm. Isotopic analysis of SOC size fractions revealed that at 0--2.5 cm, the forest soil fraction >212 ¿m was -25.7?. The fraction <2 ¿m had a 13C isotope ratio of -17.0? at the same depth, reflecting the predominance of residual prairie C in the smallest fraction. In spite of fast dynamics of soil C turnover, there was no net change in SOC amounts over 40--60 years (cumulative mineral and organic SOC in forest, 8782 g C/m2 ¿ 810; in grassland, 7699 ¿ 1004). Thus as junipers expand into mesic areas of the Great Plains, juniper forests will provide little additional soil C storage.