Afforestation Effects On Soil Carbon Storage: An Assessment for the United States Based On Meta-Analysis, Stable Isotopes, and a Geospatial Soil Carbon Database.

Afforestation is a leading management strategy for increasing terrestrial carbon (C) sequestration and mitigating rising atmospheric carbon dioxide, because compared to previous, non-forested land uses, afforestation significantly increases C storage in aboveground pools. However, because terrestrial ecosystems typically store most of their C in soils, afforestation impacts on soil organic C (SOC) storage are critical components of whole-ecosystem C budgets. We applied synthesis methods to identify the magnitude and drivers of afforestation impacts on SOC, and the temporal and vertical distributions of SOC change during afforestation in the U.S. Meta-analysis of 39 papers from 1957-2010 indicated that previous land use drives afforestation impacts on SOC (overall average = +21%), with mining industry (+173%) and wildlands (+31%) showing significant increases. Patterns of SOC increase and variability over 50-130 yr on former mining industry and agricultural lands suggest significant increases require ≥15 and 30 yr, respectively. Meta-analysis of ¹³C data demonstrated the greatest SOC changes occur nearest the top of the profile, a finding supported by a geospatial analysis of 409 profiles from the National Soil Carbon Network database. Across these profiles, transition from cultivation to forest increased A-horizon C stocks by 32%. In sum, our findings indicate significant potential for increased SOC sequestration during afforestation of U.S. lands, although site-to-site variation and sometimes questionable additionality of SOC increases are important considerations for any in-depth C budgets developed for afforested sites.