Significance Forestland in the United States is a carbon (C) sink, offsetting ∼10% of annual greenhouse gas emissions and mitigating climate change. Most of the C in forests is held in soils, and the capacity of forest soils to sequester C makes them a major component of the US forest C sink. Where reforestation is presently occurring, either through deliberate replanting after forestland is disturbed (e.g., burned), or where previously nonforested lands (e.g., cultivated) are converting to forestland, topsoils are accumulating C. However, these C accumulation rates are poorly constrained; quantifying them with empirical data are critical to accurately represent the role of reforestation in the US C budget and forecast the longevity of the US forest C sink.