Post agricultural grasslands are thought to accumulate soil organic carbon (SOC) after cultivation cessation. The Conservation Reserve Program (CRP) in the U.S. is a wide-scale, covering approximately 8.9 Mha as of 2020, example of row-crop to grassland conversion. To date, changes in SOC stock in CRP lands have mostly been evaluated at local scales and focused on the surface 20–30 cm of the soil profile. Thus, we lack knowledge of SOC dynamics in CRP lands on a continental scale, especially in the subsurface soil, after agricultural cessation. The Rapid Carbon Assessment (RaCA) project is the most recent effort by the United States Department of Agriculture (USDA) to systematically quantify C stock in the 0–100 cm soil profile across the conterminous US. Here we analyzed data from RaCA to evaluate the SOC stocks of both surface and subsurface soil of the CRP on a continental scale. We found there was no difference in SOC stock between croplands and CRP lands when comparing the 0–100 cm soil profiles, which indicates that the C sequestration in CRP lands is insignificant overall. We did find that CRP lands have higher SOC stocks in the surface soil (0–5 cm). However, such higher SOC levels in surface (0–5 cm) soil were offset by the lower SOC stock in the subsurface (30–100 cm) of the CRP. We also found that CRP lands in humid and warm regions may have net soil C sequestration because they have much more SOC in the surface as compared with croplands in the same regions. Whether the lower SOC in the subsurface of CRP lands is caused by legacy effects or is a result of C losses needs to be verified by long-term repeated sampling in both surface and subsurface soil. This analysis highlights the importance of examining C dynamics in subsurface soil after agricultural cessation to accurately measure and improve C sequestration rates in CRP lands.

农业后的草地被认为在停止耕作后会积累土壤有机碳 (SOC)。美国的自然保护区计划 (CRP) 规模广泛，截至 2020 年覆盖面积约为 8.9 Mha，是行作物向草地转化的示例。迄今为止，CRP 土地中 SOC 储量的变化大多是在局部尺度上评估的，并集中在土壤剖面的表面 20-30 厘米。因此，我们缺乏对大陆范围内 CRP 土地的 SOC 动态的了解，特别是在农业停止后的地下土壤中。快速碳评估 (RaCA) 项目是美国农业部 (USDA) 最近为系统量化美国本土 0-100 厘米土壤剖面中的碳库所做的努力。在这里，我们分析了来自 RaCA 的数据，以评估大陆尺度 CRP 地表和地下土壤的 SOC 储量。我们发现，在比较 0-100 cm 土壤剖面时，农田和 CRP 土地之间的 SOC 存量没有差异，这表明 CRP 土地中的 C 固存总体上不显着。我们确实发现 CRP 土地在表层土壤（0-5 厘米）中具有更高的 SOC 储量。然而，表层（0-5 厘米）土壤中较高的 SOC 水平被 CRP 的地下（30-100 厘米）中较低的 SOC 储量所抵消。我们还发现，潮湿和温暖地区的 CRP 土地可能具有净土壤 C 固存，因为与相同地区的农田相比，它们在地表中具有更多的 SOC。CRP 土地的地下 SOC 较低是由遗留效应引起的还是由于碳损失的结果，需要通过在地表和地下土壤中长期重复采样来验证。该分析强调了在农业停止后检查地下土壤中的 C 动态以准确测量和提高 CRP 土地中 C 固存率的重要性。