Depth Profile of Soil Carbon and Nitrogen Accumulation over Two Decades in a Prairie Restoration Experiment

Prairies converted from agriculture are known to accumulate carbon (C) and nitrogen (N) and are an important contribution to terrestrial C sequestration. However, estimates of decadal accumulation rates of C and N and their vertical distribution in the soil profile are highly variable among studies, in part due to the lack of repeated inventories of soil C and N stocks over long time periods. We determined the depth profile of soil C and N accumulation and bulk density following the transition from agriculture to planted prairie. Using 13 contiguous plantings with similar land-use histories, planted sequentially from 1995 to 2007, we sampled soil C, N, and bulk density three times over the course of two decades (2000, 2010, and 2019), combining a chronosequence approach with repeated inventories through time. In the top 20 cm of the profile, we found consistent accumulation of C and N, corresponding to 58% (0–10 cm) and 29% (10–20 cm) increases in soil C concentrations and 3.18% (0–10 cm) and 2.7% (10–20 cm) increases in soil N concentration over 19 years. In contrast, we found no change in C or N concentrations at 20–65 cm depth. A chronosequence approach did not detect C or N accumulation in any single sample year. Rather, initial soil C and N content appeared to be the best predictor of final concentrations. Our results suggest that the majority of C and N accumulation is occurring in the top portion of the profile and that restored prairies continue to sequester C two decades after initial planting.