Prairies converted from agriculture are known to accumulate carbon (C) and nitrogen (N) and are an important terrestrial C sink. The cessation of tillage and the transition back to perennial vegetation facilitate the accumulation of C and N in the soil. 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. Over the course of two decades, we determined the depth profile of soil C and N accumulation and bulk density following the transition from agriculture to planted tallgrass prairie. Using 13 contiguous prairies plantings with similar land-use histories, planted sequentially from 1995-2007, we sampled soil C, N, and bulk density three times (in 2000, 2010, and 2019) to combine a chronosequence approach with repeated inventories through time.
Results/Conclusions
In the top 20 cm of the soil profile, we found consistent accumulation of C and N, corresponding to 68% (0-10 cm) and 76% (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 from 20-65 cm depth. Soil C and N concentrations were correlated with prairie age in the top 20 cm of the soil profile when all sampling dates were pooled together, but a chronosequence approach did not predict C or N accumulation in any single sample year at any depth. Accumulation rates were also not correlated with variation in plant species richness. Rather, initial soil C and N content appeared to be the best predictor of final concentrations. We found marked decreases in bulk density over 19 years at every depth, likely due to the cessation of tillage, the development of root systems, and recolonization of soil fauna. Our results suggest that the majority of C and N accumulation is occurring in the top portion of the profile and that reconstructed prairies continue to sequester carbon two decades after initial planting, highlighting the potential role of prairie ecosystems in natural climate solutions.