Deep soil organic carbon stocks in pastures do not reflect surface soil properties or land use

Abstract

Optimizing soil organic carbon (SOC) sequestration in pasture systems requires understanding the carbon storage capacity across the entire soil profile. Though SOC concentrations typically decline with depth, the large volume of subsurface soil means it can constitute more than half of the total SOC pool. Therefore, it is important to consider deeper soil layers beyond those typically sampled (i.e., 0–30 cm). Here, we quantified the SOC stocks to 1 m on 12 Vermont (USA) pastures. This SOC stock averaged 12.9 kg C m ⁻² (range 8.8–19.2), which is consistent with both SOC storage in US grasslands and average stocks across Vermont, including forests. Environmental variables, including temperature, precipitation, and pH, were generally poor predictors of SOC stocks, though there was a positive relationship between clay content and surface SOC. We found that surface SOC stocks in these pastures were not a good predictor of deeper SOC stocks. Variability in SOC stocks, which increased with depth, may instead be influenced by untested factors. Depth to bedrock and accurate bulk density measurements substantially influence total SOC estimates and are important considerations for quantifying SOC. Our results suggest that the labor‐intensive collection of deep soil cores is necessary to accurately quantify soil carbon. Pastures are tractable landscapes for managing soil carbon compared to forests, and intensive sampling provides a baseline for evaluating soil carbon sequestration in these systems.

Plain Language Summary

We studied how much soil carbon is stored in 12 pastures in Vermont. While most soil studies only sample the top 30 cm, we measured down to 1 m and found that deeper soils can hold more than half of the total soil carbon. On average, these pastures stored 12.9 kg of carbon m⁻², similar to other grasslands across the United States and forests in Vermont. We discovered that surface carbon levels did not predict how much carbon was deeper in the soil. Variability in the soil organic carbon increased in deeper soil. Factors like soil texture, depth to bedrock, inorganic carbon, and bulk density were more important. Our results show that to understand and manage carbon storage in pasture soils, we need to sample deeper layers, even though it requires more work.

Citation

Lane, Erin; Lang, Ashley; Goldsmith, Sarah; Hicks Pries, Caitlin. 2026. Deep soil organic carbon stocks in pastures do not reflect surface soil properties or land use. Soil Science Society of America Journal. 90(3): e70270. 18 p. https://doi.org/10.1002/saj2.70270.