Quantifying and Comparing Soil Carbon Stocks: Underestimation with the Core Sampling Method

Changing climate, land use, and management can impact both surface and deep soil organic carbon (SOC) stocks on decadal timescales, highlighting the importance of accurate measurements of SOC stocks and comparisons. This study compared three soil sampling methods for estimating SOC stocks: clod, core, and excavation. The excavation method was used as the standard by which the other methods were compared. Sampling took place at an intensively managed Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] plantation in northwestern Oregon, USA. Soil samples were collected by depth to 150 cm. Clod and core method soil bulk density measurements were significantly different at all depths, with the core method consistently resulting in lower soil bulk density. The core method significantly underestimated soil bulk density at all depths deeper than 20 cm and underestimated the SOC stock to a depth of 150 cm by 36%. Most of this difference occurred deeper than 20 cm, where the majority of SOC stocks were contained across all soil sampling methods. The underestimation of soil mass by the core method similarly affected the fixed depth, genetic horizon, and mass based approaches to quantify SOC stocks. This study demonstrated that (1) commonly used soil sampling methods for measuring soil properties should not be assumed to be interchangeable; and (2) regional and global SOC stocks may be largely underestimated due to shallow sampling and the frequent use of core methods.