Estimating Carbon and Nitrogen Pools in a Forest Soil: Influence of Soil Bulk Density Methods and Rock Content

Soils with high rock content are common in many US forests, and contain large amounts of stored C. Accurate measurements of soil bulk density and rock content are critical for calculating and assessing changes in both C and nutrient pool size, but bulk density sampling methods have limitations and sources of variability. Therefore, we evaluated the use of small-diameter soil cores (SD), irregular soil volume excavation (IR), and a nuclear density gauge (ND) to measure bulk density and rock content, and estimate C and N pools in three 10-cm increments to a 30-cm depth in a glacial till soil in northern Wisconsin. Total and fine bulk densities were lower when measured with SD cores than with larger soil volume IR and ND methods. No differences in C pools among bulk density sampling methods were found in the 10-cm increments, but when combined to 30 cm, the C pool estimate with IR (81.6 Mg ha–¹) was significantly higher than those of ND (75.3 Mg ha–¹) and SD (73.4 Mg ha–¹). No significant differences in N pools were detected in the 10-cm increments, but the 0- to 30-cm N pool estimates by IR (5.65 Mg ha–¹) and ND (5 Mg ha–¹) were higher than that of SD (4.22 Mg ha–¹). Surface rocks could lower soil C and N pools by 20% or more. Overall, the bulk density method had little effect on soil C and N pool estimates in the surface 20 cm of this soil but did when sampled to 30 cm soil depth.