Maintaining long-term soil productivity is a critical requirement of sustainable forest management on private and public lands. Managers and climate change researchers use estimates of carbon pools (C) to indicate soil productivity changes, alteration of biological activity, impacts from fire, or C storage potential. Reliable measurements for bulk density, and carbon and nitrogen (N) concentrations are needed to assess soil productivity but rocky soils comprise about 33 percent of United States soils and are difficult to sample.
This study compared differences among three different bulk sampling methods for estimating soil bulk density, and C and N pools in rocky forest soil. The methods: small diameter cores (SD), irregular soil excavation (IR), and a nuclear density gauge (ND). The goals were to:
Compare the three sampling methods for measuring soil bulk density;
Evaluate the effect of the bulk sampling method for measuring C and N pools; and,
Evaluate the relative importance of factors used in soil C and N pool calculations (bulk density, rock content, and C and N concentration).
The study site was a forest in north-central Wisconsin where approximately 60 percent of soil surface was covered in rocks. Rocky soils are especially difficult to sample for bulk density.
Total bulk density increased as soil depth increased for all three methods, but this difference was much less when only fine bulk densities (soil fraction <2 millimeters) were calculated, i.e. when corrected for rock-fragment content.
Total and fine bulk densities measured with soil density cores were lower than measurements made with larger soil volume irregular soil excavation and nuclear density methods.
Bulk density sampling methods had little effect on C and N pool estimates in the surface 20 centimeters of soil.
When the soil was sampled to 30 centimeters, the different soil densities detected by the sampling methods led to differences in C and N estimates:
- The C pool estimate by irregular soil excavation was significantly higher than the nuclear density and small diamater core estimates.
- The N pool estimates by irregular soil excavation and nuclear density were higher than the small diamater core estimates.
Small diamater cores were likely to have underestimated the C and N pool sizes, perhaps because of lower bulk density measurements.
High rock content at the 20-30 centimeter depth made it difficult for the irregular soil excavation method to obtain adequate soil volume in some excavations.