New techniques in soil sampling: Methods to improve long-term ecological research

Background/Question/Methods   A major source of uncertainty in the calculation of nutrient budgets results from imprecise and inaccurate soil sampling techniques. Traditional methods of soil sampling, such as soil coring and excavation of quantitative soil pits, are limited by rocky soil conditions. In addition, the removal of soil for analysis prevents repeated sampling in sites that may have high spatial variability. To improve these limitations, we are testing two novel methods of soil collection and analysis at the Bartlett Experimental Forest in New Hampshire, Bear Brook Watershed in Maine, and Mt. Ascutney in Vermont. We tested a gas-powered, diamond-bit drill which can core through large roots and rocks to a depth of 90 cm or more, thereby eliminating the depth and density biases of manual coring. The samples we collected with this technique were compared to soils collected from quantitative soil pits located within 2 m of the core to determine if exchangeable cation concentrations were consistent between power-cored soils and pit soils. We also tested an inelastic neutron scattering (INS) approach, which non-destructively measures belowground nutrient concentrations through in-situ spectroscopy of gamma radiation, to determine the machine's capability of reproducing repeatable measurements.

Results/Conclusions   We expected to find higher cation concentrations in the soil cores as compared to the pits, as a result of ground rock that enters the core during sampling. We found significantly higher concentrations of Al (p=0.006), Si (p=0.02), and Fe (p=0.04) in the cores as compared to the pits. Analyses of different segments of the cored samples revealed significantly higher concentrations of Na (p=0.002), Mn (p=0.002), and Sr (p=0.002) at the edge of the core, presumably due to this contamination by ground rock; however, the magnitude of this contamination source is small enough that it is not reflected in the difference between the coring and quantitative pit methods. Using the INS method, we made three repeated measurements at one site and found < 4% uncertainty in C (4% for Si, 2% for O). At a second site, two repeated measures agreed poorly (12% for C). Each of these methods offers important improvements in speed and reproducibility over traditional methods of soil sampling. These new methods are applicable to a wide variety of ecosystems, and their utilization can help improve our understanding of both short-term and long-term changes in belowground ecological processes.