- Lawrence, Gregory B.
U.S. Geological Survey
- Bailey, Scott
USDA Forest Service, Northeastern Research Station
Environmental disturbances such as acidic deposition can alter soils, but little data have been collected to document soil change. The few retrospective soil studies to date have shown that repeated sampling of soil over time can provide valuable information on soil change. To test the feasibility of repeated soil sampling to monitor soil change we resampled soils at 16 locations in the Northeastern U.S., Ontario and Quebec, where previous sampling was done 12 to 24 years ago. Investigators involved in the original sampling directed the resampling of their respective site(s) to ensure consistency of methods between previous collections and those to be done in 2009-2010. Original soil samples collected at each of the sites had been archived and were available for reanalysis to evaluate possible storage effects and analytical inconsistencies. A variety of sampling approaches and replication were used at the 16 sites, which enabled a comparison of collection methods. The same methods of chemical analysis were used for original and recently collected samples. A standard soil (mineral and organic horizons) provided by the Northeastern Soil Monitoring Cooperative was routinely analyzed by all labs for quality assurance.
Statistically significant changes were detected in all of the chemical measurements for at least one site and soil horizon. In general, organic horizons (Oe, Oa, F and H) showed the most changes and tended to become less acidic over time (decreases in exchangeable aluminum concentrations and or pH). There were some exceptions, including East Bear Brook ME, where exchangeable aluminum concentrations in the Oa horizon showed pronounced increases, and pH and exchangeable calcium concentrations decreased. Mineral horizons, (A and B) tended to become more acidic (increases in exchangeable aluminum concentrations, and or decreased calcium concentrations), although both increases and decreases in pH were observed and decreased concentrations of exchangeable Al were observed in B horizons at most Ontario sites. Some of the observed changes in soils, such as decreases in exchangeable Al in organic horizons, were related to decreasing acidic deposition, but some sites did not fit the overall patterns.
Results of this project substantiated soil resampling as a method for detecting changes in soil over varying time periods, in a variety of forest types with varying soil chemistry and acidic deposition history. This work also showed that recovery of acidified soils in response to decreasing acidic deposition had begun at some sites in some horizons, but at other sites, soils continued to acidify during the study period.