Long-term Changes in Soil and Stream Chemistry across an Acid Deposition Gradient in the Northeastern United States.

Declines in acidic deposition across Europe and North America have led to decreases in surface water acidity and signs of chemical recovery of soils from acidification. To better understand the link between recovery of soils and surface waters, chemical trends in precipitation, soils, and streamwater were investigated in three watersheds representing a depositional gradient from high to low across the northeastern United States. Significant declines in concentrations of H (ranging from -1.2 to -2.74 microequivalents [µeq] L yr), NO (ranging from -0.6 to -0.84 µeq L yr), and SO (ranging from -0.95 to -2.13 µeq L yr) were detected in precipitation in the three watersheds during the period 1999 to 2013. Soil chemistry in the A horizon of the watershed with the greatest decrease in deposition showed significant decreases in exchangeable Al and increases in exchangeable bases. Soil chemistry did not significantly improve during the study in the other watersheds, and base saturation in the Oa and upper B horizons significantly declined in the watershed with the smallest decrease in deposition. Streamwater SO concentrations significantly declined in all three streams (ranging from -2.01 to -2.87 µeq L yr) and acid neutralizing capacity increased (ranging from 1.38 to 1.60 µeq L yr) in the two streams with the greatest decreases in deposition. Recovery of soils has likely been limited by decades of acid deposition that have leached base cations from soils with base-poor parent material.

The response of soil and stream chemistry to decreases in acid deposition in the Catskill Mountains, New York, USA.

The Catskill Mountains have been adversely impacted by decades of acid deposition, however, since the early 1990s, levels have decreased sharply as a result of decreases in emissions of sulfur dioxide and nitrogen oxides. This study examines trends in acid deposition, stream-water chemistry, and soil chemistry in the southeastern Catskill Mountains. We measured significant reductions in acid deposition and improvement in stream-water quality in 5 streams included in this study from 1992 to 2014. The largest, most significant trends were for sulfate (SO42-) concentrations (mean trend of -2.5 µeq L-1 yr-1); hydrogen ion (H+) and inorganic monomeric aluminum (Alim) also decreased significantly (mean trends of -0.3 µeq L-1 yr-1 for H+ and -0.1 µeq L-1 yr-1 for Alim for the 3 most acidic sites). Acid neutralizing capacity (ANC) increased by a mean of 0.65 µeq L-1 yr-1 for all 5 sites, which was 4 fold less than the decrease in SO42- concentrations. These upward trends in ANC were limited by coincident decreases in base cations (-1.3 µeq L-1 yr-1 for calcium + magnesium). No significant trends were detected in stream-water nitrate (NO3-) concentrations despite significant decreasing trends in NO3- wet deposition. We measured no recovery in soil chemistry which we attributed to an initially low soil buffering capacity that has been further depleted by decades of acid deposition. Tightly coupled decreasing trends in stream-water silicon (Si) (-0.2 µeq L-1 yr-1) and base cations suggest a decrease in the soil mineral weathering rate. We hypothesize that a decrease in the ionic strength of soil water and shallow groundwater may be the principal driver of this apparent decrease in the weathering rate. A decreasing weathering rate would help to explain the slow recovery of stream pH and ANC as well as that of soil base cations.

Methods of Soil Resampling to Monitor Changes in the Chemical Concentrations of Forest Soils.

Recent soils research has shown that important chemical soil characteristics can change in less than a decade, often the result of broad environmental changes. Repeated sampling to monitor these changes in forest soils is a relatively new practice that is not well documented in the literature and has only recently been broadly embraced by the scientific community. The objective of this protocol is therefore to synthesize the latest information on methods of soil resampling in a format that can be used to design and implement a soil monitoring program. Successful monitoring of forest soils requires that a study unit be defined within an area of forested land that can be characterized with replicate sampling locations. A resampling interval of 5 years is recommended, but if monitoring is done to evaluate a specific environmental driver, the rate of change expected in that driver should be taken into consideration. Here, we show that the sampling of the profile can be done by horizon where boundaries can be clearly identified and horizons are sufficiently thick to remove soil without contamination from horizons above or below. Otherwise, sampling can be done by depth interval. Archiving of sample for future reanalysis is a key step in avoiding analytical bias and providing the opportunity for additional analyses as new questions arise.