Effects of Acidic Deposition

Acid deposition and its effects on ecosystems is a long-standing research interest.  Acid deposition is elevated concentrations of strong acids deposited to the Earth’s surface as a result to emissions of fossil fuels and agricultural activity. Acid deposition has acidified soil and water in sensitive regions resulting in adverse effects on soils, trees and aquatic organisms. Emissions of acidifying air pollutants peaked in the U.S. and have decreased markedly in recent decades, due to air quality legislation and rules. This year I wrote a synthesis of acid and mercury deposition and its effects for the Encyclopedia of Biodiversity (Driscoll 2022).

We completed an analysis of target loads of acidity for protection of soil and streams of forest watershed in the Adirondacks (Shao et al. 2021, McDonnell et al. 2021; Figure 1). Target loads are a determination of levels of atmospheric deposition, below which will alleviate adverse ecosystem effects for a specific period. There were several innovative aspects to this work. We determined site specific targets for soil and stream recovery based on the inherent sensitivity of a watershed. For stream response, we developed a procedure to extrapolate the stream target load to hydrologic conditions of interest (e.g., spring high flow, summer low flow). Finally, we applied biological algorithms to determine the target load necessary to protect regional fisheries.  In Shao et al. (2021) we examined the interplay between decreases in acid deposition and changing climate in Adirondack watersheds.  This is one of the first studies to characterize and quantify the extent to which changing climate will delay the recovery of acid impacted surface waters.

I completed a critical loads project with funding from the Electric Power Research Institute. Working with Sonoma Technologies and partnering with a joint federal agency task force, including EPA, Research Triangle International, the National Park Service and the US Forest Service (US FS), we applied machine learning techniques to the USFS Forest Inventory Analysis to quantify critical loads of sulfur and nitrogen deposition for tree growth and survival for the coterminous US (Figure 2). This is an exciting project and may establish an approach that will be used to determine a secondary air quality standard for sulfur and nitrogen. This work will continue through a new contract with the USEPA. This project opens new opportunities for research involving machine learning techniques.

Figure 1. Time-series of model simulations of acid neutralizing capacity (ANC) for Archer Creek (a) and Buck Creek (b) in the Adirondack region of NY, comparing deposition only scenarios with combined climate and deposition scenarios. The solid lines represent the scenarios in which climate remains at current conditions with deposition change (business as usual, 20% increase and 100% reduction from current levels). The dashed lines represent scenarios in which predicted changes in climate (average of 34 model simulations) are simulated together with changes in deposition (Shao et al. 2021).   
Figure 2. Percentage of tree species in the coterminous US exceeding two representations of nitrogen critical loads for growth (left) and survival (right). The three horizontal rows of maps represent the maximum (upper 95% confidence interval), medium and lower (lower 95% confidence interval) bounds of critical loads for nitrogen using a bootstrapped confidence interval. The white areas indicate locations where tree species considered are not none to occur (Pavlovic et al. in review).

I have expanded my research on acid deposition to contrast with effects of agricultural amendments of sulfur. Over the past few years, I have worked with Eve-Lyn Hinckley (University of Colorado) to characterize and quantify inputs and effects of agricultural application of sulfur on ecosystems, contrasting these with the well-established effects of atmospheric sulfur deposition (i.e., acid rain). We found in the Midwestern region of the U.S. fertilizer sulfur application rates for corn and soy have increased in recent decades to make up for decreases in atmospheric sulfur deposition (Hinckley and Driscoll in review). The sulfur addition rate for agricultural applications is more intense at the local scale but the areal extent is smaller than for atmospheric sulfur deposition. This work will continue as we examine the roles of climate, hydrology, fertilizer application rates and other element cycles in modifying sulfur processes and flows within and down gradient of agricultural source areas.

Following the publication of Hinckley et al. (2020), we were contacted by a group from Switzerland working on a similar issue that we addressed for sulfur involving selenium. This resulted in a new collaboration and paper, involving the first global model of atmospheric selenium (Feinberg et al. 2021).


*Driscoll, C. T. In press. Acid and Mercury Deposition Effects on Forest and Freshwater Aquatic Ecosystems. Pages 1-14 in S. A. Levin, editor. Encyclopedia of Biodiversity, third edition. Elsevier Inc., Waltham, MA: Academic Press.

Feinberg, A., A. Stenke1, T. Peter, E. S. Hinckley, C. T. Driscoll and L. H. E. Winke. 2021. Reductions in the deposition of sulfur and selenium to agricultural soils pose risk of future nutrient deficiencies. Nature Communications Earth & Environment, 2(101). doi:10.1038/s43247-021-00172-0.

Hinckley, E. S., J. T. Crawford, H. Fakhraei and C. T. Driscoll. 2020. A shift in sulfur-cycle manipulation from atmospheric emissions to agricultural additions. Nature Geoscience, 13:597–604. doi: 10.1038/s41561-020-0620-3.

McDonnell, T. C., C. T. Driscoll, T. J. Sullivan, D. A. Burns, B. P. Baldigo and S. Shao. 2021. Regional Target Loads of Atmospheric Nitrogen and Sulfur Deposition for the Protection of Stream and Watershed Soil Resources of the Adirondack Mountains, USA. Environmental Pollution, 281:117110. doi: 10.1016/j.envpol.2021.117110.

Pavlovic, N. R., S.Y. Chang, J. Huang, K. Craig, C. Clark, K. Horn and C. T. Driscoll. In review. Empirical nitrogen and sulfur critical loads of U.S. tree species and their uncertainties with machine learning. PNAS Nexus.

Shao, S., D. A. Burns, H. Shen, Y. Chen, A. G. Russell and C. T. Driscoll. 2021.The response of streams in the Adirondack region of New York to projected changes in sulfur and nitrogen deposition under climate change. Science of the Total Environment, 800:149626. doi:10.1016/j.scitotenv.2021.149626