Acid rain mitigation experiment shifts a forested watershed from a net sink to a net source of nitrogen

Abstract

Decades of acid rain have acidified forest soils and freshwaters throughout montane forests of the northeastern United States; the resulting loss of soil base cations is hypothesized to be responsible for limiting rates of forest growth throughout the region. In 1999, an experiment was conducted that reversed the long-term trend of soil base cation depletion and tested the hypothesis that calcium limits forest growth in acidified soils. Researchers added 1,189 kg Ca 2+ haas the pelletized mineral wollastonite (CaSiO 3 ) to a 12-ha forested watershed within the Hubbard Brook Experimental Forest in the White Mountains of New Hampshire. Significant increases in the pH and acid-neutralizing capacity of soils and streamwater resulted, and the predicted increase in forest growth occurred. An unanticipated consequence of this acidification mitigation experiment began to emerge a decade later, with marked increases in dissolved inorganic nitrogen (DIN) exports in streamwater from the treated watershed. By 2013, 30-times greater DIN was exported from this base-treated watershed than from adjacent reference watersheds, and DIN exports resulting from this experiment match or exceed earlier reports of inorganic N losses after severe ice-storm damage within the study watershed. The discovery that CaSiO 3 enrichment can convert a watershed from a sink to a source of N suggests that numerous potential mechanisms drive watershed N dynamics and provides new insights into the influence of acid deposition mitigation strategies for both carbon cycling and watershed N export.calcium | acid deposition | nitrate

Recovery of Soils From Acidic Deposition May Exacerbate Nitrogen Export From Forested Watersheds

“…The ongoing wollastonite addition experiment in New Hampshire has provided some of the most relevant and recent results on a possible link between Ca availability and watershed N release. This one‐time addition to a Ca‐depleted watershed had minimal effect on N release for about a decade, but over the ensuing 3 to 4 years, stream NO 3 − concentrations increased abruptly, shifting the watershed from an N sink to an N source (Rosi‐Marshall et al, ). Further work in this watershed indicated that release of N from soil to stream became more sensitive to hydrologic events during the period of high N release (Marinos et al, ).…”

“…Despite these potential links between Ca and N availability, Ca‐N interactions remain poorly understood. For example, experimental addition of Ca in the form of wollastonite (CaSiO 3 ) to a Ca‐depleted watershed in New Hampshire, USA, resulted in increased forest growth (Battles et al, ) and decreases in microbial and inorganic N pools (Groffman et al, ) but also eventual increases in watershed export of N (Rosi‐Marshall et al, ).…”

Unexpected stasis in a changing world: Lake nutrient and chlorophyll trends since 1990

“…In some regions, surface water nitrate concentrations have not responded strongly to declines in deposition (Garmo et al, 2014;Skjelkv ale et al, 2005), and nitrate leaching from N-saturated watersheds can increase even under constant N deposition (Curtis, Evans, Helliwell, & Monteith, 2005). Calcium additions to artificially induce acid rain recovery converted an experimental watershed from an N sink to source, suggesting future ecosystem responses to declines in atmospheric deposition may include N increases in surface waters (Rosi-Marshall, Bernhardt, Buso, Driscoll, & Likens, 2016). Additionally, N declines were observed in agricultural regions with extremely high TN concentrations (e.g., Iowa), where positive ammonium deposition trends have offset negative nitrate deposition trends (Li et al, 2016;Stoddard et al, 2003), and N deposition likely comprises a small fraction of the total N budget for each lake.…”

How landscape organization and scale shape catchment hydrology and biogeochemistry: insights from a long‐term catchment study

“…Throughout the recent past, time‐series of data from small catchment ecosystems have provided key mechanistic insights into how hydrology and biogeochemistry interact to shape surface water chemistry, and thus how different landscapes may respond to natural and anthropogenic change . Given the suite of environmental pressures and challenges society currently faces, the termination of empirical data collection from field research infrastructures is troublesome, as the emergence of new conditions have historically had rapid and unforeseen consequences for catchment dynamics . A sample not collected is empirical data lost forever, which can be problematic as insights gleaned from the past may not necessarily be useful for predicting the future, especially if mechanistic understanding is lacking.…”