Abstract
Reductions in exchangeable calcium and magnesium and increase in exchangeable aluminum concentrations have been shown in soils impacted by acid deposition, including at four sites on the Allegheny Plateau, PA, USA, sampled in 1967 and 1997 during a period of peak deposition. We repeated sampling at these sites in 2017 to evaluate changes in soils during the more recent period when there has been a strong decline in acid deposition. The uppermost horizons, including the Oa and A horizons where humified organic matter transitions to mineral soil, were thicker, had higher concentrations of organic carbon and exchangeable calcium and magnesium, and lower concentrations of exchangeable aluminum in 2017 compared to 1997, approximating values measured in 1967. Below the Oa/A horizons, 2017 soil chemistry was more similar to the 1997 results, with some reduction of Ca in the recent measurements. These results suggest recovery of base cation–aluminum balance in surface horizons and may indicate a reduction of aluminum mobilization and increased efficiency of vegetation recycling of nutrients with decreased acid anion concentrations. These changes are consistent with a partial recovery from acid deposition. However, the increase in humified soil organic matter may also be affected by coincident increases in temperature and soil moisture.
Highlights
In the absence of active management or direct disturbance, soils develop over millennial time scales and may be quite stable at the decadal and shorter time scales of the human perspective
The exception to this pattern was observed in the uppermost portions of the profile, primarily at a depth less than 10 cm, where the 2017 exchangeable Ca and Mg values were similar to the 1967 measurements while exchangeable Al concentrations in 2017 were intermediate between 1967 and 1997
Protected long-term monitoring sites are rare but are invaluable in documenting the results of a changing chemical and physical environment, providing feedback to policy makers on the success of pollution control policy and to land managers in informing sustainable management practices. As these plateau top sites are relatively insulated from lateral groundwater inputs and depauperate of weatherable primary minerals, mineral weathering processes, a primary mechanism contributing base cations and buffering pH, is very limited and recovery may be an exceedingly slow process
Summary
In the absence of active management or direct disturbance, soils develop over millennial time scales and may be quite stable at the decadal and shorter time scales of the human perspective. The forested unglaciated Allegheny Plateau in Pennsylvania (PA) hosts some of the most northern Ultisols found in eastern North America [2] Soil forming processes such as primary mineral weathering, secondary clay formation and translocation, and fragipan development over many tens to hundreds of thousands of years have resulted in Ultisols in this district [3,4]. Soil chemical properties controlled by ion exchange sites on the surface of organic matter and clay particles may change more rapidly. These reflect changes in solute fluxes and organic matter turnover on seasonal to annual time scales, following changes in vegetation cover, land management, or disturbance. Growing appreciation for response to disturbance or changes in the environment has led to increased attention to shorter time scale soil dynamics [5] and fostered the establishment of soil monitoring programs [6,7]
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