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Status of Soil Acidification in North America

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Status of Soil Acidification in North America JOURNAL OF FOREST SCIENCE, 52,2006 (Special Issue): 3-1 3 Status of soil acidification in North America 'u.s.Department ofAgriculture, Forest Service, Pacific Southwest Research Station, Riverside, California, LlSA LI.S. Geological Survey, Augusta, Maine, LISA 30akRidge National Laboratory, Division ofEnvironmenta1 Sciences, Oak Ridge, Tennessee, USA *US. Department ofrlgriculture, Forest Service, Northeastern Research Station, Durham, New Hampshire, USA '~ole~iode Postgraduados, Programa Forestal, Montecillo-Chapingo, Edo. de Mkxico, Mkxico ABSTRACT: Forest soil acidification and depletion of nutrient cations have been reported for several forested regions in North America, predominantly in the eastern United States, including the northeast and in the central Appalachi- ans, but also in parts of southeastern Canada and the southern U.S. Continuing regional inputs of nitrogen and sulfur are of concern because of leaching of base cations, increased availability of soil Al, and the accumulation and ultimate transmission of acidity from forest soils to streams. Losses of calcium from forest soils and forested watersheds have now been documented as a sensitive early indicator and a functionally significant response to acid deposition for a wide range of forest soils in North America. For red spruce, a clear link has been established between acidic deposition, alterations in calcium and aluminum supplies and increased sensitivity to winter injury. Cation depletion appears to contribute to sugar maple decline on some soils, specifically the high mortality rates observed in northern Pennsylvania over the last decade. While responses to liming have not been systematically examined in North America, in a study in Pennsylvania, restoring basic cations through liming increased basal area growth of sugar maple and levels of calcium and magnesium in soil and foliage. In the San Bernardino Mountains in southern California near the west coast, the pH of the A horizon has declined by at least 2 pH units (to pH 4.0-4.3) over the past 30 years, with no detrimental effects on bole growth; presumably, because of the Mediterranean dirnate, base cation pools are still high and not limiting for plant growth. Keywords: calcium depletion; acidic deposition; base cations; red spruce; sugar maple; liming; winter injury; forest health Studies on the effects of acidic deposition in North case of calcium (Ca), magnesium (Mg) and potassium America increasingly point towards base cation (K), their importance as essential nutrients for tree depletion from forest soils as one of the principal growth and health. In addition, depletion of base cati- mechanisms by which air pollutants can affect forest ons from the soil is the process that leads to alurni- sustainability and productivity; particularly in areas num (Al) mobilization (LAWRENCEet al. 1997). Low where soils are inherentIy low in base cations and Ca/Al ratios in soil allows the A1 to interfere with root where cation reserves have been further depleted uptake of Ca in forest soils (SHORTLE,SMITH 1988). by harvesting or other past land use activities, Base The accumulation and ultimate transmission of acid- cation depletion is a cause for concern because of the ity from forest soils to streams is of concern as is the role these ions play in acid neutralization, and in the increase in Al concentrations in surface waters. J. FOR. SCI., 52,2006 (Special Issue): 3-13 3 Losses of Ca from forest soils and forested water- northern Pennsylvania and Quebec, Canada over sheds have now been documented as a sensitive early the last decade has been linked to cation depletion indicator of the soil response to acid deposition for a from those soils (DUCHESNEet al. 2002; LONGet wide range of forest soils in the US. (LAWRENCEet al. 1997; HORSLEYet al. 2000; MOOREet al. 2000; al. 1999; HUNTINGTONet al. 2000). The high mobili- DROHANet al. 2002). ty of Ca in soils, which makes it susceptible to leach- Acid deposition is also contributing to the deple- ing, is a primary reason for these effects, leading to tion of base cations in many poorly buffered soils concerns for longer-term decreases in forest ecosys- supporting southern pines. Soil acidification and re- tem productivity and function (Table 1).In addition, ductions in base saturation have also been observed the relatively high fraction of total Ca supply that is in highly-polluted southern California forests and exchangeable increases its susceptibly to depletion chaparral ecosystems, but cation reserves are still in some forests systems (ADAMSet al. 2000). For large in these Mediterranean climate ecosystems acid sensitive systems, perhaps the best indication (POTH,WOHLGEMUTH 1999). In contrast to these of the chemical imbalance created by continued studies, at one site in New Hampshire researchers inputs of acid deposition is evidenced when strong inferred (not direct re-measurement) no decline in acids derived from S and N (NO, and SOJ cause Ca in the forest floor in a chronosequence study in measurable alterations in the amount of chemical New Hampshire (YANAIet al. 1999; HAMBURGet elements in plant tissues (foliage, roots, and wood), al. 2003). Recently it was reported that in forests such as decreasing levels of base cations (e.g. Ca), growing on granitoid parent materials in New increasing levels of acidic cations (e.g. Al, Mn, and Hampshire, upstate New York, and Maine, apatite Fe), or both (SHORTLE,SMITH 1988; MCLAUGHLIN,provides a source of readily weathered and plant- KOHUT1992; HORSLEYet al. 2000). available Ca for forest growth (YANAIet al. 2005). Acid deposition has contributed to a regional Forests growing on these soil types may not be as decline in the availability of soil Ca and other base susceptible to Ca depletion as previously predicted. cations in many different types of forest ecosys- At sites where there may be appreciable amounts of tems. Studies in New Hampshire (LIKENSet al. dolomitic parent material (JOHNSON,TODD 1998) 1996; BAILEYet al. 1996), the Catskill Mountains in that are accessible to trees, soil Ca may plot be de- New York (LAWRENCEet al. 1999), West Virginia pleted to the extent it can be in nearby soils that lack (ADAMS1999), North Carolina (KNOEPP,SWANK a dolomitic source (TRETTINet al. 1999). In the case 1994), South Carolina (MARKEWITZet al. 1998), studies cited above, and in other examples of Ca Ontario, Canada (WATMOUGH2002; WATMOUGH, depletion in forest soils in Europe (HUNTINGTON DILLON2003), and Quebec, Canada (DUCHESNE 2000; JONSSON et al. 2003), it has not been possible to et al. 2002) have documented nutrient base cation quantify the component of depletion attributable to (e.g. Ca, Mg, K and others) depletion in soils due anthropogenic acidic deposition versus tree uptake. to acid deposition and forest regrowth. Resampling of forest soils in northwestern Pennsylvania shows Base cation depletion in North American forests an increase in soil acidity of 0.78 and 0.23 pH units in the 0 and A horizons after 36 years (DROHAN, A number of lines of evidence are consistent with SHARPE199'7). The high mortality rates and reduced decreasing soil Ca in recent decades. The failure of vigor reported for sugar maples experienced in stream water quality to recover in many parts of Table 1. Summary of linkages between acid deposition, biogeoc:hemica1 processes that affect Ca, physiological processes that are influenced by Ca, and effect on forest function Biogeochemical response to acid deposition Physiological response Effect on forest function Leach Ca from leaf membrane Reduce the cold tolerance of needles in Loss of current-year needles red spruce in red spruce Reduce the ratio of Ca to Al in soil Dysfunction in fine roots of red spruce Decreased growth and increased and in soil solutions blocks uptake of Ca susceptibility to stress in red spruce Reduce the ratio of Ca to A1 in soil More energy is used to acquire Ca Decreased growth and increased and in soil solutions in soils with low Ca: Al ratios photosynthate allocation to roots Reduce the availability of nutrient Sugar maples on drought-prone Episodic dieback and growth cations in marginal soils or nutrient-poor soits are less able impairment in sugar maple to withstand stresses 4 J. FOR, SGL, 52,2006 (Special I northeastern North America in spite of major de- Soil acidification may increase the rate of weath- creases in both sulfate emissions and deposition is ering, but the incremental increase in Ca release not fully understood but has been at least partially is likely to be small in soils that are already acidic. attributed to depletion of exchangeable base cat- Studies that have attempted to quantify Ca weath- ions including Ca (STODDARDet al. 1999; LIKENS ering rates in acidic forest soils in the eastern US et al. 1996; LAWRENCEet al. 1999). Many streams generally have found that rates are low (TURNER in the northeastern U.S. exhibit long-term trends et al. 1990; JOHNSON,LINDBERG 1992; KENNEDY of declining Ca concentrations (HUNTINGTONet al. 2002; DIJKSTRAet al. 2003) in comparison 2000). Streamwater Ca concentrations may be de- with typical combined rates of tree uptake and clining due to decreases in deposition of both sul- soil leaching (HUNTINGTONet al. 2000). A recent fate and Ca (LYNCHet al. 2000) as well as decreases study using strontium isotopes to investigate base in exchangeable soil Ca (LAWRENCEet al. 1999; cation nutrition in acid deposition impacted forests LIKENSet al. 1996; FERNANDEZet al. 2003). These raises new concerns about the capacity of mineral findings indicated that both base cation depletion weathering to replenish exchangeable base cations. and stream water chemistry were related to acid Unpolluted temperate forests can become nutrition- deposition in this area. The accumulation of S and ally decoupled from deeper weathering processes, N in forest soils from decades of elevated atmo- virtually functioning as atmospherically fed ecosys- spheric acidic deposition will delay recovery as the tems (KENNEDYet al.
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