Experimental Acidification Causes Soil Base‐Cation Depletion at the Bear Brook Watershed in Maine

Abstract

There h concern that changes i n atmospheric deposition, climate, or land use have altered the biogeochemistry o f forests causing soil base-cation depletion, particularly C e The Bear Brook Watershed i n Maine (BBWM) is a paired watershed experiment with one watershed subjected to elevated N and S deposition through bimonthly additions of (NH,)aO,.Quantitative soil excavations i n 1998 measured soil pools of exchangeable base cations9 yr after treatments began. Stream sampling at the weirs on a weekly and event basin, and weekly precipitation sampling, were used for input-output estimates. The treated watershed had lower concentrations of exchangeable Ca and M g i n ail horizons, with evidence for the greater depletion i n the 0 horizon compared to underlying m i n e d soh, and i n softwoods compared to hardwoods. This difference between watersheds is interpreted to be treatment-induced baseation depletion, which was reinforced by model simulations. The difference between watersheds was 66 and 27 kg ha-' of exchangeable Ca and Mg, respectively, after accounting for soil m w differences between watersheds. ?his was comparable with tbe total armulative excess stream Ca and Mg export i n West Bear after 9 yr of treatment o f 55 and 11 kg ha-', respectively. Model simulations o f watershed response to treatments predicted excess soil exchangeable Ca and M g losses in the treated watershed o f 47 and 9 kg ha-', respectively. These results indicate that the response to a step-incmase i n N and S deposition during Ule first d d e of treatments in t h i s experimental forested watershed was to invoke cationexchange buffering, resulting in a net decline in soil exdungeable base cations. CCELERATED LEACHING of base cations from forest

Status of soil acidification in North America

“…Many streams in the northeastern U.S. exhibit long-term trends of declining Ca concentrations (HUNTINGTON 2000). Streamwater Ca concentrations may be declining due to decreases in deposition of both sulfate and Ca (LYNCH et al 2000) as well as decreases in exchangeable soil Ca (LAWRENCE et al 1999;LIKENS et al 1996;FERNANDEZ et al 2003). These findings indicated that both base cation depletion and stream water chemistry were related to acid deposition in this area.…”

Nitrogen deposition contributes to soil acidification in tropical ecosystems

“…3 and 4). These conditions are common in tropical forest ecosystems (Krusche et al, 2003;Vogt et al, 2006;Quesada et al, 2010), and contrast sharply with most temperate ecosystems, wherein exchangeable base cations generally dominate soil cation pools (Watmough & Dillon, 2003;Fernandez et al, 2003;Högberg et al, 2006 …”

Nitrogen additions mobilize soil base cations in two tropical forests

“…Over decadal timescales, numerous studies have shown that N fertilization and acid deposition can lead to declines in soil extractable cations (Hogberg et al 2006;Huntington 2005;Saarsalmi and Malkonen 2001), reduced foliar cation concentrations (Elvir et al 2005;Minocha et al 2000), and watershed-scale cation export (Fernandez et al 2003), although some sites show no change in cation cycling (Ingerslev et al 2001;Nilsen 2001;Nohrstedt 2001;Yanai et al 1999). Overall, a meta-analysis of 107 primarily temperate and northern sites showed that N additions generally increase mobilization of exchangeable base cations into soil solution and stream water, with parallel declines in soil extractable cations (Lucas et al 2011).…”