Weathering acid deposition

Driscoll CT, Lawrence GB, Bulger AJ, Butler TJ, Cronan CS, Eagar C, Lambert KF, Likens GE, Stoddard JL, Weathers KC. 2001. Acidic deposition in the northeastern U.S. sources and inputs, ecosystems effects, and management strategies. BioScience 51 180-198. 

PROFILE is a biogeochemical model developed specially to calculate the influence of acid depositions on soil as a part of an ecosystem. The sets of chemical and biogeochemical reactions implemented in this model are (1) soil solution equilibrium, (2) mineral weathering, (3) nitrification and (4) nutrient uptake. Other biogeochemical processes affect soil chemistry via boundary conditions. However, there are many important physical soil processes and site conditions such as convective transport of solutes through the soil profile, the almost total absence of radial water flux (down through the soil profile) in mountain soils, the absence of radial runoff from the profile in soils with permafrost, etc., which are not implemented in the model and have to be taken into account in other ways. 

Class 5 (the least sensitive) soil include Kashtanozem, Brown soil and Sierozem soil zones in the Plateau of Inner Mongolia and the Loess Plateau, Desert soil zones in He-xi-zou-lang and the Talimu River Basin, Subalpine Steppe soil, Alpine Steppe soil and Alpine Desert soil in the Plateau of Tibet. These kinds of soils, belonging to the soil class of Xerosol or Alpine soil, consist of easy weathering minerals such as carbonate. They show alkaline reactions, with weak leaching and sparse vegetation. Those kinds of soils are insensitive to acid deposition. 

Sulphate, beside nitrate, is the main acid anion in acid deposition. The trend is significantly negative in 14 lakes. The absence of decreasing sulphate concentrations in the other lakes may be caused by weathering of geogenic sulphur. In fact, in most of these lakes sulphate concentrations are relatively high. 

Blum et al. (2002) and Probst et al. (2000) have found, in areas similarly cation-depleted by acid deposition, that a considerable proportion of calcium released by weathering came from apatite dissolution. This apatite was utilized directly by ectomycorrhizal tree species (spruce and fir) bypassing the soil exchange complex. In the Blum et al. (2002) study, ectomycorrhizal fungi associated with the roots of conifers provided 95% of the calcium found in the foliage of the trees and 35% of the Ca leaving the mixed conifer hardwood watershed in stream water. 

Atmospheric deposition has been repeatedly shown to be the major source of sulfur deposition in many forest ecosystems, especially in regions significantly impacted by acid deposition (Mitchell et al., 1992b). Sulfur inputs to forests derived from mineral weathering generally play a minor role however, this is dependent upon the local geology. Regions underlain by sulfide-rich... 

Geochemical constraints on acid deposition come from the foram record of a spike in Sr/ Sr across the K/T boundaiy and from base cation leaching at terrestrial K/T sites in eastern Montana. A statistical analysis of the foram data of Martin and Macdougall [1] yields an enhancement in Sr/ Sr of 25 x 10 9x10 We considered two sources of this enhancement Sr leaching from impact ejecta and increased continental weathering due to acid deposition. Impact ejecta with the Sr concentration and ratio of Chiexulub melt rock, 300... 

Extrapolation to the K/T boundary requires consideration of the time scales of acid deposition. Nitric acid formation occurs rapidly by aqueous phase reaction of NO and NO2 with liquid water produced by tlie incident K/T bolide on both impact and infall of ejecta. For tlie quantities of NO produced by the K/T impact ( 10 5 moles), conversion to HNO3 occurred wiUiin days, assuming sufficient liquid water was available in the posl-K/T atmosphere. The nitric acid will form an acid rain of pH 0 for a liquid water content of 1 g/m (typical of tropospheric clouds) but will contain enough protons to weather only 3 x 10 moles of Sr, for Sr/(Ta -0.003 in soil and bedrock minerals. Sulfuric acid formation occurred on a time scale of years [7] due to the slow rate of gas phase SO2 oxidation. Spread evenly over 10 years, 10 moles of SO2 produced a global acid rain of pH —4, and released —3 x 10 moles of Sr. 

As Figure 15.3 illustrates, aggrading vegetation (forests and intensive crop production) produces acidity because as more cations are taken up by the plants (trees) H" is released through the roots. The protons released react with the weatherable minerals to produce some of the cations needed by the plants. The H" balance in soils (production through the roots versus consumption by weathering) is delicate and can be disturbed by acid deposition. 

Schnoor, J. L. and W. Stumm (1986) The role of chemical weathering in the neutralization of acid deposition. Schweiz. Z. Hydro . 48,171-95. 

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