Acidification from atmospheric inputs

Acidification of freshwater is most marked in upland areas with high rainfall (hence high acid flux), steep slopes (resulting in a short residence time for water in the soil) and crystalline rocks (which weather, and supply cations, slowly). Thus, while acid rain is a widespread phenomenon, acidified freshwaters are less common and are controlled both by rates of atmospheric input and by rock types (Fig. 5.7). All weathering processes, except sulphide oxidation (see Sections 4.4.2 5.4.2), consume hydrogen ions, driving pH toward neutrality. Hence, mature rivers, which drain deeper, cation-rich lowland soils, have higher pH and lower aluminium concentrations. 

The effects of upland acidification of freshwaters can be dramatic. Between 1930 and 1975 the median pH of lakes in the Adirondack Mountains of northeastern USA decreased from 6.7 to 5.1, caused by progressively lower pH in rainwater (Fig. 5.7). The acidified lakewater killed fish and other animals by several mechanisms. The problem for fish is that the dissolved Al3+ in the acidic water precipitates as an insoluble Al(OH)3 gel on the less acidic gill tissues, preventing normal uptake of oxygen and suffocating the animal. Similar problems have occurred in Scandinavia and Scotland. In addition to problems in freshwaters, the loss of forests in high-altitude areas has been linked to acid leaching, which leads to impoverishment of soils coupled with direct loss of cations from plant leaves. 

---

A comparison of element inputs from the atmosphere and element outputs by leaching of the subsoil (matter balances) provides evidence about the storage of elements in the ecosystem. The S- and N-balances provide information about the acidification rate, which is caused by air pollution (Liu et al. 1993). To diminish the impacts of acid inputs, selected forest sites should be limed with calcium carbonate or dolomite at a level of 3 t ha , with a replication after 3 to 5 years. 

Figure 5.3. Forest N cycling and acidification. Atmospheric N inputs and N outputs to drainage waters are shown in italics. Soil processes (left) and plant processes (right) are clustered within ovals. Dashed lines indicate soil-plant exchanges (plant N uptake or organic N return to soil). Solid lines show processes within soils or plants. Dotted lines show fluxes into or out of forests. Values in brackets refer to net consumption [-] or production [+] of 1 mol associated with the transformation of 1 mol N. When forest N cycles are closed (small N inputs and outputs), the sum of consumed and produced by soil and plant processes is zero and no acidity is generated. When 1 mol of organic N is mineralized (1 mol consumed) and subsequently nitrified (2 mol produced), 1 mol remains to acidify soil or drainage water if nitrate is not removed from soil and converted to organic form by plants. Denitrification to any of three gaseous products consumes 1 mol H. Direct inputs of acidity can also result from ammonium and nitrate deposition. Reprinted from Nadelhoffer (2001) with permission from Elsevier...

---