Whilst there is a considerable body of evidence to support the contention that anthropogenically-derived atmospheric pollutants can result in the acidification of surface waters, several authors such as Krug and Frink (1983), argue that the effects of these pollutants are minor in comparison with the impact of land-use changes, and in particular coniferous afforestation.
2.4.1 Coniferous Afforestation
The association between coniferous afforestation and streamwater acidification is reported by several authors in upland Britain (Harriman and Morrison 1982, Stoner et al. 1984) and is described by Miller (1985) as a uniquely British feature of the acidification debate. Several general mechanisms have been put forward to account for the acidification of surface waters by forests: i) increased interception deposition, ii) changes to soil properties and iii) changes in soil hydrology.
The interception deposition mechanism has already been reviewed in Section 2.2.2 and it is sufficient to repeat that whilst coniferous afforestation will no doubt lead to an increase in solute inputs, the available evidence for the influence of this process on acidification is inconclusive since evidence exists for neutralisation as well as acidification below coniferous canopies (Miller 1985). In either situation there will be a net transfer of acidity to the soil, with possibly a consequent increase in drainage water acidification. Also if there is a significant increase in ’seasalt' interception, the 'salt effect' (Wilklander 1975) may lead to a temporary increase in drainage water acidity.
In terms of changes in soil properties the growth of coniferous forests will, over a few years, lead to the production of a highly acid litter layer and surface organic horizon, with a commensurate decrease in the pH of surface and litter flow (Hornung and Newson 1986). The development of this organic layer may also decrease the rate of flow into the subsoil, thereby increasing the acidity of runoff as
an increasing proportion of flow is deflected through the acid surface horizons.
If the supply of mineral nitrogen in the soil is dominated by ammonium, as is generally the case in upland areas, then forests will remove an excess of base cations from the soil. This excess is then balanced by a flux of protons from the roots to the soil (Miller 1985). Cation exchange at roots is thus likely to be a soil acidifying process, a conclusion supported by an examination of available evidence by Miles, (1978). This report suggests that the tendency to replace indigenous Beech and Oak forests with coniferous plantations has lead to a pH decline of 1 unit in the surface horizons of underlying soils. Nilsson et al. (1982) calculate that such soil acidification will be greatest during the early stages of forest development when humus build-up and cation accumulation are at their most rapid. If the trees are left on site, however, then much of the acidification will be temporary as the base cations return to the soil as the trees die and decompose. In contrast, removal of the trees will result in the soil acidification becoming permanent, the effect being greatest if the trees are young when felled. Nilsson et al. (1982) further argue that such root-generated acidity is unlikely to lead to surface water acidification due to the lack of a mobile anion. However, Hornung and Newson (1986) point out that the balancing anions may be supplied from increased interception deposition, particularly in the form of occult deposition.
Turning now to changes in soil hydrology, if waters are able to percolate to the base rich mineral soil then almost all soils will neutralise incoming precipitation (Bache 1984). Hence changes in the
soil physical properties, associated with initial drainage improvements or with subsequent growth, such that drainage waters are less likely to enter the 'C' horizon, is the most attractive hypothesis to explain the effects of afforestation on streamwater acidification (Miller 1985). The presence of an initial drainage system established prior to planting will increase the rate at which water is evacuated from a catchment, particularly during storms (Robinson 1984). This drainage flow will be particularly acidified if the drains are covered in acidic litter deposits (Cresser and Edwards 1987). However, pre-afforestation ploughing may also breakup indurated horizons thereby increasing flow through the mineral horizons and hence reducing drainage water acidification. Improvements in drainage may also lead to the drying of anaerobic horizons allowing oxidation reactions to produce protons, typical reactions being:
4FeS + 9O2 + 10H2O <—> 4Fe(OH)3 + 4SO42- + 8H+ (2.4)
RNH2 + 2O2 <—> ROH + NO3- + H+ (2.5)
In the absence of an initial drainage network the hydrological effects of afforestation are contradictory. The greater interception and evapotranspiration from coniferous canopies (Calder and Newson 1979) are likely to reduce rapid throughflow (Cresser and Edwards 1987). However, in contrast the drying out of forest soils may be significant in the summer leading to the contraction of organic matter and the consequent development of macropore systems, which may also be produced by root development (Hornung et al. 1987). These macropores act as a pathway for rapid water transfer and consequently may lead to increased runoff acidity.
2.4.2 Deforestation
If,as alleged, afforestation is linked with streamwater acidification it may be expected that deforestation will result in an increase in stream pH. This arises because the removal of trees will result in a decrease in the interception deposition of pollutants and neutral salts which in turn will increase the pH of drainage waters (Miller 1985). Ciearfelling may also lead to an increase in base cation concentration at depth as root uptake is reduced, thereby neutralising soil and hence drainage water acidity (Nilsson et al. 1982). On the other hand, removal of the trees will prevent the return of base cations, locked in the vegetation, to the soil as the trees die and decay, hence root-generated acidification will be permanent. Clearfelling will also reduce evapotranspiration and interception losses and thereby increase flow through the surface horizons so that runoff acidity may actually increase. If the trees are not totally removed then increased mineralisation and nitrification in the humus layer may lead to an increase in the flux of sulphate, nitrate and associated cations through the soil and a possible increase in drainage water acidity (Rosen and Lindmark-Thelin 1987). The effects of deforestation will also depend on the vegetation which colonises the area afterwards, if such vegetation is an acidophilic moorland vegetation such as Calluna vulgaris,, then the catchment acidification may be little changed (Krug and Frink 1983). Overall the impact of deforestation on streamwater acidity is far from clear, much depending upon the manner of deforestation and local site factors.
2.4.3 Heather Burning and Forest Fires
The burning of moorland vegetation in order to encourage young growth for hunting and shooting is a common practice in much of upland Britain. The effects on stream acidification may be considerable especially in the short term (Starr 1985). A particularly fierce fire may have a beneficial effect on stream acidification by returning base cations in the plant ash to the soil surface layers and, if the organic horizons are destroyed, by reducing the input of organic acids to the soil. Rosenquist (1981) reports that after a particularly fierce fire in Norway where the humus layers were totally lost, the pH of streams in the burnt area was 1-3 units higher than those in the surrounding unburnt area. The reduction in surface vegetation by burning will, however, also reduce evapotranspiration and interception rates and thereby increase flow through the surface horizons and the acidity of runoff. Runoff acidity may also be increased if the burn modifies soil structure so that the surface layers remain hydrophobic for several months. Hence the influence of fires on stream acidification is complex as the liming effect of the plant ash is mitigated to a varying extent by modifications to hydrological pathways.
2.4.4. Fertiliser Inputs
The addition of fertilisers to upland catchments is generally associated with commercial afforestation as the areas are too marginal for agriculture which would necessitate fertiliser application. The use of nitrogen fertilisers is generally accepted as leading to soil acidification at locations where the soil has a low buffering capacity
and/or lime has not been added. This acidification occurs as the ammonium in the added fertilisers is nitrified, with each mole of ammonium producing two moles of hydrogen (van Breeman and Jordens 1983), typical reactions for urea and anhydrous ammonium
are;
(NH2)2CO + 4O2 —> 2NO3- + 2H+ + H2CO3 (2.6)
NH3 + 2O2 —> NO3-+ H+ + H2O (2.7)
It has been estimated that the addition of protons in an application of 100 Kg of ammonium-N per hectare may be 1.4 times that in 1000mm of rain at pH 4.0 (Cresser and Edwards 1987). In lowland agricultural areas the potential acidification is neutralised by the addition of lime to the soil which retains base saturation and pH. However, as upland afforested catchments are rarely, if ever, limed (Department of the Environment 1976) the use of fertilisers is another possible contributing agent to surface water acidification in these areas.