It is apparent that the acid neutralising capacity in catchments is strongly influenced by soil type and geology. Additionally, the response of surface water chemistry to different vegetation types within catchments has been addressed by numerous studies (e.g. Johnson
and Swank, 1973; belong etal., 1990). In anthropogenically modified catchments in the UK
the influence of land management practices are particularly important. These can significantly alter the chemistry of the soil as well as influencing deposition efficiency (Fowler
at a!., 1989). With regard to the acidification of surface waters two types of land use change have major consequences, particularly in the more sensitive upland areas. These are conifer afforestation and acid amelioration programmes based on the application of neutralising agents (Reynolds and Ormerod, 1993).
2.7.2.1 Conifer afforestation
A number of authors have observed increased levels of acidity and toxic dissolved aluminium in streams draining areas of conifer forest compared with those draining
equivalent moorland areas (Harriman and Morrison, 1982; Stoner et a!., 1984; Stoner and
Gee, 1985; Reynoids at al., 1986). This has also been supported by analysis of diatom
assemblages in the sedimentary records of lakes with and without coniferous forestry in the
catchment (Kreiser at a!., 1990). There are a number of mechanisms which may be
responsible for this and the impact of these will, to some extent, be dependent on the species of tree (Hornung, 1985).
Increases in acid input can resuit from cloud droplet deposition. Compared to moorland vegetation, the forest canopy provides more surfaces onto which acidic water droplets can
impact. This causes increased concentrations of solutes in throughfali (Fowler at a!., 1989).
Solute concentrations will also be increased by evaporation in the canopy (Reynolds and Ormerod, 1993). Concentrations in throughflow and stemfiow to the forest floor may exceed
that of rain input by a factor of five (Cape at a/., 1987) although it is aiso suggested that
trees can reduce the acidity of water passing through the canopy layer (Miller at a/., 1987).
The increased levels of évapotranspiration also affects the annual runoff and baseflow levels of afforested streams relative to moorland streams. The proportion of well buffered baseflow
in the streamflow of afforested catchments is reduced thus iowering the pH (Bird at a/.,
1990a).
Prior to planting, ploughing and drainage can alter the chemistry of the soil and influence the hydrological regime of the affected catchment (Waters and Jenkins, 1992). The soil becomes drier following drainage. This increases organic matter decomposition and aeration, releasing sulphate which, in turn, increases the level of soil water acidity (Bache, 1984). The furrows from ploughing and drainage channels also influence soil water hydrology so that acidic runoff enters the stream network, bypassing the buffering potential of the soil system, although a number of management techniques have been identified which may overcome this problem (Miller, 1985). This latter effect becomes subordinate to that of soil drying as
the forest matures (Bird et al., 1990a).
The planting of coniferous forests on uplands previously characterised by moorland vegetation is likely to cause an increase in base cation uptake. Following harvesting the base cations are lost to the system thus decreasing the base saturation of the soil although forest management practices such as reducing the intensity of the harvest (Hornbeck, 1992) and the addition of fertilizer (Kreutzer, 1988) may lessen the impact. An understanding of the long term effects of clearfelling is inhibited by the practice of replanting felled areas
(Reynolds etal., 1995). Short term streamwater responses are driven by factors which may
exacerbate or ameliorate acidity. Harvesting may impact on flow pathways with more water
flowing through acid upper horizons, reducing the potential for buffering (Neal et al., 1992;
Reynolds et al., 1992). Elevated concentrations of NOg', Ca^^ and labile monomeric
aluminium, with concomitant decreases in pH, have been observed for two years following whole tree harvesting (Lawrence, 1988), a result of increased soil nitrification and reduced uptake. Conversely canopy removal is likely to lead to a reduction in dry and cloudwater
deposition of 8 and N species (Fuller etal., 1987; Adamson and Hornung, 1990). Generally,
the acidifying potential of forest growth is particularly problematic in sensitive catchments (Sverdrup and Warvfinge, 1990).
There is considerable debate as to whether observed increases in stream acidity in forested
catchments is due to forest growth or as a result of acid deposition (Billet et al., 1990,
Eriksson et al., 1992). Recent applications of the dynamic MAGIC model to forested sites
suggest that a combination of afforestation and high sulphate deposition on acid sensitive
soils is responsible for acidification of soils and surface waters (Cosby et al., 1990; Jenkins
2.T.2.2 Upland agricultural improvement
A variety of techniques exist to improve pasture in upland areas depending on the type of soil being treated (Newbold, 1985). These include stock control, the addition of artificial fertiliser and reseeding with more productive grasses (Reynolds and Ormerod, 1993). However, the application of lime is by far the most widespread management practice used to overcome the natural acidity of many upland soils.
2.7.2.3. Catchment liming
The application of lime to sensitive upland soils has major consequences for the exchangeable cation complex causing increases in calcium and decreases in aluminium
(Hornung et al., 1986). Soil pH also increases and with it the pH dependent CEC.
Liming causes changes in soil water chemistry and can alter the susceptibility of upland catchments to acidification (Boon and Kay, 1990). Following the application of lime to the experimental catchment at Llyn Brianne there was a rapid increase in pH, alkalinity and calcium concentrations in surface horizons (Reynolds and Ormerod, 1993). Increases in ammonium encourages nitrification and increased nitrate concentrations which, if leached, can lead to acidification (Meiwes, 1995). If NOg' is leached with base cations the soil is more
susceptible. Surface waters are affected if NOg' is leached with H^ or Al^. These changes
in soil water chemistry are also reflected in improvements in the surface water, with increases in pH, calcium and magnesium levels and reductions in aluminium concentration
in comparison with streams draining unimproved catchments (Hornung at a!., 1990c).
Consequently, agricultural liming has lately been used to ameliorate the effects of surface water acidification (Crawshaw and Diamond, 1988). The benefits to surface waters of catchment liming are dependent on the type of materials used, the rate (Gasser, 1985) and
method of application and the placement within the catchment (Hornung et al., 1990c). Additionally the effectiveness in combatting episodic acidification needs to be considered
(Porcella etal., 1989).
The utility of this approach is still under review. Although pasture improvement has been shown to reduce surface water acidity and increase base cation concentrations (Adams and Evans, 1989), evidence suggests that the major hydrologically active areas in the catchment
should be specifically targeted for treatment (Waters et al., 1991). Deleterious effects on
flora (Mackenzie 1989; Larsson, 1995) and fauna (Mackenzie and Shore, 1989) following liming have also been noted.
2.7.2A. Lake liming
Liming direct to lakes and streams is now widely practised, particularly in Sweden and Norway, and a number of long term studies have been undertaken to assess its impact on surface water acidity (Henriksen and Brodin, 1995; Svenson ef a/., 1995). Generally, these programmes have enhanced water quality and increased species richness and diversity
(Appleberg, 1995). A variety of agents, doses and strategies are employed (Henriksen et
al., 1995). Problems can be encountered with direct applications to streams as the supply
of base cations from slowly dissolving lime may not adequately counteract acid pulses (Milner and Varello, 1990). Lake liming is more widespread and less expensive than stream
liming (Svenson et al., 1995). The efficacy of this approach is strongly dependent on the
turnover time (Werritty and Maucotel, 1992).