Interception and attenuation of atmospheric pollution in a lowland ash forested site, Old Pond Close, Northamptonshire, UK

A study of interception of chemicals at an ash plantation forest in southern-central England shows the modification of acidic pollution as it passes from precipitation, through the vegetation cover and the soil to generate surface runoff. Precipitation is highly acidic ŽpH 3.7 4.8, alkalinity 16 to 200 Eq l. and it is enriched in the strong acid anions associated with acidification Žsulfate and nitrate. as well as ammonium and the trace elements aluminium and zinc. The concentration of both sea-salt and pollutant components varies considerably over time and this is linked to washout from the atmosphere during precipitation events as marked by an inverse relationship between concentration and volume of catch. The catchment is also supplied by sea-salt and pollutant additions as dry deposition: gaseous inputs of SOx may also increase sulfate deposition. Through the vegetation, much of the acidity is neutralised and, particularly during the growth period, calcium, magnesium and potassium is cycled, while sodium and nitrate are partially removed. Within the catchment, weathering ensures that further base cation production occurs leading to enhanced neutralisation of acidity and the generation of positive alkalinities. As a result, surface runoff becomes buffered with alkalinity approximately 490 Eq l and pH approximately 7.9. Thus, although the acidification input from the atmosphere is high, this does not translate to acidic runoff due to within-canopy and within-soil processes.