Throughfall chemistry in a Sitka spruce plantation in response to six different simulated polluted mist treatments

A mixed provenance Sitka spruce plantation, planted in 1986 on a drained de ep peat, has been exposed to 6 different simulated mist treatments in 4 rep licated blocks since 1996. Treatments provided N and/or S at a concentratio n of 1.6 mol m(-3), supplying ca. 50 k g S and/or N ha(-1)yr(-1) as N (NH4N O3), S (Na2SO4), NS Acid (NH4NO3 + H2SO4 at pH 2.5), 2NS Acid (double dose by application at twice frequency), a control treatment supplied with addit ional rainwater only and a 'no treatment' set of plots. Throughfall, preser ved with thymol in the field, was collected using gutters with a surface ar ea of 1m(2) in all the replicate plots, and was analysed for all major ions . Prior to treatment in 1999, S deposition in throughfall exceeded that in rain because of dry deposition of SO2 and SO42- to the canopy; NH4+ and NO3 - ions were both retained in the canopy. During treatment, only 20-40% of t he applied N in the high-N treatments was retained in the canopy. Acidity i n the applied mist was partly neutralised by the canopy, but not primarily through exchange of base cations, leading to the conclusion that weak organ ic acids, in solution or in situ in the canopy, contributed to the bufferin g of the H+ ion deposition in the acid treatments.