NEEDLE S-FRACTIONS AND S-TO-N RATIOS AS INDEXES OF SO2 DEPOSITION

Needles of Scots pine (Pinus sylvestris L.) from 25 and 40 sampling pl ots in southern and northern Finland, respectively, that had earlier b een analysed for total sulphur concentration (S-t) were reanalysed for foliar sulphate sulphur (SO4-S) and total nitrogen (N-t). Organic sul phur content (S-o) was calculated as the difference between S-t and SO 4S. Current (c) and previous-year (c+1) needles were collected from so uthern Finland in December 1989 and c - c+2 needles from northern Finl and in September-October 1990/September 1992. The results show that th e S-t concentration and S-t/N-t ratio in Scots pine needles are good i ndices of dry deposition of SO2 in general, while SO4-S concentrations and SO4S/S-o ratios can be used in areas with low N supply from the s oil and/or low wet deposition of N. The normal S-t concentration in ne edles of Scots pines growing on a podzol with low N supply is consider ed to be 500-700 mu g g(-1) and that of SO4-S 100-200 mu g g(-1). An i ncrease of 100 mu g g(-1) in needle S-t may be attributed to a rise of 1.4 mu g m(-3) in ambient SO2 concentration in areas with relatively low SO2 concentrations (<15 mu g m(-3)). A critical level of 5 mu g m( -3) as an annual and growing season mean is proposed for forestry in n orthern Europe (north of 60 degrees N).