Effects of simulated acid precipitation on the cycling of manganese under Sitka spruce (Picea sitchensis)

Simulated acid rain at pH 3.3, 4.3 and 5.3 has been applied by overhead spraying to reconstructed soil profiles and young Sitka spruce (Picea sitchensis) trees in pots to assess the effect of rainfall acidity on the fate and recycling efficiency of Mn from (5)4Mn-labelled Sitka spruce litter. The concentration of (5)4Mn in throughfall was significantly increased by acidification of the rain, but (5)4Mn activity in drainage water remained low and was not significantly affected. The fact that only <1.5% of Mn-54 was lost in drainage water suggests that forest ecosystems efficiently retain Mn, at least in the short term. The bulk of (5)4Mn (approximately 60 to 70%) was retained in the litter layer. Compared with rain at pH 4.3, rain at pH 3.3 acidified L/F and H/A horizon soils. The amount of Mn-54 found in the different soil horizons was only influenced significantly by rainfall acidity in the E horizon soil, however, where Mn-54 activity was increased by the most acid treatment. The activity of Mn-54 in Sitka spruce needles did not differ significantly with treatments.