Cycling of nutrient base cations in a twelve year old Sitka spruce plantation in upland mid-Wales

The effects of the early growth of plantation forestry on the biogeochemica l cycling of potassium, calcium and magnesium have been investigated in a s tand of 12 year old Sitka spruce and adjacent moorland growing on acid peat y podzol soils in mid Wales. Element budgets have been calculated for both systems using measurements of soil and vegetation base cation pools and flu xes. In the moorland, the magnesium budget is approximately at steady-state with no net change to the soil store whilst the soil is accumulating potas sium. The calcium budget is approximately balanced but contains significant uncertainties due to between plot variability in calcium leaching losses. Afforestation has greatly increased the above-ground living biomass which h olds 7 to 15 times more nutrients compared to the living aerial biomass in the moorland. With the exception of magnesium, the base cation stores withi n the forest soil are being depleted as the increase in atmospheric deposit ion due to the forest canopy provides only a small offset to the much large r accumulation of base cations within the trees. The current net rate of ch ange in the soil store of calcium is sustainable for only 65 years. However , as the trees mature, their demand for calcium will be reduced and they sh ould be able to 'tap' deeper sources of calcium in the soil profile as well as in the drift and regolith material.