Effects of forest clear-cutting on the sulphur, phosphorus and base cations fluxes through podzolic soil horizons

Abstract

Clear-cutting considerably alters the flow of nutrients through the forest ecosystem. These changes are reflected in soil solution concentrations and fluxes. The effects of clear-cutting (stems only) on the fluxes of water soluble phosphorus (P), sulphur (S) and base cations (Ca, Mg and K) through a podzolic soil were studied in a Norway spruce dominated mixed boreal forest in eastern Finland. Bulk deposition, total throughfall (throughfall + stemflow) and soil percolate from below the organic (O), eluvial (E) and illuvial (B) horizons were collected for 4 years before and for 3 years after cutting. Annual deposition loads (kg ha−1) to the forest floor were less after clear-cutting, averaging 1.7 S, 0.84 Ca, 0.14 Mg, 0.64 K and 0.10 P. Before cutting, the loads were 4.6 S, 2.7 Ca, 0.70 Mg, 6.2 K and 0.20 P. Annual fluxes of total S and sulphate (SO 4 2− ) from below the O-horizon were also lower (33%) after clear-cutting, total S averaging 2.0 kg ha−1, the flux from below the B-horizon also diminished after clear-cutting. The flux of total P (mainly inorganic) from below the O-horizon increased threefold (6.9 kg ha−1; sum over the 3-year period) compared to period before cutting. The fluxes of base cations from below the O-horizon increased twofold. The flux of K+ from below the O- and E-horizons was most strongly correlated with that of phosphate (PO 4 3− ) and those of Ca2+ and Mg2+ with the DOC flux. Increased fluxes of P and base cations to the mineral soil generated only slightly increased fluxes from below the B-horizon. The retention of base cations and P in the mineral soil indicates there was little change in leaching to ground and surface waters after clear-cutting.