Sulphur balance in a broadleaf, non-polluted, forest ecosystem (central-western Spain)

Abstract

The sulphur balance in a non-polluted forested area in western Spain was estimated. The study was carried out over a period of 3 years (1990–1993) in a semi-natural, deciduous oak ( Quercus pyrenaica Willd) forest, where a permanent experimental plot was selected. Sulphate concentrations and fluxes in rainfall, throughfall, surface runoff, and drainage leachates were measured. The litterfall production, the aboveground biomass, and the S concentration in these plant fractions were measured. The S demand, S uptake, and S resorption were calculated. A positive balance was found in the ecosystem studied, since total atmospheric inputs (6.0 kg ha −1 per year), mainly through wet deposition, were higher than the outputs (4.9 kg ha −1 per year), mainly through deep-drainage. Inputs of S to the soil were higher via throughfall (5.0 kg ha −1 per year) than via litterfall (1.9 kg ha −1 per year), but almost all the SO 4 2− reaching the soil via throughfall was leached away from the root zone. Part of the tree S requirements (3.7 kg ha −1 per year) were met by retranslocation of S from senescing leaves just prior to shedding (0.8 kg ha −1 per year) and by canopy absorption (1.0 kg ha −1 per year); it is supposed that the rest of the total S demands were supplied by root uptake. Both mechanisms, S retranslocation and canopy absorption, are considered to be conservative processes that lead to nutrient economy and to a tight circulation of the nutrient in the ecosystem.