ROLE OF VEGETATION COVER IN THE BIOGEOCHEMICAL BALANCES OF SMALL AFFORESTED CATCHMENT IN SOUTHEASTERN SPAIN

The budgets of major dissolved nutrients have been determined in a sma ll micaschist catchment (0.34 km2) covered with Mediterranean shrubs a nd afforested pines at 1560 m altitude in the Filabres range (southeas t Spain). The climate is semiarid with 395 mm annual rainfall and 12-d egrees-C annual mean temperature. During the hydrological year 1988-19 89, major element concentrations in open field precipitation, throughf all, stemflow and runoff were recorded as well as the element fluxes o f the main solutes. The total rainfall recorded during the sampling ye ar was 650 mm of which 86% was lost through evapotranspiration and 14% through runoff. Mean annual rainfall interception is estimated as 15% and net rainfall reaching the soil is divided between 90% throughfall and 10% stemflow. Two methods were applied to compute bulk precipitat ion inputs to the ionic balances: (I) bulk open field deposition was e stimated from data provided by open field polyethylene collectors (bul k open field precipitation inputs); (II) net throughfall and stemflow were corrected for canopy leaching. The results show that the total de position calculated using balance (II) is higher than that calculated by (I); the effect of canopy surfaces in scavenging atmospheric dry fa llouts and gases is identified in a more efficient way than by simple polyethylene collectors. When comparing both methods of computing bala nces in several environments, using published data, the authors conclu de that the plant canopy effect is larger in sermiarid areas with dust y atmospheres and in industrial regions with higher concentrations of sulphur. These conclusions suggest that the retention of nutrients wit hin the ecosystem may be more widespread than was previously thought. This calls for more research on the assessment of biomass removal and the dynamics of intermediate pools (precipitates). Consequently, it th rows doubts on most of the figures available for catchment weathering rates; these were often estimated by difference between bulk open fiel d precipitation inputs measured with polyethylene collectors and runof f outputs.