Calculation and mapping of critical loads of sulphur and nitrogen for forest soils in Galicia (NW Spain)

Calculations of critical loads of acidity within Europe have shown Galicia to be one of the regions in Spain with the highest risk of soil acidification by deposition of atmospheric pollutants. We used the simple mass balance method (SMB) to derive critical loads of sulphur and nitrogen for forest soils in Galicia using available site-specific information. Exceedance of acidic deposition was calculated using the deposition levels of N and S registered in 2001. Most forest ecosystems analysed can support acid loads of more than 2000 eq N ha 1 year 1. The lowest critical loads of acidity (1483 eq ha 1 year 1) occurred in coniferous forest stands growing on shallow soils developed on slates, in the interior zone. The highest critical load values (9386 eq ha 1 year 1) were observed in coniferous stands in zones with elevated precipitation rates, in the littoral area. The critical load for acidity was exceeded in 5% of the forest soils. Such exceedances were mainly caused by deposition of nitrogen compounds. The contribution of sulphur deposition to acid exceedance was important in the surroundings of the city of A Corun˜a. Analysis of rainfall bulk composition revealed that N deposition in Galicia was mainly due to ammonium emissions, probably derived from agricultural and cattle breeding activities.