Calculation and mapping of critical loads for S, N and acidity in China

Critical loads of nutrient and acidifying nitrogen, as well as of sulphur and acidity, were derived for various ecosystems in China using the steady state mass balance (SSMB) equations. The weathering rates of major soils necessary for applying SSMB were calculated through the PROFILE model on the basis of mineralogical data from experimental analysis. The growth uptakes of nitrogen and base cations were also derived by multiplying the annual increases in biomass with the element contents of the vegetation. Using a geographical information system (GIS), 1 degrees (latitude)x1 degrees (longitude) critical load maps of China with different percentiles were compiled. Results indicate that low critical loads of S (< 0.5 keq(.)ha(-1.)a(-1)) occurred predominately in southwest and northeast China, and the critical loads of southeast China were intermediate and in the range of 0.5 similar to1.0 keq(.)ha(-1.)a(-1). In addition, the critical loads of N were very low for desert ecosystems in northwest China and high for agricultural ecosystems in east China. Among the ecosystems with intermediate critical load of N, coniferous forests may be more sensitive to N deposition than broad-leaf forests and temperate steppes.