Empirical critical loads of nitrogen in China

Abstract

For supplying the scientific basis of future nitrogen (N) emission control, the empirical critical loads of some forests and grasslands in China were determined by using reported field observations of N effects. Results showed that the critical loads of forest varied from 10~30 kg N ha-1 year-1for temperate deciduous forest to 170~300 kg N ha-1 year-1for subtropical coniferous plantation, and those of grassland varied from < 50 kg N ha-1 year-1for typical temperate steppe and alpine steppe to 150~250 kg N ha-1 year-1for subtropical grassland. These critical loads were much higher than 10—15 kg N ha-1 year-1and 10–30 kg N ha-1 year-1, the common values of natural forest and grassland respectively in Europe. The critical load map, based on the median of critical load range of each vegetation type, indicated that the empirical critical loads were lower in the northwest part of China, and higher in the southeast. Although the critical loads of N in the southeast part of China, where high nitrogen deposition existed, were relatively high to very high, critical loads occurred in some areas in north China and the south of northeast China, and sporadically in southwest China and east China. Nitrogen emission abatement is required in these areas to avoid exceedance of the estimated critical loads.