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...Chapter effects of nitrogen containing air pollutants critical levels the first edition these guidelines drew attention to on vegetation for were formulated as follows in presence sulfur dioxide and ozone not higher than g m respectively atmospheric concentration should be no a yearly average hour means order protect sensitive ecosystems total deposition exceed per year this new literature is reviewed judge whether guideline needs revision atmosphere earth dinitrogen n which equals about kg above each hectare s surface unpolluted conditions small fraction ha converted by fixing microorganisms biologically more active forms ammonium nh nitrate natural compounds other much less itself contains five times but weathering solid rock negligible source denitrification reduction lesser extent nitrous oxide o ammonia recycled from human activities both industrial agricultural have strongly increased amount thereby disturbing cycle various pollute mainly nitric dry wet another contribution occul...