自然界中的氮循环( The Nitrogen Cycle) by some anaerobic bacteria Amino acids nd other reduced synthesis in nitrogen-carbon plante and microorganisms degradation by animals and Ammonia anIsms Nitrate NO NHA nitrogen fixation by some bacterin (e. g, Klebsiella, Azotobacter Rhizobium) nitrification nitrifcation by soil bacteria by sod bacteria Nitrite Figure 21-1 NO2 of nitrogen fix 10 kg The total amount of nitrogen fixed annually in the biosphere exceeds 10kg自然界中的氮循环（The Nitrogen Cycle ） The total amount of nitrogen fixed annually in the biosphere exceeds 1011 kg