第一节 The nitrogen cycle The formation of nH4 from gas is termed nitrogen fixation. fixation is an exclusively prokaryotic process. No animals are capable of either nitrogen fixation or nitrate assimilation Animals release excess nitrogen in a reduced form. either as nhat or as organic nitrogenous compounds such as urea. The release of n occurs both during life and as a consequence of microbial decomposition following death.第一节 The nitrogen cycle The formation of NH4 + from N2 gas is termed nitrogen fixation. N2 fixation is an exclusively prokaryotic process. No animals are capable of either nitrogen fixation or nitrate assimilation. Animals release excess nitrogen in a reduced form, either as NH4 + or as organic nitrogenous compounds such as urea. The release of N occurs both during life and as a consequence of microbial decomposition following death