▁of ▁the ▁carbon ▁into ▁their ▁cells . ▁If ▁the ▁C : N ▁ratio ▁of ▁the ▁decom pos ing ▁plant ▁material ▁is ▁above ▁about ▁ 3 0 : 1 ▁the ▁soil ▁micro b ial ▁population ▁may ▁take ▁nit ro gen ▁in ▁min eral ▁form ▁( e . g . ▁nit rate ). ▁This ▁min eral ▁nit ro gen ▁is ▁said ▁to ▁be ▁imm obil ized . ▁Micro organ isms ▁out - comp ete ▁plants ▁for ▁N H 4 + ▁and ▁NO 3 - ▁during ▁imm obil ization , ▁and ▁therefore ▁plants ▁can ▁easily ▁become ▁nit ro gen ▁def ic ient . ▁ ▁As ▁carbon ▁dio x ide ▁is ▁released ▁via ▁decomposition ▁the ▁C : N ▁ratio ▁of ▁the ▁organ ic ▁matter ▁decre ases , ▁and ▁the ▁micro b ial ▁demand ▁for ▁min eral ▁nit ro gen ▁is ▁decre ased . ▁When ▁the ▁C : N ▁ratio ▁falls ▁below ▁about ▁ 2 5 : 1 ▁further ▁decomposition ▁results ▁in ▁simultane ous ▁min eral ization ▁of ▁nit ro gen ▁which ▁is ▁in ▁excess ▁to ▁that ▁required ▁by ▁the ▁micro b ial ▁population . ▁ ▁When ▁decomposition ▁is ▁virt ually ▁complete ▁soil ▁min eral ▁nit ro gen ▁will ▁be ▁higher ▁than ▁it ▁was ▁initially ▁due ▁to ▁min eral ization ▁of ▁the ▁plant ▁resid ue ▁nit ro gen . ▁ ▁Mechan isms ▁of ▁nit ro gen ▁imm obil ization ▁ ▁There ▁are ▁two ▁mechan isms ▁of ▁nit ro gen ▁imm obil ization : ▁N it ro gen ▁accum ulation ▁in ▁micro b ial ▁bi om ass ▁and ▁accum