Microbes like Rhizobium, Nitrosomonas and Nitrobacter are used for:

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Step 1: Rhizobium fixes atmospheric nitrogen in root nodules, converting molecular nitrogen to ammonia: $N_2 \xrightarrow{	ext{nitrogenase}} 2\,NH_3$.  
Step 2: Nitrosomonas oxidizes ammonia to nitrite (simplified): $NH_3 ightarrow NO_2^-$. A balanced oxidation: $NH_3 + 1.5\,O_2 ightarrow NO_2^- + H^+ + H_2O$.  
Step 3: Nitrobacter oxidizes nitrite to nitrate: $NO_2^- + 0.5\,O_2 ightarrow NO_3^-$.  
Step 4: All three microbes participate in transformations of nitrogen compounds (nitrogen fixation and nitrification), so they are involved in the nitrogen cycle, not primarily in carbon, water, or sulphur cycling.  
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