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Agronomy QUESTION #2096
Question 1
The nitrogen cycle in agricultural soils involves multiple transformation processes. Which sequence correctly represents the pathway of urea fertilizer nitrogen from soil application to crop uptake?
  • Urea → Denitrification → \(N_2O\) → Plant uptake
  • Urea → Hydrolysis (\(NH_4^+\) via urease) → Nitrification (\(NO_3^-\) via Nitrosomonas/Nitrobacter) → Plant uptake or leaching/denitrification losses✔️
  • Urea → Direct uptake as intact molecule → Hydrolysis inside root cells
  • Urea → Volatilization as \(NH_3\) → Redeposition → \(NO_3^-\) formation → Plant uptake
Correct Answer Explanation
Urea undergoes rapid hydrolysis by the soil enzyme urease: \(CO(NH_2)_2 + H_2O \rightarrow 2NH_3 + CO_2\), producing \(NH_4^+\) in soil solution. Nitrification then converts \(NH_4^+ \rightarrow NO_2^- \rightarrow NO_3^-\) (Nitrosomonas, Nitrobacter). \(NO_3^-\) is mobile and plant-available but vulnerable to leaching and denitrification. This sequence explains why urease and nitrification inhibitors improve N efficiency.