The oxidation number of nitrogen atoms in NH₄NO₃ are:

$\stackrel{-3}{{\mathrm{NH}}_4}\stackrel{+5}{{\mathrm{NO}}_3}$ $\to$${\mathrm{NH}}_4^{+} +{\mathrm{NO}}_3^{-}$

$$\begin{gathered} {\mathrm{NH}}_4^{+} \mathrm{X}+4(+1)=+1 \\ \mathrm{X}+4=+1 \\ \mathrm{X} =-3 \\ {\mathrm{NO}}_3^{-} \mathrm{X}+3(-2)=-1 \\ \mathrm{X}-6=-1 \\ \mathrm{X}=+5 \end{gathered}$$