\begin{equation}
\mathit{t_{nitrogen}}=\sum_{l=1}^\mathit{nbdl} \mathit{tsoil_{month}} \cdot \mathit{rpc} \cdot \left( e^{  -\frac{\mathit{z_{soil}(l-1)}}{\mathit{z_{nitrogen}}}  } 
- e^{  -\frac{\mathit{z_{soil}(l)}}{\mathit{z_{nitrogen}}}  } \right)
\end{equation}
