# Baryon number conservation

Can someone explain me why the Hamiltonian for the nucleon field (derived from the corresponding Lagrangian) $$H_N=\int d^3xN^\dagger(\textbf{x})\Big(-\frac{\nabla^2}{2M_0}+M_0\Big)N(\textbf{x})$$ becomes $$H_N=\sum_{i=1}^A\Big[\frac{P_i^2}{2M_0}+M_0\Big]$$ (where the sum is over the nucleons) because of baryon number conservation?