We give a non-trivial upper bound for the critical density when stabilizing i.i.d. distributed sandpiles on the lattice $\mathbb{Z}^2$. We also determine the asymptotic spectral gap, asymptotic mixing time and prove a cutoff phenomenon for the recurrent state abelian sandpile model on the torus $\left( \mathbb{Z} / m\mathbb{Z} \right)^2$. The techniques use analysis of the space of functions on $\mathbb{Z}^2$ which are harmonic modulo 1. In the course of our arguments, we characterize the harmonic modulo 1 functions in $\ell^p(\mathbb{Z}^2)$ as linear combinations of certain discrete derivatives of Green's functions, extending a result of Schmidt and Verbitskiy.