Given a rank $2r$ matrix $M\in\Bbb Q_{\geq0}^{n\times n}$ can we find two matrices $M_+\in\Bbb Q_{\geq0}^{n\times n}$ and $M_-\in\Bbb Q_{\geq0}^{n\times n}$ each of rank at most $r$ such that $M=M_+-M_-$ holds? Variant of https://mathoverflow.net/questions/290126/what-is-this-matrix-decomposition-called-and-does-it-exist-always and $M$ is non-negative here and so the counter example there does not work.