We consider a certain linear combination $S(\mathbf{ s},\mathbf{ y};I;\Delta)$ of zeta-functions of root systems, where $\Delta$ is a root system of rank $r$ and $I\subset\{ 1,2,\ldots,r\}$. Showing two different expressions of $S(\mathbf{ s},\mathbf{ y};I;\Delta)$, we find that a certain signed sum of zeta-functions of root systems is equal to a sum involving Bernoulli functions of root systems. This identity gives a non-trivial functional relation among zeta-functions of root systems, if the signed sum does not identically vanish. This is a genralization of the authors' previous result proved in \cite{ KMTLondon}, in the case when $I=\emptyset$. We present several explicit examples of such functional relations. A criterion of the non-vanishing of the signed sum, in terms of Poincar{ \'e} polynomials of associated Weyl groups, is given. Moreover we prove a certain converse theorem, which implies that the generating function for the case $I=\emptyset$ essentially knows all information on generating functions for general $I$.
Categories: math.NT