The role of C:N ratio in compost feedstock is similar to that of soil feedstock. The recommendation is around 20-30:1. The microbes prefer a ratio of 30-35:1, but the carbon is usually not completely digested (especially in the case of lignin feedstock), hence the lowered ratio.
An imbalance of C:N ratio causes a slowdown in the composting process and a drop in temperature. When the C:N ratio is less than 15:1, outgassing of ammonium may occur, creating odor and losing nitrogen. A finished compost has a C:N ratio of around 10:1.