Magneto-capacitance spectra of quantum wires clearly reflect the energy quantization of the electronic system in one-dimensional subbands. We present results of simulation calculations of the wire capacitance, in which interaction effects are included by local density approximation. The simulation calculations show that a peak observed in the experiments close to the wire threshold is caused by exchange and correlation effects in the one-dimensional electron system. Spin effects are calculated in the local spin density approximation. At densities, where the second spin level starts to be occupied, we find effective g-factors similar to values in two-dimensional electron systems at the spin gap.