We investigated a plasma excited by high power pulsed microwaves (MWs) (pulse duration 2.5 mikrosecond, repetition rate 400 Hz, peak power 100 kW, frequency 9.4 GHz) in nitrogen at reduced pressure (pressure range 10-2000 Pa) with the aim of a better understanding of such types of discharge. The construction of the experimental device suppresses the plasmawall interactions and therefore the volume processes are predominant. To obtain the temporal evolution of the electron density we used two MW interferometers at frequencies of 15 and 35 GHz with dielectric rod waveguides which gives them the capability of localized measurements. We estimated the effective collision frequency from the absorption of a measurement beam. Time resolved optical emission spectroscopy of the 1st negative system and the 2nd positive system was carried out, too. Due to a high power input the discharge dynamics was fast and the steady state was typically reached in 1 mikrosecond. We found that the effective collision frequency has the same temporal behaviour as the 2nd positive system of N2, including a characteristic maximum at the beginning of the pulse.