The plt dropped off parachutists at 8,500 ft and began to descend back to arpt. The a/c was descending at 145 mph, 19 inches mp, and 2300 rpm in a long circular pattern. The flt was on final between 500 and 1000 ft msl when the plt tried to apply pwr. The eng failed to respond. After switching the fuel selector and attempting to restart the eng twice, the plt decided to lnd in a field short of the runway. During landing roll, the nose gear broke off and the a/c nosed over. Later the eng was operated to full pwr with no evidence of failure or malfuction. Calculations indicated enough fuel for operation and fuel was found in carb bowl. According to icing probability curves, the a/c was flying in conditions conducive to carb icing. The plt stated that carb heat was not applied during descent.