The official purchasing managers' index (PMI) fell to 49.0 in December from 49.4 the previous month, according to the National Bureau of Statistics, below the 50-mark separating growth from contraction and weaker than a median forecast of 49.5 in a Reuters poll. China's central bank said on Thursday it would step up policy adjustments to support the economy and promote a rebound in prices, amid signs of rising deflationary pressures.