(Click to enlarge.) The USA is not on the above figure; their number of papers is about a factor 4 higher still: During the period shown the USA output increased from 265,000 to 340,000 publications. Note that Russia's output actually decreased during that period. The increase in the number of Chinese publications is even more dramatic when you normalize each curve to the 1982 level:
China's share on the number of world publication (in 2004-2008) is the largest in material science (20.83%), chemistry (16.90%), and physics (14.16%).
For details on the figures, download the full report on China here, and the report on Russia here.