China in July held almost $847 billion in U.S. Treasuries, more than double the amount it held four years earlier.
U.S. economic growth slowed to an annual rate of 1.7 percent in the second quarter from 3.7 percent in the first period. The growth figures are adjusted for changes in prices; nominal gross domestic product doesn't adjust for inflation.
The economy probably expanded at a 1.9 percent pace in the quarter ended Sept. 30, according to the median estimate of 62 economists surveyed by Bloomberg News.
Krugman, a New York Times columnist, wrote last year during a previous clash that Ferguson "hasn't bothered to understand the basics" of economics. Ferguson has said Krugman's economic assumptions are based on a "flawed" reading of John Maynard Keynes' model.