By buying bonds with a negative yield, investors are essentially prepared to pay Germany for the right to hold its debt if they retain those bonds until maturity.

Image Jens Weidmann, president of the Bundesbank, at the German central bank's headquarters in Frankfurt. Credit... Kai Pfaffenbach/Reuters

Investors, however, can potentially sell those bonds for a gain in the short term if corporate and government spending takes off in Europe after the E.C.B. begins its bond-buying program this year, said Alberto Gallo, the head of European macro credit research at the Royal Bank of Scotland. “Obviously, this could be a positive or a very big negative. It depends on how corporates and sovereigns react,” Mr. Gallo said.

Beginning in March, the E.C.B. is aiming to make €60 billion a month in purchases of government bonds and other debt to try to stimulate growth.

Negative-yield bonds are the fastest-growing asset class in Europe, with about 30 percent of European sovereign bonds trading at a negative yield, Mr. Gallo said. Some corporate bonds have also fallen to a yield below zero in the secondary market, he said.

For some large financial institutions, buying German debt at a negative yield is still a better option than keeping money on deposit in Switzerland or with the E.C.B., both of which are charging banks to hold money above certain thresholds, Mr. Gallo said.