Asset purchases flatten the yield curve and reduce the returns on maturity transformation; and the negative interest rate policy de facto flattens the curve further, since the zero lower bound that banks typically apply on deposits means that a large share of bank liabilities no longer reprice when market rates fall. Bank lending rates however typically do, compressing loan-deposit margins.
Consistent with this, margins between loan and deposit rates on new business narrowed in 2016 and may tighten further as loan rates continue to decline but deposit rates increasingly stack up against zero. More than half of the new deposits held by corporations and close to 40% of those held by households are de facto no longer remunerated.