As a bonus, a 10% annual interest rate is awarded to Steem dollar holders.
If I have understood right, there is no fixed interest rate. It can be changed by witnesses so it can adapt to market conditions if necessary. This might be good to make clear so that people don't feel like they are betrayed when they get some day less than 10 %.
Ok thx @samupaha! I'll remove that for now. I thought I heard the 10% interest somewhere, but the whitepaper doesn't specify any fixed rate so you're probably right.
10% is just the default value but @samupaha is correct it can be changed by witness vote according to market conditions.