The IRS considers your distributive share of partnership income as being income from "self-employment."  Consequently, on that income you'll pay the 15.3% self employment tax as well as income tax.

So if you're in, say, the 15% tax bracket, then $6897 in SE income will generate over $2000 in tax, though you do get to deduct half the SE tax from your gross income (thus saving you 15% of the amount deducted, in that bracket).

That's how IRS taxes SE income.