I would blindly guess N. Protein exporting and all.
here's an analysis that was done on skimmate and other exports by Shimek Down the Drain
He found P in skimmate of 37 mg/kg and N of 421 mg/kg so mass ratio of 11.4 N to P.
In moles, a ratio 25.1 N to P.
Not as heavy N to P as I expected but maybe enough to say disproportionately N?
(the sample standard deviations are huge, so take with a grain of salt.)