Experiments and monitoring studies have shown that nitrogen
and phosphorus retention in bioretention systems is variable. For instance,
in a column study where stormwater was passed through soil
media, the Total Kjeldahl Nitrogen (TKN) was reduced to 65–75% and
ammonium was reduced to 60–80%, but nitrate concentrations increased
in the leachate (Davis et al., 2001). Export of phosphorus
was reported from rain gardens, which was explained by decomposition
of fresh plant materials during soil disturbance (Dietz and
Clausen, 2005).