Leachable phosphorus from senesced green ash and Norway mapleleaves in urban watersheds

In urban watersheds, street tree leaf litter is a critical biogenic source of phosphorus (P) in stormwater runoff.
Stormwater extracts P from leaf litter and transports it, through the storm sewer network, to a receiving
waterbody potentially causing downstream eutrophication. The goal of this study is to understand P leaching dynamics of two prevalent tree species (Norway maple (Acer platanoides) and green ash (Fraxinus pennsylvanica))
in three urban residential watersheds in Madison, Wisconsin, USA. Leaf litter was collected from the three basins
during Fall 2017 and 2018. Laboratory experiments showed an initial rapid total dissolved phosphorus (TDP) release that gradually plateaued over a 48-hour period. The total TDP released from Norway maple (2.10 mg g−1
)
was greater than from green ash (1.60 mg g−1
).Within the same species, increased fragmentation of leaves led to
more rapid initial TDP release, but not greater total TDP release. Increased aging of senescent leaves decreased
total TDP release. Incubation temperature and volume of water in contact with leaves may not be critical factors
affecting TDP leaching dynamics. Predictive equations were derived to characterize time-variable TDP release of
both Norway maple and green ash leaves. Potential TDP release from leaf litter estimated using these equations
was compared with field-measured end-of-pipe TDP loads in one of the study watersheds. Our results indicate
that preventing leaf litter from accumulating in streets is an important stormwater quality control measure.