Urban Grasslands: Source or Sink of Phosphorus in Urban Watersheds.

Are urban grasslands a source or sink of phosphorus (P) in urban watersheds? Regulation at the local and state level have been implement to attempt to protect urban water quality by limiting or banning the use of phosphorus containing fertilizers applied to the largest urban plant based ecosystem, urban grasslands. Many urban watersheds also have total maximum daily limits established requiring landscape scale implementation plans to meet target levels. The in a nationwide survey on surface waters by USGS it was determined that the concentration of total P is generally as high in urban/suburban streams as in agricultural streams. More than 70 % of sampled urban streams exceeded the USEPA desired goal (0.1 mg L-1 of total P). To determine the role of urban grasslands (including lawns, sports fields, parks, cemeteries and golf courses) in water quality of urban watersheds, small scale-controlled research studies involving: rates and sources of P fertilizers, grass species, other landscape types and management variables. From all the studies, annual P loading rates are in the order of 0.5 Kg/ha, often independent of P application rate. Several factors affect the conditions when urban grasslands are a source of P in urban watersheds including: high soil P levels, days after treatment, season of year, and grass/perennial weed species composition.