Elevated phosphorus: dynamics during four years of green roof development
Mitchell, Mark E.; Matter, Stephen F.; Durtsche, Richard D.; Buffam, Ishi
Green roofs are emerging engineered ecosystems that provide multiple benefits, but many are constructed with nutrient-rich substrate and have been found to leach out high levels of phosphorus (P) in runoff. It is unclear, however, how long green roofs act as sources of P or what mechanisms are responsible for these net losses. We measured P concentrations in runoff water over 4 years from a 1-5 year old extensive green roof in Cincinnati, OH, USA, produced a model to predict runoff P levels into the future, and validated predictions using runoff from 2 nearby extensive green roofs. P concentrations in runoff from the focal green roof were on par with heavily fertilized agroecosystems and displayed strong seasonal dynamics and a rapid decline over the 4-year study. Runoff measurements and changes in substrate P content over a 2-year period were used to estimate a mass balance for green roof P. P loss from the substrate was substantial (4.55 +/- 2.3 g P/m(2)/yr), but only a small portion of the loss was attributable to leaching of P in runoff (0.19-0.65 g P/m(2)/yr). Missing P may be attributed to a combination of plant uptake and altered P form and binding strength, but further research is needed to precisely identify the mechanisms of P depletion. Our results also suggest that these and similar extensive green roofs are likely to act as environmentally significant sources of P for 10 or more years following roof installation, highlighting the need for reductions in initial substrate P content.
Phosphate; Nutrient cycling; Nutrient retention; Time-series; Urban green infrastructure
2017, Volume: 20, number: 5, pages: 1121-1133
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