Vegetation's Role in Mitigation of Phosphorus From Agricultural Ditch Systems.

Concern over phosphorus levels in agricultural runoff have driven landowners and conservationists to search for new ways to reduce the impact of this nutrient on downstream aquatic receiving systems.  An often-overlooked management practice is the use of drainage ditch vegetation to filter runoff water before entering rivers and lakes.  For the last six years, scientific efforts have targeted plant-specific ability to mitigate phosphorus in both mesocosm and field scale studies.  In mesocosm experiments, three different emergent aquatic macrophytes, typically found within drainage ditches, were examined in monocultures for their ability to decrease phosphorus concentration and load in runoff.  Study results demonstrated Typha latifolia (common cattail) and Leersia oryzoides (rice cutgrass)  decreased dissolved phosphate loads by 53±6% and 51±6%, respectively, while Sparganium americanum (American bur-reed) decreased the load by 14±6%.  Unvegetated control systems actually had an net increase in phosphorus load of 6±4%.  Additionally, Sparganium and unvegetated control  replicates had the highest phosphorus release after a clean water flushing event (27±8% and 42±4%, respectively).  Field scale studies examined phosphorus retention in drainage ditches populated primarily with L. oryzoides and T. latifolia.  Decreases in phosphorus concentrations from inflow to ditch outflow ranged from 64-98%, while loads were decreased 95-99%.  Recent studies have focused on the use of rice (Oryza sativa) as a phytoremediation tool in ditches.  While beneficial in removing certain organic pollutants, no differences have been noted in phosphate mitigation with rice (93±3%) when compared to unvegetated systems (96±1).  Efforts continue to examine individual plant species' ability to remove phosphorus from the water column.  By utilizing drainage ditch vegetation, in addition to other successful in-field and edge-of-field management practices, landowners can begin to decrease the amount of phosphorus entering water bodies, thereby improving the Nation's river and lake water quality.