A large amount of land is under pine production in the U.S., with loblolly pine (Pinus taeda L.) widely used for timber and other wood products. Intercropping switchgrass (Panicum virgatum L.), as a celluolosic bioenergy crop in loblolly pine stands is hypothesized to provide bioenergy feedstock without competing for land currently in food production. Switchgrass is a native perennial grass that uses water efficiently and prevents soil erosion. However, in order to recommend this novel intercropping practice to forest managers and landowners, there is a critical need for information about its impacts on water quantity and quality. Accordingly, six recent years (2009-2014) of data from a watershed-scale field experiment conducted on Weyerhaeuser's managed pine forest in coastal North Carolina suggest that fully grown switchgrass between pine tree beds improved downstream water quality by reducing nitrate and phosphate concentrations and loads compared to the traditional pine forest. However, the watershed planted only with switchgrass yielded higher nitrate concentration and loads than the intercropped site, possibly due to greater outflow from the former site. Pine-switchgrass intercropping may thus be a viable method of producing a sustainable bioenergy crop on pine forest lands while improving water quality, except for its site preparation period.