Water quality response after two years of short-rotation pine management for bioenergy in the southeastern U.S.

We are using a watershed-scale experiment to examine the water quality effects of growing woody crops for bioenergy in the southeastern United States. Three adjacent watersheds in the Upper Coastal Plain of South Carolina were instrumented and baseline conditions were established for two years (2010-2012). In spring 2012, approximately 40% of the 2 treatment watersheds were harvested while the third watershed served as an unmanipulated control. Loblolly pine seedlings were planted in spring 2013, and silvicultural activities (herbicide and fertilizer applications) were implemented following a short-rotation (8-12 year) schedule. The chemistry (nitrogen, phosphorus, dissolved organic carbon, herbicides) of stream water, soil water (interflow), groundwater, and precipitation was measured weekly, monthly, or on an event basis. Baseline chemistry and hydrological measurements together showed that groundwater is the dominant flowpath in these watersheds, and thus we predicted that any effects of pine management on stream water quality may be lagged by several years. After two years of post-treatment water quality monitoring, there have been no changes in stream water quality in the two treatment watersheds. Ammonium concentrations varied temporally in all 3 watersheds, and this pattern may be driven by seasonal variation in nitrification rates. Stream nitrate concentrations were generally low (<500 μg N/L), and concentrations peaked in all 3 watersheds after stream flow resumed following a dry year. Ammonium and nitrate concentrations in deep groundwater increased for several months after harvest; however, concentrations were low (primarily <200 μg N/L for ammonium and <1,500 μg N/L for nitrate). Longer-term measurements are needed to determine if this pattern of increased nutrients in groundwater persists.