Water Quality Effects of Switchgrass Intercropping on Pine Forest in Coastal North Carolina

Abstract

Abstract. Interplanting a cellulosic bioenergy crop (switchgrass, L.) between loblolly pine ( L.) rows could potentially provide a sustainable source of bio-feedstock without competing for land currently in food production. The objectives of this study were to: (1) quantify the concentrations and loads of drainage water nitrogen (N) and phosphorus (phosphate) associated with establishment and growth of switchgrass treatments and compare them with those for a mid-rotation pine forest (control), and (2) quantify the treatment effects on drainage water N and phosphate (IC) and switchgrass only (SG). Thinned mid-rotation loblolly pine with natural understory (MP) was used as the control. Pretreatment calibration equations for nutrients were obtained using a paired watershed approach and bootstrap geometric regression with 2007-2008 data, when pine on all sites had reached canopy closure. Treatment effects were calculated as the difference between expected values from the pretreatment relationship and measured data for the treatment period. Precipitation, outflow, and N and phosphate concentrations in the outflow were measured during calibration (Jan. 2007 to Dec. 2008), site preparation for switchgrass establishment (Nov. 2009 to Mar. 2012), and switchgrass growth (Apr. 2012 to Apr. 2014). Mean NO3-N concentrations and loads were significantly (a = 0.05) greater for SG than for IC during the switchgrass growth period. Average treatment concentrations with standard errors and total load effects during switchgrass growth for NO3-N followed the trends SG (-0.002 ±0.01 mg L-1) > IC (-0.12 ±0.04 mg L-1) and SG (0.75kg ha-1) > IC (0.23kg ha-1), respectively. For phosphate average concentrations and loads, the treatment effects during switchgrass growth followed the trends SG (-0.004 ± mg L-1) >IC ( -0.02 ± mg L-1) and IC (-0.43 kg ha-1) > SG (-0.70 kg ha-1), respectively. Average concentration effects for NO3-N and phosphate and total load effects for phosphate significantly (a = 0.05) decreased for IC compared to the MP control. These results suggest that the intercropping treatment (IC) with loblolly pine and switchgrass improved water quality by reducing NO3-N and phosphate concentrations and phosphate loads. Keywords: Bioenergy crop, Bootstrap geometric regression, Loblolly pine, Nutrients, Paired watershed.