Impacts of Bioenergy Feedstock Production on Soil Physical Properties, Soil Water and Nitrogen Dynamics, and Shallow Groundwater Quality of a Drained Forest

Intercropping bioenergy crops such as switchgrass (Panicum virgatum) with managed loblolly pine (Pinus taeda L.) has emerged as a source of biomass for biofuel production that does not affect the supply of food, feed and fiber. An experimental research study was conducted in order to quantify the impacts of the proposed land use on: 1) soil physical properties, 2) soil water distribution in the vadoze zone and shallow groundwater dynamics, 3) soil nitrogen dynamics, and 4) shallow groundwater quality of an intensively managed loblolly pine plantation in eastern North Carolina, U.S.A. Experimental treatments include traditional pine with biomass in place (PINE), pine-switchgrass intercropped with biomass in place (PSWITCH), switchgrass only (SWITCH), and a 38-year old loblolly pine stand (REF). Each treatment is replicated three times on 0.8 ha plots drained by parallel ditches that are 1.0 to 1.2 m deep and 100-m apart. The effects of pine-switchgrass intercropping on soil physical properties, including bulk density, total porosity, saturated hydraulic conductivity, soil water characteristic, and drainable porosity, were investigated by collecting intact soil cores (7.62 x 7.62 cm) during the third harvesting operation (December 2011) from three random points within each plot and at three depths per sampling point: 0-15, 15-30, and 30-45 cm. In PSWITCH, intact soil cores were collected before and after harvest from the center of the interbed and at the portion of the interbed that is likely traversed by the equipment. In PINE and REF, intact soil cores were only collected once at the MIDDLE after post-harvest soil core collection from PSWITCH. Standard procedures were used to measure and calculate soil the soil physical properties. The effect of the proposed land use on the soil water distribution within the vadoze zone was studied using a hand-held soil moisture probe AP 827 (AquaPro) to measure relative saturation (Sr) at least once a week from June 30, 2011 to January 15, 2013 at six depths (15, 22.5, 30, 45, 60, and 75 cm) from ground surface (g. s.) across three locations in each plot including middle, quarter point, and near the ditch. At each location, the Sr was measured near a tree (NT), between two trees on the same bed (BT), between two trees of two adjacent beds (B2T), and in the middle of four trees of two adjacent beds (M4T). NT and BT were grouped as "BED," while B2T and M4T were lumped together as "INTERBED." The effect of the proposed land use on shallow groundwater dynamics was studied by measuring and recording the water table depth (dgw) using U20 HOBO water logger (Onset Computer Corp.) on a 15-minute interval from September 10, 2009 to January 15, 2013. The loggers were placed in a 5.08 cm (I.D.) polyvinyl chloride (PVC) pipes installed approximately 2.4 m from the g. s. The effect of the proposed land use on soil N availability, assessed in terms of net mineralization and net nitrification rates, was investigated using both field and laboratory measurements. At each location, three wells were spaced at approximately 1 m and were installed at three depths: 0.75-1.0 m, 1.25-1.5 m and 1.75-2.0 m. Water samples were collected monthly or more frequently after fertilizer application (depending on rainfall events) from fall of 2009 to fall of 2012 in PSWITCH, PINE, and SWITCH, and from winter of 2011 to fall of 2012 in REF and were analyzed for total Kjeldahl nitrogen (TKN), ammonium nitrogen (NH4+ - N), nitrate + nitrite nitrogen (NO3 - + NO2- - N), orthophosphate phosphorus (OP), total organic carbon (TOC), and chloride (Cl-). Soil physical properties were significantly affected at the top layer. The results indicate that the initial site preparation for switchgrass planting has a larger impact on soil physical properties than annual harvesting operations. Intercropping did not significantly affect the soil water distribution in the vadoze zone nor did it affect shallow water table fluctuation. SWITCH had consistently shallower water table among the treatments throughout the study period. This observed trend support the fact that loblolly pine trees and the understory vegetation have higher evapotranspiration than switchgrass due to their higher leaf area indices and deeper rooting system. Laboratory results suggest that litter quality differences did not significantly affect net N mineralization and nitrification rates. Field data indicate that PSWITCH and SWITCH tend to confine soil mineral N in ammonium form relative to PINE. Shallow groundwater quality was not significantly affected by intercropping. Nutrient (especially NO3- + NO2 - - N) concentrations were elevated within two years (2009-2010) of establishment, but reverted back to background levels as early as the third year (2011) relative to REF.