Impacts of forest-based bioenergy feedstock production on soil nitrogen cycling

We investigated impacts of simultaneous production of biomass for biofuel and quality timber on soil nitrogen (N) cycling in a poorly drained forest soil of eastern North Carolina, U.S.A. Treatments included traditional loblolly pine (PINE) and pine-switchgrass intercropping (PSWITCH). Treatments were replicated three times on 0.8 ha plots drained by parallel open ditches which were 1.2 m deep and spaced 100 m apart. Net N mineralization (N-m) and nitrification (N-n) rates were measured in the field using sequential in-situ technique over two years with multiple measurements in each year and laboratory by incubating soil samples for one-, two-, eight-, and thirteen weeks. Soil incubation in-situ or sample collection for laboratory incubation was conducted at nine sampling points within a 30 x 40 m subplot at each plot center and 20 cm from the soil surface. Soil samples were composited by location including near tree (NT), between two trees on the same bed (BT), and in the middle of four trees on two adjacent beds (M4T). Composite samples from NT and BT were categorized as treebed (BED), while those from M4T were grouped as interbed (INT). Field results showed that total soil N availability and its temporal variations over two years were not significantly affected by PSWITCH. However, it significantly reduced N-n rates, particularly in the INT. The plot-level mean N-m rates in PINE were 0.21 and 0.26 mg N kg soil(-1) d(-1), while in PSWITCH they were 0.10 and 0.21 mg N kg soil(-1) d(-1) in 2011 and 2012, respectively. The plot-level mean N-n rates in PINE were 0.09 and 0.10 mg N kg soil(-1) d(-1) in 2011 and 2012, respectively, while in PSWITCH they remained at 0.03 mg N kg soil(-1) d(-1) across these two years. At the INT, mean N-n rates in PINE were 0.11 and 0.12 mg N kg soil(-1) d(-1) in 2011 and 2012, respectively, while in PSWITCH, N-n rate remained at 0.02 mg N kg soil(-1) d(-1) over two years. Laboratory results indicated that change in litter quality inputs (changing from mixed species to switchgrass) in the INT did not significantly affect N-m rates. Results of this study contributed to a better understanding of the changes in soil N cycling due to loblolly pine-switchgrass interactions, which is important in sustainable nutrient management of this new land use. Further, the results suggested that growing switchgrass as intercrop to managed loblolly pine has positive water quality implication since ammonium N is less mobile in soil than nitrate N.