Temporal Patterns of Soil Nutrient Availability In Southern Loblolly Pine Forests Intercropped with Switchgrass.

Intercropping of loblolly pine (Pinus taeda L.) with switchgrass (Panicum virgatum L.) offers an opportunity to utilize interbed space on southern pine plantations for the production of a biomass feedstock.  Demand for essential nutrients by pine and switchgrass must be met to ensure maximized productivity of both plant species.  In order to evaluate various biofuel intercropping systems, four blocks of seven plots were established in the summer of 2008 on a recently harvested 25-year-old loblolly pine plantation near Dover, NC.  Treatments included: 1) pine with biomass left in place; 2) pine with biomass removed; 3) pine intercropped with switchgrass with biomass left in place; 4) pine intercropped with switchgrass with biomass removed; 5) pine intercropped with “extra” row of flat-planted trees with biomass left in place; 6) pine intercropped with “extra” row of flat‐planted trees with biomass removed; and 7) switchgrass only.  Ion exchange membranes were deployed in the top 10 cm of mineral soil starting in June 2009 and collected and replaced continuously every 4-6 weeks through fall 2011.  Here we present 2.5 years of data on exchangeable anions and cations including NH₄, NO₃, PO₄, Al, Fe, Ca, Zn, B, Cu, Mg, and Mn.  Ammonium and NO₃ availability were reduced by 72% and 66% respectively under switchgrass during the first 1-1.5 years after switchgrass establishment, after which no differences were observed.  A time effect was found for all nutrients in the bed and interbed and time by treatment effect for NH₄, NO₃, and PO₄ in the interbed only.  Temporal changes in nutrient availability were likely influenced by the assart effect as well as seasonal changes in precipitation, temperature, and physiological demands by pine and switchgrass.  These results suggest that switchgrass intercropping may efficiently utilize available nutrients in the interbed when pine trees are not accessing this nutrient pool.  Therefore, inorganic N is being efficiently used to produce a bioenergy crop rather than being lost from the system due to uptake by competing vegetation.