Soil N Cycling Processes Under Pine and Switchgrass As Influenced By Intercropping.

Space between rows of loblolly pine trees (Pinus taeda L.) in southern pine plantations could be utilized to produce an herbaceous bioenergy feedstock, such as switchgrass (Panicum virgatum L.).  Belowground competitive interactions between pine and switchgrass may impact resource availability of both pine and switchgrass.  Previous research at this site indicates that switchgrass intercropping has impacted soil carbon (C) and nitrogen (N) biogeochemistry in adjacent pine beds compared to pine beds without intercropped switchgrass.  We were also interested in comparing the intercropped switchgrass treatment to the switchgrass field treatment, in order to evaluate effects of intercropping on soil C and nutrient dynamics under switchgrass.  Similar site preparation occurred for intercropped strips of switchgrass and fields of switchgrass, which included post-harvest residue removal, root raking, mechanical seed planting, and herbicide and fertilizer application.  Data collected included KCl-extractable NH₄⁺ and NO₃^- concentrations, gross and net N transformations, dissimilatory nitrate reduction to ammonium (DNRA), N₂O evolution from short-term laboratory incubations (same used for gross and net N transformations), and microbial biomass N.  Nitrogen cycling and availability was increased under pines when intercropped with switchgrass.  No effect of intercropping on N cycling or availability was found under switchgrass compared to the switchgrass field, but total soil N was reduced and δ¹⁵N values were enriched.  Based on these results, intercropping of pine and switchgrass impacts N cycling and availability under pines but not switchgrass.  While intercropping of switchgrass has had minimal effects on N cycling and availability compared to the pure field of switchgrass it may reduce soil organic N.  Belowground interactions between pine and switchgrass in the intercropped treatment are a likely driver of changes in soil N cycling under pine.  Pines may have increased root growth to more heavily mine beds for soil N in response to belowground pressure from intercropping.