Switchgrass intercropping reduces soil inorganic nitrogen in a young loblolly pine plantation located in coastal North Carolina

• Loblolly pine forests managed for woody and herbaceous bioenergy production.
• Switchgrass intercropped between bedded rows of pine trees.
• Soil NH4+ and NO3- concentrations reduced under switchgrass.
• Elevated NO3- concentrations in pine beds adjacent to switchgrass.
• Switchgrass utilized soil N when N supply was greater than pine N demand.

As biofuel production continues to increase, so will demand for forests to provide sources of biomass feedstocks. Intensively managed loblolly pine (Pinus taeda L.) plantations cover 15.8 million ha of the southeastern United States. Intercropping of switchgrass (Panicum virgatum   L.) within loblolly pine stands offers an opportunity to use interbed space to produce an herbaceous biomass feedstock. Furthermore, removal of post-harvest woody residues could act as another forest-based biomass feedstock. Understanding how managing forests for biofuel production influences soil nitrogen (N) cycling and availability is crucial given the critical role N plays in terrestrial ecosystem productivity. Therefore, our objective was to study effects of harvest residue removal and pine-switchgrass intercropping on soil extractable NH4+ and NO3-. We used a randomized complete block design, consisting of four blocks of seven plots (0.8 ha) established in the summer of 2008 on a recently harvested 34-year-old loblolly pine plantation in the Lower Coastal Plain of North Carolina, USA. Ion exchange membranes were deployed in the top 10 cm of mineral soil starting in June 2009 and replaced continuously every 4–6 weeks through December 2011. Presence of switchgrass significantly reduced soil extractable NH4+ and NO3-, amounting to a total reduction of 39% and 60%, respectively, over the course of the timeframe (30 months) of this study. There was evidence that intercropping of switchgrass increased extractable NO3- in the adjacent pine bed, although this result was only found in the final 6 months of the study. Presence or absence of harvest residues and/or interbed pines in the interbeds generally had no effect on soil inorganic N pools. These results indicate that switchgrass production effectively utilized inorganic N during a time when mineral N supply was greater than N demand by loblolly pines. Assessment of the long-term effects of switchgrass intercropping on soil nutrient cycling and availability and pine health and productivity will be essential to determine environmental and economic sustainability of intercropping.