Study examines soil nitrogen availability associated with switchgrass intercropping systems

Switchgrass (Panicum virgatum) is a native grass that can be used as a biofuel feedstock. Some landowners are investigating alley cropping systems in which switchgrass is grown between the rows of pine or other tree species in plantations managed for more traditional forest products.

Such systems have potential to provide an additional income source for landowners during forest maturation should markets for switchgrass as a biofuel feedstock develop.

Introduction of switchgrass as an alley crop could alter soil properties, nutrient availability, and management requirements, yet very little work has investigated these relationships.

A recent publication presents results from a study supported by NCASI in collaboration with Weyerhaeuser Company in which researchers examined soil nutrient availability and microbial biomass in pine-switchgrass systems in Louisiana, North Carolina, and Mississippi, and in a cottonwood-switchgrass system in Arkansas and Louisiana. Dr. Michael Blazier of the Louisiana State University Hill Farm Research Center led the investigation, along with scientists from Weyerhaeuser Company, Mississippi State University, and the University of Arkansas.

The investigators found that switchgrass influenced nitrogen supply in a manner similar to typical understory vegetation within loblolly pine plantations.

The abstract for the paper follows.

“Switchgrass has favorable characteristics as a biofuel feedstock, and it may be feasibly grown on sites currently forested or retired from agricultural production. It is ecologically important to understand the impacts of establishing and managing switchgrass on site nutrient cycling. Four studies conducted in the southeast United States tested switchgrass as a bioenergy crop grown in monoculture, in alley cropping systems, and as an alternative to conventional agricultural rotations. Each trial included comparisons of switchgrass with the prevailing land uses. In each trial, nitrogen mineralization (Nmin), nitrification (Nnit), microbial biomass carbon (Cmic), microbial activity, and labile carbon were measured because of their sensitivity to management practices that alter soil nutrient cycling. In all studies of loblolly pine and switchgrass, switchgrass was apparently the dominant substrate source for Cmic, and Nmin increased when juvenile loblolly pine was isolated with herbicide as the sole vegetation. However, Cmic and Nmin of loblolly pine-switchgrass alley cropping systems were similar to those observed in conventional juvenile loblolly pine plantation conditions. In a trial of converting former agricultural fields into switchgrass or cottonwood biofuel plantations, this conversion reduced Nnit, and therefore the potential for nitrogen losses compared with those for a soybean-sorghum rotation typical for such sites.”



Blazier, M.A., T.R. Clason, H.O. Liechty, Z.H. Leggett, E.B. Sucre, S.D. Roberts, K. Krapfl, and E.D. Vance. 2015. Nitrogen and carbon of switchgrass, loblolly pine, and cottonwood biofuel production systems in the southeast United States. Forest Science 61:522-534.