Soil Response to Perennial Grass Monocultures and Polycultures for Bioenergy and Nitrogen, Phosphorus and Potassium Fertilization.

Dedicated energy crops including perennial grass monocultures and polycultures may provide abundant biomass for biofuel production while maintaining soil and environmental quality. Perennial grasses may have different impacts on biomass production and soil properties, depending on grass species and varieties, but such impacts have not been widely studied.  The objectives of this study were to characterize soil response to select warm season grasses and NPK fertilization rates under two experiments established in 2009 at the University of Nebraska’s Agricultural Research and Development Center, Mead, NE. In the first experiment, twenty nine monocultures and polycultures were seeded, but only high yielding switchgrass varieties (Goldmine/Warrior/Shawnee, Bonanza/Scout/Shawnee, Shawnee, and Kanlow N1) including miscanthus were selected for this study. Monocultures and polycultures were replicated four times and soil sampling was done for 0-10, 10-20, 20-40, 40-60, 60-80 and 80-100 cm in spring 2014.  The second experiment had only switchgrass with two study factors: two biomass harvest dates (August and November) and NPK rates. Soil sampling was done using hand probe for the 0-10 and 10-20 cm depths. The soil samples were analyzed for soil organic carbon (SOC), bulk density, and pH. In the first experiment, results showed no significant effect of monocultures and polycultures on SOC and bulk density at all depths. Also, in the second experiment no effect of switchgrass under NPK rates on SOC, bulk density and pH were observed for both soil depths. Overall, after 4 years of perennial grasses production no effect on these soil properties can be attributed to increased mineralization rate of residues due to inorganic N application or also may be due to the short-term nature of the experiment and removal of biomass as feedstocks. Other studies have reported a decrease in pH with N fertilization unlike this study, which suggests the site-specificity of the impacts. Root biomass amount and wet aggregate stability are being analyzed and the results will be presented at the meeting.