Soil C Storage and Greenhouse Gas Emission Perennial Grasses Managed for Bio Energy Feedstock.

Switchgrass (Panicum virgatum L.) and big bluestem (Andropogon gerdardii Vitman), are potential bioenergy feedstocks. Perennial grasses managed as bioenergy feedstock require nitrogenous inputs, which can cause (nitrous oxide) N₂O emission; thereby, alter their effectiveness to mitigate greenhouse gas (GHG) emission. Few studies have measured N₂O emission from perennial grasses managed as feedstock. The objectives of this study were to compare N₂O flux and soil organic C (SOC) storage between 1) grasses with legume companion crop or with nitrogenous fertilizer, 2) two grass harvest times (Autumn, Spring), and 3) perennial systems and corn (Zea maize L.)/soybean (Glycine max) (C/S) rotation, all without tillage. Nitrous oxide flux was measured from May 2009 to May 2012 and SOC was measured in 2000, 2006, and 2011. Big Bluestem-Clover and Switchgrass-Clover treatments had dramatically reduced annual N₂O emission and yield-scaled emission compared to the respective grasses with urea fertilizer. Cumulative N₂O emission was 14 to 40% greater in the Big Bluestem-Spring and Switchgrass-Spring treatments compared to respective Autumn-harvested treatments. Average cumulative emission in fertilized Big Bluestem and Switchgrass treatments (18.5 kg N₂O-N ha^-1) exceeded that of C/S rotation (12.7 kg N₂O-N ha^-1). Emission factor (EF) for fertilized grasses averaged 2.5%, corn averaged 1.05%, and C/S rotation averaged 1.9%. Sequestration of SOC occurred at the soil surface (0-5 cm) of perennial grasses but not deeper in the profile. The SOC storage may not be adequate to offset N₂O emission. Nitrogen management refinement is recommended for herbaceous perennials to optimize biomass production and minimize N₂O emission.