Nitrous Oxide Emission and Soil Carbon Sequestration from Herbaceous Perennial Biofuel Feedstocks

Core Ideas Adding N fertilizer increases N 2 O emission from perennial grasses managed as feedstock. Interseeding clover with perennial grasses reduced N 2 O emission and yield. Soil organic C storage under grasses may be inadequate to offset fertilizer‐induced N 2 O flux. Nitrogen management to optimize yield‐scaled N 2 O flux from established grasses is needed. No soil organic C storage occurred under no‐till corn–soybean rotation to offset N 2 O flux. 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 N 2 O emission and, thereby, alter their effectiveness to mitigate greenhouse gas (GHG) emission. Few studies have measured N 2 O emission from perennial grasses managed as feedstock. The objectives of this study were to compare N 2 O flux and soil organic C (SOC) storage between (i) grasses with legume companion crops or with nitrogenous fertilizer, (ii) two grass harvest times (autumn and spring), and (iii) perennial systems and corn ( Zea maize L.)–soybean [ Glycine max (L.) Merr.] (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 ( Dalea ) and switchgrass–clover treatments had dramatically reduced annual N 2 O emissions and yield‐scaled emissions compared with the respective grasses with urea fertilizer. Cumulative N 2 O emission was 14 to 40% greater in the big bluestem‐spring and switchgrass‐spring treatments compared with respective autumn‐harvested treatments. The average cumulative emission in fertilized big bluestem and switchgrass treatments (18.5 kg N 2 O‐N ha −1 ) exceeded that of the C–S rotation (12.7 kg N 2 O‐N ha −1 ). The emission factor (EF) for fertilized grasses averaged 2.5%, corn averaged 1.05%, and C–S rotation averaged 1.9%. The SOC storage by perennial grasses was limited to 0 to 5 cm and thus may not be adequate to offset N 2 O emission. Nitrogen management refinement is recommended for herbaceous perennials to optimize biomass production and minimize N 2 O emission.