Does Soil C Accural Under Perennial Grasses Managed for Bioenergy Offset Fertilizer Induced N2O Emission?.

Perennial grasses [e.g., switchgrass (Panicum virgatum L.) and big bluestem (Andropogon gerdardii Vitman)] are often touted as being low input crops and as having a C-neutral foot print, but managing them as bioenergy feedstock means adding nitrogenous fertilizer, which can increase emission of nitrous oxide (N2O). Increases in N2O emission may reduce greenhouse gas mitigation from soil organic carbon (SOC) accrual, which is anticipated by growing perennials instead of annuals. Alfalfa (Medicago sativa), a perennial legume typically grown as animal feed, is also expected to accrue SOC but few studies have addressed if the SOC accrual was adequate to offset N2O emission. The objectives of this study were to compare SOC storage and N2O emission between 1) two perennial grasses managed as bioenergy crop and a no till perennial legume rotation - corn (Zea maize L.)/soybean (Glycine max)/wheat (Triticum aestivum L.)+Alfalfa (C/S/W+A/A/A); 2) perennial grass systems and a no till - C/S rotation with all residue returned; 3) a perennial legume rotation and no till - corn (C/S) rotation with all residue returned; and 4) the annual (C/S/W+A) and perennial (W+A/A/A) phase of a long-term perennial rotation. All crops were managed with recommended fertilizer rates. Soil was sampled for SOC in 2000, 2006 and 2011 incrementally to 100 cm. N2O flux was assessed between 2 June 2009 and 30 March 2012 using closed-vented chambers. Comparison of SOC accrual, N2O flux, cumulative emission, and N2O offset among these four treatment combinations will be presented.