Impacts of managing perennial grasses in the northern Midwest United States for bioenergy on soil organic C and nitrous oxide emission

Authors: Johnson, Jane; Barbour, Nancy

Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: 9/7/2017
Publication Date: 9/7/2017
Citation: Johnson, J.M., Barbour, N.W. 2017. Impacts of managing perennial grasses in the northern Midwest United States for bioenergy on soil organic C and nitrous oxide emission [abstract]. Proceedings of the 6th International Symposium on Soil Organic Matter. p. 436.

Interpretive Summary:

Technical Abstract: In the USA perennial grasses [e.g., switchgrass (Panicum virgatum L.) and big bluestem (Andropogon gerdardii Vitman)] are proposed as cellulosic feedstock. Perennial grasses are often touted as being low input and as having a C-neutral foot print, but managing them as bioenergy feedstock means adding nitrogenous fertilizer or inter-cropping with legumes, 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. Few studies have reported SOC storage and N2O emission from managed perennial grasses. The objectives of this study were to compare SOC storage and N2O emission between 1) grasses with legume companion crop or with nitrogenous fertilizer, 2) two grass harvest times (Autumn, Spring), and 3) perennial systems and the most common row crop system - corn (Zea maize L.)/soybean (Glycine max) (C/S) rotation. This study was conducted in northern Midwest US at the Swan Lake Research Farm. An experimental site established in 2000 with perennial grass and no till row crops was used to assess SOC accrual and N2O emission from established grassed, and a C/S managed without tillage and with all residue returned. Crops were managed with recommended fertilizer rates. Soil was sampled for SOC three times (2000, 2006 and 2011) and N2O flux was assessed between 2009 and 2012 using with closed-vented chambers. In Perennial grasses, SOC accrual was only measured in the surface 0-5 cm, but in the C/S rotation SOC declined over the same frame even though it was managed without tillage. For both grasses, intercropping with a legume instead of adding nitrogen as urea reduced annual and yield-scaled annual N2O emission and yield-scaled emission compared to their respective counterpart. Delaying grass harvest until Spring increased N2O emission 14 to 40% compared to Autumn harvest. Emission from the fertilized grasses exceeded that of C/S rotation, such that the emission factor was almost doubled from fertilized grasses compared to fertilized corn. It was concluded that SOC accrual by the grasses was not enough to compensate for the increase in N2O emission. Management strategies to improve SOC accrual, minimize N2O emission and enhance biomass production for bioenergy managed grasses are needed.