Carbon footprint of perennial bioenergy crop production receiving various nitrogen fertilization rates

Authors: Sainju, Upendra; Allen, Brett

Submitted to: Science of the Total Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/29/2022
Publication Date: 12/7/2022
Citation: Sainju, U.M., Allen, B.L. 2022. Carbon footprint of perennial bioenergy crop production receiving various nitrogen fertilization rates. Science of the Total Environment. 861. Article 16063. https://doi.org/10.1016/j.scitotenv.2022.160663.
DOI: https://doi.org/10.1016/j.scitotenv.2022.160663

Interpretive Summary: Concerns about carbon dioxide emissions to the atmosphere continue to grow throughout the world. Moreover, the recent increase in the cost of petroleum-based energy emphasizes the need to continue to research renewable energy. Switchgrass, smooth bromegrass and intermediate wheatgrass are perennial crops with the potential to produce bioethanol fuel and to increase carbon sequestration in soil. Determining the carbon footprint of perennial bioenergy cropping systems requires quantification of the amount of carbon stored in shoot, root, and soil, as well as the amount of carbon lost through soil respiration. Comparing the amount of carbon stored to that which is lost indicates if an agroecosystem is a carbon source or sink. ARS researchers in Sidney, Montana reported that shoot carbon was greater for switchgrass, but root carbon varied with perennial grass species and nitrogen fertilization rates. Carbon dioxide emission was greater for smooth bromegrass and switchgrass than intermediate wheatgrass. Carbon balance was greater for switchgrass than other perennial grasses at higher nitrogen fertilization rates. Shoot, root, and soil C as well as C balance were lower and carbon dioxide emission was greater for spring wheat than perennial bioenergy crops. Results show that perennial bioenergy crops reduce greenhouse gas emissions compared to annual crops and can replace a significant portion of fossil fuels. Switchgrass grown with recommended amounts of nitrogen fertilizer will store more carbon in the plant and soil than conventional cropping systems and enhance the effectiveness of the agroecosystem as a carbon sink.

Technical Abstract: Perennial bioenergy crops can reduce greenhouse gas emissions compared to fossil fuels, but little is known about their C footprints. We evaluated C footprint and C balance of perennial bioenergy crops receiving various N fertilization rates and compared them with an annual crop from 2012 to 2014 in the semiarid region of US northern Great Plains. Perennial bioenergy crops were intermediate wheatgrass (Thinopyrum intermedium [Host] Barkworth and Dewey, IW), smooth bromegrass (Bromus inermis L., SB), and switchgrass (Panicum virgatum L., SG), and N fertilization rates were 0, 28, 56, and 84 kg N ha-1. The annual crop was spring wheat (Triticum aestivum L., WH). The CO2 flux increased in the summer when air temperature and precipitation were greater. Cumulative CO2 flux was greater for SB and SG than IW in 2012-2013 and greater for SB than IW and SG in 2013-2014. Shoot C increased with increased N fertilization rate and was greater for SG than IW and SB at most N fertilization rates in both years. Root and rhizosphere C varied with N fertilization rates and were lower for SG than IW and SB at 0 kg N ha-1, but greater at 84 kg N ha-1. Carbon balance also varied with N fertilization rates, being lower for SG than IW and SB at 0 kg N ha-1, but greater at other N rates. Cumulative CO2 flux was higher, but shoot, root, and rhizosphere C as well as C balance were lower for WH than perennial bioenergy crops. Because of greater total C input but lower CO2 flux, SW with N fertilization can be C positive, retaining more C in plant residue and soil than other perennial bioenergy crops. Spring wheat remained C negative compared to perennial bioenergy crops, losing more C as CO2 flux than total C input.