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

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 visually 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 annual 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, SG 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.