Researchers Evaluate Greenhouse Gas Reductions from Different Cropping Systems

3 April 2007

Researchers from Colorado State University and the US Department of Agriculture, Agricultural Research Service have conducted a complete lifecycle analysis of greenhouse gas emissions from biofuel production from a variety of crops.

Study results revealed that when compared with the lifecycle of gasoline and diesel, ethanol and biodiesel from corn and soybean rotations reduced greenhouse gas emissions by nearly 40%. Use of reed canarygrass reduced emissions by 85% and use of switchgrass and hybrid poplar reduced emissions by about 115%. Hybrid poplar and switchgrass were found to offset the largest amounts of fossil fuels and therefore reduced emissions the most out of the studied crops.

Biofuels have a great potential to reduce our dependence on imported gasoline and diesel fuel. We have performed a unique analysis of the net biofuel greenhouse emissions from major biofuel cropping systems by combining ecosystem computer model data with estimates of the amount fossil fuels used to grow and produce crops for biofuels.

—William Parton, Colorado State University

Different crops vary with respect to length of plant life cycle, yields, biomass conversion efficiencies, required nutrients, net soil carbon balance, nitrogen losses and other characteristics which in turn impact management operations. Additionally, crops have different requirements for farm machinery inputs from planting, growing, soil tillage, applying fertilizer and pesticide and finally harvesting.

Parton, Stephen Del Grosso and Paul Adler from the USDA used the DAYCENT biogeochemistry model, developed by Parton and Del Grosso, to account for all of these factors as well as integrate climate, soil properties and land use to accurately evaluate the impact of bioenergy cropping systems on crop production, soil organic carbon and greenhouse gas fluxes. The study assessed soil greenhouse gas fluxes and biomass yields for corn, soybean, alfalfa, hybrid poplar, reed canarygrass and switchgrass.

DAYCENT simulates flows of carbon and nitrogen using daily weather, soil texture and land management as inputs.

Although fossil fuel inputs are required to produce and process biofuels, hybrid poplar and switchgrass converted to ethanol compensate for these emissions and actually remove greenhouse gasses from the atmosphere when the benefits of co-products are included. Greenhouse gas savings from biomass gasification for electricity generation are even greater. This research provides the basis for evaluating net biofuel greenhouse gas emissions and highlights the need to improve the technologies used for large scale conversion of biomass to energy and to more fully exploit agricultural co-products.

—Stephen Del Grosso

Bioenergy crops are able to offset carbon dioxide emissions by converting atmospheric carbon dioxide into organic carbon in biomass and soil, but the production of biofuels requires fossil fuels and impacts greenhouse gas fluxes. The primary sources of greenhouse gas emissions associated with crop production are soil nitrous oxide emissions and the CO₂ emissions from farm machinery, farm inputs and agricultural processes. Colorado State and USDA scientists quantified all of these factors to determine the net effect of several bioenergy crops on greenhouse gas emissions.

The study is published in the April 2007 issue of Ecological Applications.