The Best Bioenergy Grass Crops to Accumulate Carbon in the Soil

The Science

The extensive root systems of certain types of perennial bioenergy crops like switchgrass and big bluestem can accumulate significant amounts of carbon in the soil. How do characteristics of the roots of these grasses affect that ability? A multi-institutional team of scientists evaluated how differences in root traits in and among three varieties of switchgrass and three of big bluestem affected the amount of carbon accumulation in soil. Their results showed that careful selection of both species and varieties can help improve bioenergy crop management while maximizing the amount of carbon in soil and lowering carbon emissions.

The Impact

Plant roots are the main conduit to transfer atmospheric carbon to soil. Therefore, renewable bioenergy crops like switchgrass and bluestem can contribute to mitigating global carbon concentrations by increasing soil carbon storage. These grasses also have low resource requirements, reducing the need for energy use associated with farming operations such as irrigation and fertilization. Advancing the understanding of how grass root traits influence soil carbon storage will yield insights into the mechanisms of this storage at the root-soil interface and allow selection of the best bioenergy crops to increase storage of carbon in the soil.

Summary

A multi-institutional team of scientists collected soil cores from the Department of Energy (DOE) National Environmental Research Park (NERP) in Batavia, Illinois, from the six varieties of grass. They measured root biomass, root diameter, specific root length, bulk soil carbon, and carbon associated with coarse particulate organic matter and fine particulate organic matter plus silt- and clay-sized fractions of soil. They then characterized organic matter chemical class composition using high-resolution Fourier-transform ion cyclotron resonance mass spectrometry at EMSL, the Environmental Molecular Sciences Laboratory, a DOE Office of Science User Facility. Because the NERP had been supporting grassland for nearly 50 years, the scientists could study the natural abundance of stable carbon isotopes to quantify plant-derived carbon. They found that big bluestem grass had higher plant-derived carbon compared to switchgrass in the coarse particulate organic matter fraction of the soil to a depth of 10 cm, while switchgrass had higher plant-derived carbon in the clay fraction between 10 and 20 cm deep. The findings suggest that, after 10 years of growth, the large root system in big bluestem helps rapidly increase soil carbon formation in the form of particulate organic matter, while switchgrass root structure and chemistry contribute to building a mineral-bound clay carbon pool. Thus, both species and cultivar selection can help improve bioenergy management to maximize soil carbon gains and lower carbon emissions.

Contacts

Megan Kelly-Slatten, Boise State University, megankelly-slatten@u.boisestate.edu

Marie-Anne de Graaff, Boise State University, Marie-AnnedeGraaff@boisestate.edu

Julie Jastrow, Argonne National Laboratory, jdjastrow@anl.gov

Malak Tfaily, EMSL and University of Arizona, tfaily@arizona.edu

Funding

This work was supported in part by the DOE Office of Science, Office of Biological and Environmental Research and the U.S. Department of Agriculture National Institute of Food and Agriculture. Through a grant, some research was conducted at EMSL, the Environmental Molecular Sciences Laboratory, a DOE Office of Science User Facility.

Publications

M.J. Kelly-Slatten, et al., “Root traits of perennial C₄ grasses contribute to cultivar variations in soil chemistry and species patterns in particulate and mineral-associated carbon pool formation.”

Global Change Biology Bioenergy 15, 613 (2023). [DOI: 10.1111/gcbb.13041]