Root traits of perennial C-4 grasses contribute to cultivar variations in soil chemistry and species patterns in particulate and mineral-associated carbon pool formation

Recent studies have indicated that the C-4 perennial bioenergy crops switchgrass (Panicum virgatum) and big bluestem (Andropogon gerardii) accumulate significant amounts of soil carbon (C) owing to their extensive root systems. Soil C accumulation is likely driven by inter- and intraspecific variability in plant traits, but the mechanisms that underpin this variability remain unresolved. In this study we evaluated how inter- and intraspecific variation in root traits of cultivars from switchgrass (Cave-in-Rock, Kanlow, Southlow) and big bluestem (Bonanza, Southlow, Suther) affected the associations of soil C accumulation across soil fractions using stable isotope techniques. Our experimental field site was established in June 2008 at Fermilab in Batavia, IL. In 2018, soil cores were collected (30 cm depth) from all cultivars. We measured root biomass, root diameter, specific root length, bulk soil C, C associated with coarse particulate organic matter (CPOM) and fine particulate organic matter plus silt- and clay-sized fractions, and characterized organic matter chemical class composition in soil using high-resolution Fourier-transform ion cyclotron resonance mass spectrometry. C-4 species were established on soils that supported C-3 grassland for 36 years before planting, which allowed us to use differences in the natural abundance of stable C isotopes to quantify C-4 plant-derived C. We found that big bluestem had 36.9% higher C-4 plant-derived C compared to switchgrass in the CPOM fraction in the 0-10 cm depth, while switchgrass had 60.7% higher C-4 plant-derived C compared to big bluestem in the clay fraction in the 10-20 cm depth. Our findings suggest that the large root system in big bluestem helps increase POM-C formation quickly, while switchgrass root structure and chemistry build a mineral-bound clay C pool through time. Thus, both species and cultivar selection can help improve bioenergy management to maximize soil carbon gains and lower CO2 emissions.