Root traits of perennial C4 grasses contribute to cultivar variations in soil chemistry and species patterns in particulate and mineral-associated carbon pool formation

<strong>Abstract:</strong> Recent studies have indicated that the C4 perennial bioenergy crops switchgrass (<em>Panicum virgatum</em>) and big bluestem <em>(Andropogon gerardii</em>) accumulate significant amounts of soil carbon (C) owing to their extensive root systems. Soil C accumulation rates under these grasses are likely driven by inter- and intra-specific variability in plant traits. However, the mechanisms that underpin this variability in soil C storage remain unresolved. In this study we evaluated how inter- and intra-specific 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; 4.8 cm diameter) from the root zone of all cultivars. We measured root biomass, root diameter, specific root length, bulk soil C and C associated with coarse and fine particulate organic matter (CPOM, FPOM) plus silt- and clay-sized fractions. Cultivar monocultures of both C4 species were established on soils that supported C3 grassland for 36 years before planting, which allowed us to use differences in the natural abundance of stable C isotopes to quantify C4 plant-derived C. We also characterized organic matter chemical class composition in root-zone soil using high resolution FTICR mass spectrometry. We found that species accumulated C through different mechanisms. Big bluestem cultivars had larger root systems that increased C4 plant-derived C in the POM-C pool, while switchgrass cultivars increased the C4 plant-derived C in the clay fraction via differences in root morphology and soil chemistry. This highlights the importance of both POM-C and mineral associated C in building soil carbon pools. <br> <br> <strong>Abbreviations:</strong> <strong>Species:</strong> BB = Big Bluestem SG = Switchgrass <strong>Cultivars:</strong> BO = Bonanza BSL = BB Southlow ST = Suther CR = Cave-in-Rock KA = Kanlow SSL = SG Southlow <strong>Depth:</strong> A = 0-10 cm B = 10-20 cm C = 20-30 cm