Abstract
AbstractPerennial bioenergy crops have been shown to increase soil organic carbon (SOC) stocks, potentially offsetting anthropogenic C emissions. The effects of perennial bioenergy crops on SOC are typically assessed at shallow depths (<30 cm), but the deep root systems of these crops may also have substantial effects on SOC stocks at greater depths. We hypothesized that deep (>30 cm) SOC stocks would be greater under bioenergy crops relative to stocks under shallow‐rooted conventional crop cover. To test this, we sampled soils to between 1‐ and 3‐m depth at three sites in Oklahoma with 10‐ to 20‐year‐old switchgrass (Panicum virgatum) stands, and collected paired samples from nearby fields cultivated with shallow rooted annual crops. We measured root biomass, total organic C, 14C, 13C, and other soil properties in three replicate soil cores in each field and used a mixing model to estimate the proportion of recently fixed C under switchgrass based on 14C. The subsoil C stock under switchgrass (defined over 500–1500 kg/m2 equivalent soil mass, approximately 30–100 cm depth) exceeded the subsoil stock in neighboring fields by 1.5 kg C/m2 at a sandy loam site, 0.6 kg C/m2 at a site with loam soils, and showed no significant difference at a third site with clay soils. Using the mixing model, we estimated that additional SOC introduced after switchgrass cultivation comprised 31% of the subsoil C stock at the sandy loam site, 22% at the loam site, and 0% at the clay site. These results suggest that switchgrass can contribute significantly to subsoil organic C—but also indicated that this effect varies across sites. Our analysis shows that agricultural strategies that emphasize deep‐rooted grass cultivars can increase soil C relative to conventional crops while expanding energy biomass production on marginal lands.
Highlights
Soil horizons deeper than 30 cm contain the majority of Earth’s soil organic carbon (SOC)—possibly holding well over 1000 Pg of C globally (Harrison et al, 2010; Jobbágy and Jackson, 2000)
We explore C storage in marginal lands cultivated with switchgrass (Panicum virgatum, L.), a deeply rooted perennial grass grown as forage and as a cellulosic bioenergy feedstock
Clay content and exchangeable cation concentrations were lowest at the Sandy Loam site and highest at the Clay site (Table 2)
Summary
Soil horizons deeper than 30 cm contain the majority of Earth’s soil organic carbon (SOC)—possibly holding well over 1000 Pg of C globally (Harrison et al, 2010; Jobbágy and Jackson, 2000). While the bulk of deep soil C tends to exchange slowly with the atmosphere (Mathieu et al, 2015; Trumbore, 2009), SOC losses from deep soil horizons following land use change have been substantial—accounting for the majority of the 133 Pg of SOC lost following the global expansion of agriculture (Sanderman et al, 2017). A range of processes introduce C to subsoils, including dissolved C transport in percolating water, burial of aboveground litter via physical mixing, and C fluxes from root exudates and root turnover at depth (Rumpel and Kögel-Knabner, 2011). A large fraction of SOC is root derived, and the depth distribution of SOC correlates with rooting distributions across biomes in natural ecosystems (Grayston et al, 1997; Jobbágy and Jackson, 2000; Rasse et al, 2005). Dead roots and root exudates fuel production of microbial biomass, which subsequently becomes a primary source of mineral-associated C that can persist over long timescales (Sokol et al, 2019)
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