3 m up, >1.3 m under: Tracking carbon fluxes of a change from Midwestern row crops to perennial grasses
The use of biofuels in place of petroleum-derived carbon is mostly justified on the basis of carbon savings, but the majority of US biofuel production comes from annually tilled maize, and most maize ecosystems are net carbon sources. Some researchers have suggested that a shift to perennial grasses as feedstocks could produce large carbon savings both through reduced management emissions and through storage of carbon in the soil. For a perennial ecosystem to store carbon, it must either increase C inputs or reduce losses (ideally both) relative to the annual system.
We evaluated the soil carbon storage potential of three possible biofuel crops (Miscanthus x giganteus, Panicum virgatum, and a mix of native tallgrass prairie species) alongside that of a typical annually-tilled maize-maize-soybean rotation. We quantified changes in root biomass using minirhizotron cameras, tracked respiration from soil using infrared gas analyzers coupled to automated soil chambers, and distinguished root-respired from microbe-respired carbon dioxide by measuring carbon isotope signatures using cavity absorption spectrometry.
Perennial grasses had greatly increased (up to 300%) root volume compared to both maize and soybeans, indicating that more carbon was placed belowground in these systems and thus potentially stored. Respiration from soil increased, but isotopic measurements suggested that this was mostly from living roots; breakdown of soil organic matter was reduced in all perennial crops, probably in part of because of reduced tillage in the perennial systems. In addition, perennials allocated more root carbon to the very deep subsoil, where it may be better protected from breakdown. In sum, management transitions form annual corn-soy to perennial grasses may have climate benefits from ecosystem carbon storage as well as from fossil fuel combustion offsets.