Very Fine Roots Differ Among Switchgrass (Panicum Virgatum L.) Cultivars and Differentially Affect Soil Pores and Carbon Processes

Abstract

Switchgrass (Panicum virgatum L.) is a promising feedstock for biofuel production, with diverse cultivars representing several ecotypes adapted to different environmental conditions within the contiguous USA. Multiple field studies have demonstrated that monoculture switchgrass cultivation leads to slow to negligible soil carbon (C) gains, an outcome unexpected for such a deep-rooted perennial. We hypothesize that different switchgrass cultivars have disparate impacts on soil C gains, and one of the reasons is variations in physical characteristics of their roots, where roots directly and indirectly influence formation of soil pores. We tested this hypothesis at Great Lakes Bioenergy Research Center’s research site in Michigan using two lowland cultivars (Alamo and Kanlow) and four upland cultivars (Southlow, Cave-in-Rock, Blackwell, and Trailblazer). Three types of soil samples were collected: 20cm diameter (Ø) intact cores used for root analyses; 5cm Ø intact cores subjected to X-ray computed tomography scanning at 18µm resolution used for pore characterization; and disturbed soil samples used for microbial biomass C (MBC) and soil C measurements. Path analysis was used to explore interactive relationships among roots, soil pores, and their impact on MBC, and ultimately, on soil C contents across six cultivars. The abundance of very fine roots (<200µm Ø) was positively associated with fractions of pores in the same size range, but negatively with distances to pores and particulate organic matter. Higher abundance of such roots also led to greater MBC, while greater volumes of medium pores (50-200µm Ø) and shorter distances to pores led to greater MBC. Results suggest that the greater proportion of very fine roots is a trait that can potentially stimulate soil C gains, with pore characteristics serving as links for the relationship between such roots and C gains. However, 7 years of cultivation led to no significant C gains in any of the studied cultivars.

Keywords: Switchgrass cultivar, fine root, distance to pore, distance to particulate organic matter, X-ray computed micro-tomography