Root traits of modern alfalfa lines with contrasting stem fiber digestibility

Abstract

Alfalfa (Medicago sativa L.) is a perennial legume forage crop with high-quality forage and a robust root system. Alfalfa roots support forage growth and contribute to soil carbon and nitrogen supplies. Novel alfalfa lines selected for stem fiber digestibility have been developed, but the consequences of these selections on root traits are understudied. Tradeoffs between selections for aboveground traits and belowground outcomes can have implications for root inputs to soils. We evaluated five alfalfa lines ranging in stem fiber digestibility for root biomass production, morphological traits, and chemistry. The roots of third-year alfalfa stands were excavated from replicated field trials at two Minnesota locations. Root biomass beneath the crown and the pectin and lignin in their root cell walls were measured. Fine root biomass from the space between plants was collected and analyzed for morphological traits, C and N concentrations. The high lignin (HL) line had ∼35% more root biomass than the low fiber digestibility (LFD) and the high fiber digestibility (HFD) lines. The LFD line had 11% more cell wall pectin in roots than the high HL line, and 25%–33% more lignin in the root cell walls than all other alfalfa lines. Additionally, the LFD line had a lower N concentration and a higher C:N ratio than the HL and HFD lines. The data suggest that the LFD alfalfa line produced fewer, less degradable roots than other lines, which has implications for the rate and persistence of soil organic matter.