Elevated CO₂ and Temperature Affect Composition and in vitro Degradability of Timothy Roots

Climate change is expected to affect the growth and metabolism of both aboveground and belowground parts of most crop species including timothy ( Phleum pratense L.), a major forage grass species in northern regions. Our objective was to assess the effects of predicted increases in CO 2 concentration and temperature on root chemical composition and degradability of timothy grown under contrasted N fertilization. Timothy was grown in growth chambers under either 400 or 600 µmol mol −1 of CO 2 at day and night temperatures of either 22 and 10°C or 25 and 15°C and was fertilized with 0, 60, or 120 kg N ha −1 . Root degradability was characterized using two in vitro enzymatic methods: in vitro true digestibility (IVTD) and enzyme‐released glucose from cell walls (CW), while root composition was determined for N and C concentrations and enzyme‐released hemicellulose and pectin concentrations. Increasing temperature and atmospheric CO 2 concentration generally decreased N concentration and increased the C/N ratio and decreased root degradability assessed by the two enzymatic methods under all three N rates. When both factors were considered together, root N concentration was 13% lower and the C/N ratio was 19% greater under predicted future conditions than under current conditions at first harvest. Both enzymatic approaches showed that root degradability was lower under elevated CO 2 and temperature compared with current conditions with an estimated average reduction of 6.3% with the IVTD method and 15.5% with the enzyme‐released glucose method. Predicted future conditions of elevated CO 2 and temperature decreased timothy root degradability across a wide range of N fertilization compared with current conditions and this could increase its potential for soil C sequestration.