Root traits and carbon input in field-grown sweet pearl millet, sweet sorghum, and grain corn

Little information exists on root morphological characteristics of agricultural crops under field conditions, which can be a major determinant of plant N uptake efficiency and C input to soil. Sweet pearl millet [Pennisetum glaucum (L.) R.BR.] and sweet sorghum [Sorghum bicolor (L.) Moench] are envisioned as energy crops in eastern Canada, to complement corn starch ethanol. Th e aims of this study were to characterize and compare root biomass and root traits of field-grown sweet pearl millet, sweet sorghum, and grain corn (Zea mays L.), and to estimate their annual C input to soil. At two sites in eastern Canada, root samples recovered from 30-cm deep soil cores in sandy loams were weighed and scanned. Image analysis was performed with the WinRhizo soft ware. Roots and shoots were analyzed for C concentration. Estimated C input to soil at harvest was higher for corn (243 g C m–2) than for sorghum and millet (197 and 131 g C m–2, respectively). In contrast, millet and sorghum had the greatest specific root lengths (83, 39, and 22 m g–1 for millet, sorghum, and corn, respectively), and a larger contribution of very fine roots (<0.5 mm diam.) to the total root surface (60–63% for millet and sorghum, and 45–55% for corn). Th e longer and finer roots of millet and sorghum could contribute to their high N uptake efficiency. However, compared to grain corn, their lower C input to soil needs to be recognized to ensure a balanced C budget.