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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. The 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 software. 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). The 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.
Thivierge et al. (Fri,) studied this question.