Dryland Crop Root Biomass and Carbon and Nitrogen Contents in Relation to Soil Water Content.

            Crop roots are important C and N inputs for soil C and N sequestration and are essential for water and nutrient uptake. In semiarid regions, root biomass yield and C and N contents may depend on soil water availability. We quantified root biomass and C and N contents of dryland crops in continuous alfalfa (CA), durum-barley hay (D-B), durum-foxtail millet hay (D-M), durum-(Austrian winter pea + barley) hay (D-P/B), and durum-fallow (D-F) sequences and related them with soil water content at 0- to 150-cm depth in eastern Montana. Root biomass decreased with soil depth and 34 to 61% of total root biomass at 0- to 120-cm was located within the surface 15 cm. Root biomass was greater in CA (376 kg ha^-1) than in other cropping sequences at 0- to 15-cm and greater (617 kg ha^-1) than in D-F at 0- to 120-cm. Root C concentration was similar for all crops but root N concentration was greater or C/N ratio was lower in alfalfa than in durum following fallow and millet following durum. Root C content (214 kg C ha^-1) was greater in CA than in D-B and D-F. Similarly, root N content was greater in CA (11.0 kg N ha^-1) than in other cropping sequences. When sampled after crop harvest, root biomass decreased linearly (R² = 0.77) with soil water content (0-150 cm) which was lower in CA than in other cropping sequences. Because of greater root biomass and C and N contents, CA may be potentially more effective in increasing soil C and N sequestration and water use than D-B and D-F.  Durum following millet produced greater root biomass and C and N contents than durum following fallow.