Forages grown in rotation with or without cereals to sustain dryland soil water content and crop production may influence N dynamics. We evaluated the effect of alfalfa (Medicago sativa L.) and durum (Triticum turgidum L.)–annual forage cropping sequences on above- (stems + leaves) and belowground (roots) biomass N, dryland soil total N (STN), particulate organic N (PON), microbial biomass N (MBN), potential N mineralization (PNM), NH4–N, and NO3–N contents at the 0- to 120-cm depth in northeastern Montana from 2002 to 2005. Cropping sequences were continuous alfalfa (CA), durum–barley (Hordeum vulgare L.) hay (D-B), durum–foxtail millet (Setaria italica L.) hay (D-M), durum–Austrian winter pea (Pisum sativum L.)/barley mixture hay (D-P/B), and durum–fallow (D-F). From 2002 to 2005, total above- and belowground biomass N was 20 to 97 kg N ha−1 greater under CA than other treatments. In 2005, STN, PON, and PNM were 7 to 490 kg N ha−1 greater under CA than D-M, D-B, and D-P/B at 0 to 30 cm but varied by treatment at other depths. In contrast, MBN at 0 to 15 cm and NH4–N content at 30 to 90 cm were 23 to 37 kg N ha−1 greater under D-B than D-M and D-F. The NO3–N content at 0 to 120 cm was 65 to 107 kg N ha−1 greater under D-P/B than other treatments. Even though haying removed a greater amount of N, alfalfa increased surface soil N storage and mineralization and reduced the potential for N leaching compared with durum–annual forages, probably due to increased root growth or N2 fixation. Durum–pea/barley hay, however, increased N mineralization and availability in subsoil layers, probably due to greater root N concentration or downward movement of water-soluble N.
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