Soil Nitrogen Dynamics under Dryland Alfalfa and Durum–Forage Cropping Sequences

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), NH 4 –N, and NO 3 –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 NH 4 –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 NO 3 –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 N 2 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.