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Title:   Author  item
                                            Sainju, Upendraitem
                                            Lenssen, Andrew
  Submitted to: Soil Science Society of America Journal
                                    Publication Type: Peer Reviewed Journal
                                    Publication Acceptance Date: 9/15/2010
                                    Publication  Date: 3/21/2011
                                    Citation: Sainju, U.M., Lenssen, A.W. 2011. Soil Nitrogen Dynamics under Dryland Alfalfa and Durum-Forage Cropping Sequences. Soil Science Society of America Journal. 75:669-677.Interpretive Summary: Some of the major limiting factors to sustain crop yields in dryland cropping systems are available soil water and nitrogen content. Use of alternate-year fallow with cropping can conserve soil water, increase N availability due to increased N mineralization, control weeds, sustain crop yields, and reduce the risk of crop failure. Continuous adoption of crop-fallow systems for the last several decades, however, has reduced dryland soil nitrogen storage. The system has not only decreased crop yields and became uneconomical by the absence of crops during fallow but also reduced soil quality and productivity due to loss of soil organic matter from increased soil erosion. 
One of the options to reduce the fallow period and increase water-use efficiency, crop yields, and net return is continuous cropping, such as cereal-annual forage sequences. Inclusion of annual forages in rotation with cereals maintains both cereal and forage yields because forages are harvested earlier for hay than cereals, which results in greater soil water content and succeeding crop yields. Little is known about the effect of cereal-forage sequences on soil N dynamics under dryland cropping systems in the northern Great Plains. 
We evaluated the effect of alfalfa and durum-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, and estimated N balance at the 0- to 120-cm depth. Cropping sequences were continuous alfalfa (CA), durum-barley hay (D-B), durum-foxtail millet hay (D-M), durum-Austrian winter pea/barley mixture hay (D-P/B), and durum-fallow (D-F). The experiment was conducted in a Williams loam from 2002 to 2005 in eastern Montana. 
Aboveground biomass N was lower in 2002 but greater in 2003 and 2005 in CA than in other treatments. Similarly, belowground biomass N was greater in CA than in other treatments in 2003 and 2005. In 2005, soil surface residue N was greater in CA and D-B than in other treatments. The STN at 0- to 15-cm was greater in CA than in D-M but at 30- to 60-cm was greater in D-P/B than in D-B and D-M. The PON at 15- to 30-cm was greater in CA than in D-P/B but at 60- to 120-cm varied with treatments. The PNM at 0- to 15-cm was greater in CA and at 15- to 120-cm was greater in D-P/B than in other treatments. The MBN at 0- to 15-cm was greater in D-B and D-F than in D-M but at 60- to 90-cm was greater in CA than in D-B, D-M, and D-P/B. The NH4-N content at 30- to 90-cm was greater in D-B than in D-F. The NO3-N content at 0- to 120-cm was greater in D-P/B than in other treatments, except in D-B. Estimated N balance was greater in CA than in other treatments. 
Because of greater biomass N, alfalfa stored more N in the surface residue and soil and reduced the potential for N leaching than durum-annual forage sequences. Durum-pea/barley hay, however, increased N mineralization and availability at the subsoil layers compared with other cropping sequences. Including Austrian winter pea with barley hay in rotation with durum may reduce N fertilization rate compared with barley hay alone and increase the potential for N leaching. As a result, deep rooted nonlegumes other than durum and barley hay may be needed to rotate with Austrian winter pea to capture soil NO3-N and reduce the potential for N leaching. Annual forages, such as foxtail millet, may be less desirable to rotate with durum because of its lower N uptake and greater potential of N loss through leaching, volatilization, and denitrification.
                                Technical Abstract: 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, and estimated N balance at the 0- to 120-cm depth. 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 biculture hay (D-P/B), and durum-fallow (D-F). The experiment was conducted in a Williams loam (fine-loamy, mixed, superactive, Typic Argiustoll) from 2002 to 2005 in eastern Montana. Aboveground biomass N was lower in 2002 but greater in 2003 and 2005 in CA than in other treatments. Similarly, belowground biomass N was greater in CA than in other treatments in 2003 and 2005. In 2005, soil surface residue N was greater in CA and D-B than in other treatments. The STN at 0- to 15-cm was greater in CA than in D-M but at 30- to 60-cm was greater in D-P/B than in D-B and D-M. The PON at 15- to 30-cm was greater in CA than in D-P/B but at 60- to 120-cm varied with treatments. The PNM at 0- to 15-cm was greater in CA and at 15- to 120-cm was greater in D-P/B than in other treatments. The MBN at 0- to 15-cm was greater in D-B and D-F than in D-M but at 60- to 90-cm was greater in CA than in D-B, D-M, and D-P/B. The NH4-N content at 30- to 90-cm was greater in D-B than in D-F. The NO3-N content at 0- to 120-cm was greater in D-P/B than in other treatments, except in D-B. Estimated N balance was greater in CA than in other treatments. Because of greater biomass N, alfalfa stored more N in the surface residue and soil and reduced the potential for N leaching than durum-annual forage sequences. Durum-pea/barley hay, however, increased N mineralization and availability at the subsoil layers compared with other cropping sequences.


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