Characterizing Alfalfa's N Benefit in Rotations in Irrigated Cropping Systems.

Alfalfa (Medicago sativa L.) is a highly productive legume under irrigated Mediterranean conditions and fixes significant atmospheric N₂ annually, with a range between 300 – 900 kg N ha-1.   A portion of this is available to crops in rotation, but there is limited information on N credits for alfalfa in irrigated cropping systems.    We conducted field studies in three sites on soils ranging from clay loams and a sandy loam in California to compare an alfalfa-grain rotation and a grain-grain rotation.  The grain rotations included and wheat (Triticum aestivum) sudangrass (Sorghum bicolor var. sudanense) grown for at least 1 1/2 years with zero N fertilizers, and the alfalfa rotations followed two- to four-year-old alfalfa stands.   The experimental design was a Split Plot Design with 4-5 replicates per site.  Crop rotation treatments were the main plot, and subsequent wheat N treatments of 0-280 kg ha-1 were applied to subplots to test N sufficiency in wheat.  Starter N  of 56 kg N ha -1 was applied at planting to all fertilized plots, the remaining N applied at tillering.   Control plots received zero fertilizer.  Both wheat and alfalfa were plowed down in the Fall and wheat was planted as a test crop to estimate N uptake.     N plant monitoring data taken during growth of wheat showed sufficiency in many of the zero N alfalfa-wheat plots through tillering stages, but not in the grain-grain rotation.  Yields and N concentrations of wheat following the two cropping systems were compared to estimate the N credit.  N credits were estimated to be over 125 kg ha-1 fertilizer equivalency in the first year.  A second crop of sudangrass tested residual rotation effects in a subsequent crop.  ¹⁵N analyses were also done on soil and plant materials to estimate contribution of N from alfalfa.  Better estimations of N credits in crops following alfalfa would enable lower costs and the environmental impacts from nitrogen applied to non-fixing grain and broadleaf crops.