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Alfalfa Response to Soil Water Deficits. III. Nodulation and N₂ Fixation¹

Our objective was to determine the effect of plant and soil water deficits and forage harvest on alfalfa (Medicago sativa L.) nodulation and N₂ fixation. In a field study on Typic Hapludoll (silt loam) soil, nitrogenase specific activity (NSA), nodule numbers, and shoot and root growth of irrigated and nonirrigated alfalfa were measured during a harvest/regrowth cycle. In a glasshouse study with Udorthentic Haploboroll (loamy sand) soil, NSA, nodulation, shoot and root growth, and root total nonstructural carbohydrate (TNC) concentration of nonharvested and harvested alfalfa were monitored during two successive harvest/regrowth cycles. Both harvest treatments were subjected to a range of soil and plant water deficits. Forage harvest decreased NSA and root TNC concentration for well-watered alfalfa, but changes in NSA for water-stressed plants were more associated with fluctuations in plant water (_p) and soil matric (_m) potentials than to removal of herbage or level of root carbohydrates. Nitrogenase specific activity declined linearly as _p decreased to –3.0 MPa, at which point activity approached zero. Alfalfa nodules subjected to severe water stress were able to resume activity when soil moisture levels were restored. Nodulation was inhibited by well-watered conditions (mean _m of –0.015 MPa) in the silt loam and by soil water deficits (mean _m of –0.07 to –0.51 MPa) in the loamy sand.

Key Words: Medicago sativa L. • Plant water potential • Soil matric potential • Nitrogenase specific activity • Total nonstructural carbohydrates

² Former research assistant and associate professor, Dep. of Agron. and Plant Genetics, Univ. of Minnesota, St. Paul, MN 55108.

Received for publication November 8, 1982.