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

Abstract

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 (ψp) and soil matric (ψm) potentials than to removal of herbage or level of root carbohydrates. Nitrogenase specific activity declined linearly as ψp 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 ψm of −0.015 MPa) in the silt loam and by soil water deficits (mean ψm of −0.07 to −0.51 MPa) in the loamy sand.