EFFECTS OF SOIL MOISTURE AROUND NODULES ON NITROGEN FIXATION BY WELL WATERED SOYBEANS

Abstract

Soybeans (Glycine max (L.) Merr.) were grown at various soil moisture levels around nodules so effects on nitrogen fixation could be studied. Plants were grown in a growth room in 35-cm diam pots. Pots contained two layers of loam–sand mixture separated by a layer of coarse silica, intended to restrict capillary movement of soil moisture from the bottom to the top soil layer. At the beginning of seed development, pots received 200 ml water on the surface, 200 ml in the bottom layer through plastic tubes or 100 ml each way. Plants with good root development in the bottom soil layer maintained leaf water potentials greater than − 6 bars, while soil moisture around nodules varied from 4 to 20%. In three individual experiments, in which only plants with leaf water potentials greater than − 6 bars were considered, there was no relationship between soil moisture around nodules and mg N 2 [C 2 H 2 ] fixed/g nodule dry weight × h. When results of two experiments with similar fixation means were combined, there was a barely significant (R 2 =.19*) quadratic relationship, with very dry soil or soil near field capacity around nodules decreasing fixation slightly. Percent moisture in soil around nodules did not affect nodule moisture content, indicating that nodule moisture status was maintained if plants received adequate moisture from below the nodule zone.