Effects of available soil N and rates of inoculation on nitrogen fixation by irrigated soybeans and evaluation of δ¹⁵N methods for measurement

Abstract

Nitrogen fixation by irrigated soybeans (Glycine max (L.) Merr. cv. Forrest) was studied in a field experiment on a grey clay soil at Trangie, N.S.W. during the summer of 1985-86. Cropping with oats during the previous winter diminished the concentration of plant-available nitrogen in the soil from 37.6 to 18.5 mg N kg-1 and induced differences in the natural abundance of 15N (S15N) in this nitrogen. Four rates of liquid inoculation with Bradyrhizobium japonicum strain CB 1809, interacted with soil nitrogen to produce a wide range of nodulation of the soybeans. The following main effects on growth and N2 fixation resulted: (a) Initially, growth and accumulation of plant nitrogen was lower in pre-cropped than in prefallowed soil but N2 fixation was higher. (b) Nitrogen fixation during seed development was high in pre-cropped soil and greatest at the highest rate of inoculation. It resulted in high yields of seed (3.5 t ha-1 with 100 times the normal inoculation) with significantly higher concentration of seed nitrogen than from plants grown in prefallowed soil. (c) With increasing rates of inoculation on the pre-fallowed soil, more uniform nodulation was associated with smaller variances in most of the parameters studied. Other findings included further validation of the S15N method of calculating the proportion (p) of plant nitrogen derived from N2 fixation, with good agreement between treatment effects based on such estimates and those based on the relative concentrations of ureides in vacuum-extracted xylem sap. The values of p from S15N measurements on shoot nitrogen were affected little by inclusion of root nitrogen, and similar values were obtained when uninoculated, unnodulated Forrest soybeans, a nonnodulating genotype (non-nod Clark 63) or extractable mineral nitrogen of soil were used to estimate the S15N of plant N assimilated from soil. More dry matter (flowers, young pods and older leaves) containing more nitrogen (23-26 kg N ha-1) fell from the canopy of plants during seed maturation on pre-fallowed soil (high nitrogen) than on pre-cropped soil (13-15 kg N ha-1). Several correlations between the various quantities measured were noted and are discussed. It is concluded that growing winter cereals on land newly broken from pasture, coupled with high rates of inoculation of the following soybeans, may be a profitable way of diminishing plant-available soil nitrogen, thus maximizing the contribution of nitrogen from N2 fixation with benefits in seed yield and protein content.