Evaluation of the symbiotic nitrogen-fixing potential of soils by direct microbiological means

Abstract

A method for estimating the nitrogen-fixing capacity of a population of rhizobia resident in soil is presented. Legume test plants, growing under microbiologically-controlled conditions in test tubes packed with a vermiculite substrate moistened with a nitrogen-free plant nutrient solution, are inoculated directly with a suspension of the soil under examination. Rhizobia in the soil nodulate the test plants, and the amount of foliage dry matter produced in the 28 days after inoculation is regarded as an index of their effectiveness. An inoculum of at least 30, and preferably 100, rhizobia is needed to ensure that nitrogen fixation is not masked by delayed nodulation. The new method is tentatively described as the 'whole-soil inoculation' technique.Appraisals were made with Trifolium subterraneum L. and Rhizobium trifolii and with Medieago sativa L. and R. meliloti. Soil-borne pathogens did not interfere with plant growth. The whole-soil inoculation technique was less tedious and time-consuming than an alternative method which involved extracting representative isolates from the soil and testing their effectiveness individually, and appeared to give more realistic values for the nitrogen-fixing capacity of the soil as a whole. Used in association with a field experiment, the whole-soil inoculation technique confirmed microbiologically that there had been an agronomic response to surface application of inoculant to poorly-nodulated T. subterraneum pasture.It is submitted that this technique for determining the effectiveness of rhizobia in soil, combined with a plant-infection method for counting rhizobia, can be a reliable guide to the need for inoculation in the field.

Influence of tree canopy on N2 fixation by pasture legumes and soil rhizobial abundance in Mediterranean oak woodlands

“…Using the "whole-soil inoculation technique" proposed by Beck (1993) and Brockwell et al (1988), Soares et al (in press) measured the indices of effectiveness (%E) of rhizobial selected in these pastures according to Ferreira and Marques (1992), with inoculation of subclover in a controlled environment and found that soil rhizobial populations were always effective (E N 25%), and particularly high in the youngest sown pasture at Estremoz (E = 88%). Soares et al (in press) explained this result by the more recent seed inoculation of subclover with R. leguminosarum L. compared with other sown pastures.…”

Improving N2 fixation from the plant down: Compatibility of Trifolium subterraneum L. cultivars with soil rhizobia can influence symbiotic performance

“…Single strains isolated from soils and the Australian commercial strain WSM1325 were grown on yeast mannitol agar slopes for 4 days and suspended in 0.85% NaCl (~10 6 cells/mL) prior to application at 1 mL per vial. For soil inoculation treatments, 1 mL of soil extract (10 g soil in 90 mL 0.85% NaCl, shaken for 10 mins) was applied (Brockwell et al 1988). Uninoculated treatments received only sterile saline.…”

Role of legumes in providing N for sustainable tropical pasture systems

“…As suggested by Date (1977) the simplest approach, in the absence of an ability to predict an inoculation response, is to conduct simple need-to-inoculate tests. Details of these tests are available (Brockwell et al, 1988;Date, 1977;Sylvester-Bradley, 1984;Vincent, 1970) and some have been discussed further by Giller and Wilson (1991). Briefly, these tests compare the growth of uninoculated and inoculated plants with a third treatment receiving doses of N fertilizer of 30 kg ha-1 every 2 weeks.…”