Soil pH is a major determinant of the numbers of naturally occurring Rhizobium meliloti in non-cultivated soils in central New South Wales

Abstract

Measurements were made of soil pH, frequency of occurrence of annual species of Medicago (medics) and populations of Rhizobium meliloti at 84 sites on 7 dominant soil groups of the Macquarie region of central-western New South Wales. Over all sites, soil pH (0-10 cm; 1:5 soil: water) ranged from 5.26 to 8.07, medic frequency from 0 to 100% and most probable numbers of R. meliloti from undetectable to 675 000/g soil. There was a highly significant (P<0.001) relationship between soil pH and number of R. meliloti. Above pH 7.0, the mean soil population of R. meliloti was 89000/g; below pH 6.0, it was 37/g. Medics occurred most frequently on the more alkaline soils and with least frequency on the more acid soils, but the relationship between soil pH and medic frequency was weaker than between pH and R. meliloti number. Medics were more tolerant of low soil pH than their rhizobia were; at 2 sites, of pH 5.49 and 5.35, medics occurred at 100% frequency but R. meliloti was undetected. There was an indication of some acidification in these soils over a period of 35 years but this remains to be confirmed.

Systemic Regulation of Soybean Nodulation by Acidic Growth Conditions

“…Low-pH conditions can also negatively impact the growth of plants and rhizobia (Brockwell et al, 1991;Marschner, 1991;Smit et al, 1992;Hungria and Stacey, 1997;Sadowsky et al, 1998;Morón et al, 2005) and can decrease the synthesis of legume flavonoids and rhizobia Nod factor (McKay and Djordjevic, 1993;Hungria and Stacey, 1997). These factors may have also contributed to the down-regulation of early nodulation gene expression observed here (Figs.…”

“…This is especially important considering that nitrate acts through GmNARK in a root-localized fashion to suppress nodule development, and hence, a local mechanism is in place that could have been usurped by acid to exert nodulation control. Indeed, the debilitating effects of low pH on plant and rhizobia health (Brockwell et al, 1991;Marschner, 1991;Sadowsky et al, 1998) and on their symbiotic interactions (Smit et al, 1992;Hungria and Stacey, 1997;Morón et al, 2005) have long been considered local rhizosphere effects induced directly by H + ions. This now appears to be only one component of pH-regulated nodulation control.…”

“…Bradyrhizobium spp. are generally more acid tolerant than Rhizobium spp., but the cause for this remains unclear (Brockwell et al, 1991;Sadowsky et al, 1998). Moreover, certain strains that are less potent under neutral conditions can outcompete other strains under low-pH conditions (Triplett and Sadowsky, 1992;Hungria and Stacey, 1997).…”

Assessment of indigenous Bradyrhizobia spp. population levels in low phosphorus soils of southern Cameroon using cowpea (Vigna unguiculata L. Walp) as trap crop

“…This is in agreement with earlier findings by Nambiar et al(1983) where it was shown that most cultivated tropical soils have a rhizobial population of more than 100 rhizobia cells per gram of soil capable of nodulating the legumes grown on such soils. More so, in soils where naturalized rhizobial populations are high (>10 3 rhizobium bacteria per g soil), introduction of new strains can be difficult and often unsuccessful (Thies et al, 1991;Brockwell et al, 1995).…”

Effect of Rhizobium and Intercropping Systems on Soil Nutrients and Biological Nitrogen Fixation as Influenced by Legumes (Phaseolus vulgaris and Dolichos lablab)

“…The optimum soil pH for maize/ legume plants is 6.5 to 7.0 (Landon, 1991). Brockwell et al, (1991) reported a 10 -3 decrease in the number of soil bacteria in soils with a pH<6 compared to those with a pH>7.0. There is a range of effects of soil pH on rhizobia, but relatively few grow and survive well below pH values of 4.5 to 5.0.…”