Ecological Indicators of Native Rhizobia in Tropical Soils

Abstract

The relationship between environment and abundance of rhizobia was described by determining the populations of root nodule bacteria at 14 diverse sites on the island of Maui. Mean annual rainfall at the sites ranged from 320 to 1,875 mm, elevation from 37 to 1,650 m, and soil pH from 4.6 to 7.9. Four different soil orders were represented in this study: inceptisols, mollisols, ultisols, and an oxisol. The rhizobial populations were determined by plant infection counts of five legumes (Trifolium repens, Medicago sativa, Vicia sativa, Leucaena leucocephala, and Macroptilium atropurpureum). Populations varied from 1.1 to 4.8 log1o cells per g of soil. The most frequently occurring rhizobia were Bradyrhizobium spp., which were present at 13 of 14 sites with a maximum of 4.8 log10 cells per g of soil. Rhizobium trifolii and R. leguminosarum occurred only at higher elevations. The presence of a particular Rhizobium or Bradyrhizobium sp. was correlated with the occurrence of its appropriate host legume. Total rhizobial populations were significantly correlated with mean annual rainfall, legume cover and shoot biomass, soil temperature, soil pH, and phosphorus retention. Regression models are presented which describe the relationship of legume hosts, soil climate, and soil fertility on native rhizobial populations.

Indigenous bradyrhizobia populations in tropical farm-lands

“…1 Distribution of hradyrhizohia in seven tropical farm-lands content and WHC. These observations are in conformity with the reports of Woomer et al (1988a) and Brockwell et a/. (1991).…”

“…T h e biggest influence on rhizobial numbers is the presence or history of legume cover. Lowendorf (1980) indicated a similar trend even though rhizobial presence without their legume hosts is documented (Woomer et al 1988a).…”

“…Other factors reported to affect rhizobial presence and survival are soil organic carbon, soil temperature, mean annual rain falls and phosphorous retention (Woomer et al 1988a;Lowendorf 1980;Brockwell 1991).…”

“…1992). This is so because the distribution of indigenous rhizobia populations is crucial to the successful introduction and establishment of the inoculant strain (Woomer et al 1988a). However, this basic index is often lacking for many tropical soils.…”

Mutualism as a constraint on invasion success for legumes and rhizobia

“…First various soil bacteria have been shown to interfere competitively with introduced rhizobia (Parle, 1964;Chatel et al, 1968;Lowendorf, 1980;Pugashetti et al, 1982). Secondly, when legumes are first introduced into a region that lacks closely related native legume taxa, nodule formation is often scarce or absent (Wilson, 1934;Hely, 1957;Bottomley & Jenkins, 1983;Kueneman et al, 1984;Woomer et al, 1988;Lindstrom et al, 1990;Odee et al, 1995;Sullivan et al, 1995). Finally, planting of legume crops can boost rhizobial population size by several orders of magnitude (e.g.…”