Soil acidity and Rhizobium: their effects on nodulation of subterranean clover on the slopes of southern New South Wales

Abstract

Rhizobium trifolii and root nodulation were compared in acid and limed soil to investigate mechanisms by which current soil acidity on the slopes of southern New South Wales may reduce the growth of Trifolium subterraneum. Increasing soil pH increased both the rate at which Rhizobium trifolii colonized soil and the frequency of nodules/g root (NF). Thus, numbers of R. trifolii were greater in carbonate-treated soil throughout autumn and winter compared to unlimed soil. In limed soil R. trifolii were measured at 104-105/g soil as early as seedling germination, 40-200 x more than in unlimed soil. Despite this, by spring, the numbers of rhizobia in soil without lime reached large and even similar levels as occurred in limed soil. Subsequently, with the drying of soil in summer, the numbers of R. trifolii declined markedly in both untreated and limed soil. Maximum NF could not be achieved with addition of Ca without increase in pH. From the field observations and other research, a hypothetical model is proposed to explain how differences in the rates of change in rhizobial numbers due to acidity may determine nodulation (NF). Thus, it is suggested that, when there is a repetitive decline in numbers of R. trifolii to low levels in summer, their subsequent rate of increase in the following autumn influences nodule abundance. The production of DM in an acidic soil appeared to be limited by symbiotic N2 fixation since added N fertilizer removed lime response. The limitation to N2 fixation did not appear to be due to lower occupancy of nodules by Rhizobium strains with less potential for N2 fixation than those in limed soil. It is possible that the effect of acidity on NF influences DM, though sampling limitations made it difficult to generalize in this regard.

Rhizobium -Legume Symbiosis and Nitrogen Fixation under Severe Conditions and in an Arid Climate

“…The number of R. leguminosarum bv. trifolii cells was greater in carbonate-treated soil (103); increasing the soil pH increased both the rate at which rhizobia colonized the soil and the frequency of nodule formation. It has also been reported (253) that while germination of pigeon pea was decreased at pH values of Ͼ8.8, growth of rhizobia was unaf-fected up to pH 11.5.…”

Tolerance of the annual legumesBiserrula pelecinus,Ornithopus sativa,Trifolium spumosum,T. vesiculosumandT. subterraneumto soil acidity

“…This implies that, in bladder clover more than any other species, adaptation to acid soils is restricted by the symbiosis with Rhizobium. Previous research has shown that even for an acid-tolerant species such as subterranean clover, N 2 -fixation and shoot N concentration is increased when lime is applied to a highly acid soil, suggesting that Rhizobia is relatively more sensitive to the acidic conditions than the legume genotype (Whelan & Alexander 1986;Evans et al 1988;Hayes et al 2008), albeit the legume is reliant upon effective symbiosis for optimal performance. Ballard et al (2002) postulated that for balansa clover, which was shown to have ineffective symbiosis in many field sites across a range of soil pH conditions, a plausible approach to improving the performance of this species in the field would be to select genotypes on the basis of improved symbiosis.…”

Prospects for improving perennial legume persistence in mixed grazed pastures of south-eastern Australia, with particular reference to white clover

“…It is well documented that soil acidity has significant negative effects on plant growth in legume-based pastures, hence reducing N inputs of legumes (Unkovich et al 1996). The soluble toxic Al 10 ion complex associated with low soil pH not only reduces root growth, but also supresses the proliferation of rhizobium (Evans et al 1988) and reduces the efficiency of nodulation (Richardson et al 1989) and N fixation in legume species (Peoples et al 2012). Soil acidity could represent the most widespread threat to the 15 symbiotic performance of legumes in pastures of south-eastern Australia.…”

“…The introduction and use of acid-tolerant host germplasm and rhizobial strains may provide a potential solution to poor symbiotic performance in acid soils (Howieson and Ewing 1986;Howieson et al 1988;Wigley 20 et al 2018). In conjunction, lime application to ameliorate soil acidity may increase soil populations of rhizobia and enhance root nodulation (Evans et al 1988;Richardson et al 1989), stimulate nodule nitrogenase activity (Coventry et al 1985) and manage reacidification of soil in a long-term pasture 25 (Li et al 2019).…”

“…Acidity Soil acidity may restrict root growth and root hair development of some sensitive legumes (Unkovich et al 1996;Dilworth et al 2000) Some strains are sensitive to soil acidity due effects on nutrient availability, such as reduced available P and Ca and increased Al (Evans et al 1988;Bottomley 1992) and Mn (Lowendorf 1980) Soil acidity reduces the availability of some essential plant nutrients (e.g. P and Ca) and increases the availability of other toxic elements (e.g.…”