Residual effects from lime application on soil pH, rhizobial population and crop productivity in dryland farming systems of central New South Wales

A mildly acidic (pHCa 4.79, 0–10 cm depth) red-brown earth soil (Chromosol) at Condobolin in central-western New South Wales was cultivated and limed (once only) at six rates (range 0–4 t/ha) and sown with field peas (Pisum sativumL.) with and without inoculation (once only) with Rhizobium leguminosarum bv. viciae – the rhizobium for peas. The soil already contained a very small population of pea rhizobia (<4 per g soil). The experiment embraced two parallel rotations, each over 4 years: (1) year 1, inoculated peas; year 2, wheat; year 3, wheat; year 4, uninoculated peas; and (2) year 1, inoculated peas; year 2, wheat; year 3, inoculated chickpeas; year 4, uninoculated peas. The objectives of the work were to establish whether liming had any immediate and residual benefits for rhizobia and plants and, if so, to determine if the two events were linked. Liming had an immediate effect on soil pH (0–10 cm depth). Increases in pH were greater per unit of lime at lower rates of application than at higher rates. Although lime effects existed for the duration of the experiment (four seasons of cropping), there was a small decline in soil pH over time (mean decline in unlimed plots 0.16 pHCa units, mean decline in limed plots 0.47 pHCa units). In the first year (pea crop), there was a very large and highly significant response to inoculation on populations of rhizobia in soil and rhizosphere. The number of rhizobia that occurred naturally in uninoculated plots increased rapidly in high-lime plots until, by the third year, they were substantial and, by the fourth year, equal to those in the inoculated treatment. By the end of the experiment, the mean population of rhizobia in the 4 t/ha lime treatment was 7250 per g soil, compared with <4 rhizobia per g in the nil lime treatment. It was noteworthy that, in those years in the rotations when peas were not grown, populations of R. leguminosarum bv. viciae were sustained by their ability to colonise the rhizospheres of wheat and chickpea. In the first pea crop, eight parameters of plant production responded overwhelmingly to inoculation, while there was an underlying response to liming in two of those parameters. The positive effect of inoculation on peas in the first year carried over to the wheat crop of the second year, which was interpreted as a consequence of increased soil N in the inoculated plots. By the third and fourth years, soil populations of pea rhizobia in the plus inoculation and minus inoculation treatments were approximately equal, and inoculation was no longer a determinant of crop production. On the other hand, application of lime, which had only an underlying effect on pea production in the first year, significantly enhanced several parameters of the symbiosis and growth of the chickpea and pea crops, including legume nodulation and percentage nitrogen in the seed. R. leguminosarum bv. viciae, legumes and cereals each responded differently to increasing rates of lime application. Populations of rhizobia in soil and plant rhizospheres increased with each additional rate of liming. Legume productivity responded to additional lime up to 2 t/ha. There was no significant evidence that liming per se had any effect at any time on wheat production. The practical implications of these results are discussed.

An evaluation of potential Rhizobium inoculant strains used for pulse production in acidic soils of south-east Australia

Profitability of the pulse industry relies considerably on crop nitrogen fixation because this process supplies greater than 60% of pulse crop nitrogen. Therefore the industry requires the most efficient Rhizobium symbioses and effective inoculation management. Re-appraisal of the recommended inoculant strain for field pea, SU303, in south-east Australia, was warranted by field evidence that SU303 failed to maximise grain yield at sites in Western Australia. Re-appraisal of the inoculant strain for faba bean and lentil, WSM1274, was warranted because of anecdotal evidence from Western Australia of associated crop failures. In addition, a glasshouse study in Western Australia reported greater dry matter production by faba bean and lentil inoculated with strains other than WSM1274. This paper reports trials comparing potential inoculant strains for field pea and faba bean in soils of south-east Australia. Comparisons are based on efficiency for nitrogen fixation, survival on seed and survival in soil. Additionally, because the pulse industry lacked comprehensive information to assist decision making on the need for recurring inoculation, relevant investigation of this issue is also reported. The results of 3 field experiments for efficiency for nitrogen fixation, over mildly (pHCa 5.0) to strongly (pHCa 4.3) acidic soil in south-east Australia supported replacing SU303 as the commercial inoculant. The efficiency for nitrogen fixation of WSM1274 on faba bean was not found to be inferior to alternative strains. However, its capacity for survival on seed at temperatures of 15°C and above, over a wide range of relative humidity, and perhaps its capacity for survival in acidic soil, was inferior. This provided additional evidence to justify the replacement of this inoculant strain that was agreed to by a national steering committee in 2001, based on the Western Australia reports, the early experiments in this study and those of a collaborative study in Victoria. Alternative inoculant strains to SU303 and WSM1274 were identified in the current study. Temperature and relative humidity conditions suitable for maintaining inoculant viability with extended storage of inoculated field pea and faba bean are also discussed. A survey of rhizobia surviving in soil was used to determine the time scale of persistence of Rhizobium leguminosarum bv. viciae and Bradyrhizobium sp. (Lupinus) in soils of the south-east. It was concluded that in soils of pH (CaCl2) <5.1, inoculation of field pea and faba bean should be routinely practiced; none of the strains of R. leguminosarum bv. viciae tested showed ability for survival in strongly acidic soil sufficient to obviate seed inoculation. It was further concluded that the absence of a legume host for lupin rhizobia for 4 or more years would also warrant reintroducing inoculant of B. sp. (Lupinus).