4. AUTOCUIDADOS Y CALIDAD DE VIDA EN LOS PACIENTES
4.3. Autocuidados y salud en pacientes enterostomizados
The lack of survival of the commercial inoculant at Glenmore is most likely to have been due to the high numbers of naturalised Rhizobium sp. already present in the soil out competing the commercial inoculant. The MPN from white clover with 0 t or 4 t lime was 3.3 x 102 and 9.2 x 102
cells g-1 soil, respectively at Glenmore Station. Brockwell et al. (1975) found that when seed
inoculated with R. leguminosarum strain TA1 at a rate of 135 rhizobia seed-1 was sown into a soil
containing naturalised rhizobia at 40 gram-1 of soil, almost all nodules were occupied by R.
leguminosarum strain TA1. However, on a soil with 10,000 naturalised rhizobia gram-1, less than
10% of nodules were occupied by R. leguminosarum strain TA1. The high Al3+ levels at the Glenmore
site could also have prevented the survival and growth of the commercial and any naturalised
Rhizobium sp. strains in the soil (Section 2.4.1.1).
2.4.2.2
Lucerne
The low number of background rhizobia at Glenmore explains why, despite the adverse conditions, the commercial inoculant was present in a high percentage of the nodules from both the 0 t and 4 t lime plots (Section 2.4.1.2). The MPN of rhizobia in the soil in the 4 t lime plots was 1.2 x 103 cells g-1 soil but it was undetectable in the 0 t plots. Brockwell et al. (1991) found that S. meliloti are particularly affected by acidity. At a pH >7 soil contained 89 000 cells g-1 soil and 37 cells
g-1 soil at a pH <6. Ballard et al. (2003) also found that the MPN of rhizobia in the soil was correlated
(r=0.64) with soil pH. All 13 soils with less than 1 x 102 rhizobia g-1 had a pH less than 6.7.
The higher density of naturalised rhizobia compatible with lucerne at Ashley Dene and Lincoln University is most likely because of the frequent presence of the host. The soils in the current
79 experiment had both previously been sown in lucerne. There have been many studies that show very high (>100 000 g-1 soil) clover rhizobia populations in soils that were continuously growing
pasture that contains white clover in New Zealand (Hale, 1980, 1981; Gaur and Lowther, 1982). Similar results have been shown for lucerne although the naturalised population was not as high. Ballard (2003) collected soils from 42 dryland lucerne pastures and eight irrigated multiple-use lucerne stands in the south-east of South Australia. The lucerne swards were on average 5.6 years old (range 1–28) and the MPN of lucerne rhizobia exceeded 1.0 × 103 g-1 in 23 of the soils. Even
without the host rhizobia are able to survive in the soil for 40 years providing temperature and pH are optimal (Jensen, 1941) with temperatures below 42.5°C (Bowen and Kennedy, 1959) and a pH between 5.8 – 6 (Bolton, 1962).
The current study is the first time MPN has been monitored over time in New Zealand and linked to soil temperature and soil moisture and showed the importance of both soil moisture and soil temperature for rhizobia survival in the soil. This is important for the survival of the commercial inoculant over time and because the size of the background populations of rhizobia already present in the soil influences the competitiveness of the commercial strains, particularly for white clover.
The change in soil population observed in this experiment was due to a decline in numbers as the soil moisture decreased and temperature increased over the summer and an increase in numbers as the soil moisture increased and temperature declined in the winter Figure 2.4 and 2.5. The MPN experiment showed changes in soil rhizobia numbers between seasons with the lowest numbers of rhizobia (2.0 x 102 - 4.5 x 103 cells g-1 soil) recorded in summer and the highest numbers of
rhizobia (4.9 - 5.2 x 104 cells g-1 soil) recorded in winter at each site. When repeated in the second
year at the Lincoln University site the same trend was found. The regression analysis showed an interaction between soil moisture and soil temperature in relation to MPN. With an increase in soil moisture there was an increase in the number of rhizobia in the soil but this was also dependent on the soil temperature at any given soil moisture level, i.e at a high soil moisture, MPN decreased with increasing temperature. Brockwell (1963) found similar results over a two year period. They found that R. leguminosarum populations were highest in winter and spring, and lowest in summer and early autumn. Slattery and Coventry (1993) found that in unlimed soil, soil R. leguminosarum populations fluctuated with season and smallest numbers were found in the summer-autumn months and Chatel et al. (1968) also found differences in soil populations between growing seasons. They found that rhizobia populations in the soil were lowest in the autumn at 1 x 102 to <
80 10 g-1 soil. Although these studies have looked at MPN over seasons, they have not linked soil
moisture and soil temperature to rhizobia numbers or the interaction between these two factors.
An interaction between soil moisture and soil temperature was observed. This is the first time both soil moisture and soil temperature have been linked to MPN. The current study found that rhizobia numbers were lower at higher temperatures but this was dependent on soil moisture and not influenced by soil temperature alone. For example, at low soil moisture decreasing the soil temperature will not increase MPN. Bowen and Kennedy (1959) suggest that rhizobia survival is affected by temperature and is lower at higher temperatures. However, their study did not include soil moisture as a factor. In the current study, soil moisture was also shown to have a significant effect on MPN. As soil moisture increased, MPN also increased. In contrast, rhizobia survival has been shown to be higher in soils with lower soil moisture (Section 1.4.1.1).
2.4.3
Experiment 2.3: Symbiotic potential
2.4.3.1
White clover
Inoculant strains affected shoot and root dry weight. However, all strains, naturalised and commercial, produced more dry matter compared with the uninoculated control. These results show that these strains, including R. leguminosarum strain TA1, are not limited by their ability to support dry matter production. These strains also all had pink nodules suggesting that they all effectively fixed nitrogen. R. leguminosarum strain TA1 appears to be similar in terms of dry matter production and N fixation, but unable to compete for nodule occupancy (Section 2.4.1.1). This again shows the importance of understanding rhizobia in their free living state in the soil. This could be the key to understanding what determines a strains’ competitive ability and persistence and this could be a critical element required to improve inoculant success (Chapter 3).
2.4.3.2
Lucerne
It is possible that the host plants in these studies that distinguished between effective and ineffective strains and selected for effective ones. This is a process known as sanctioning (Kiers and Denison, 2008). Some genotypes found frequently in the nodules, such as S. meliloti strains RRI128 and LU1M, were shown to support increased dry matter production compared with other genotypes. These genotypes could be more frequent because the plant placed a selection pressure on the symbiotic partner, preferentially partnering with S. meliloti and Rhizobium sp. genotypes that form effective nodules and benefit plant growth. Friesen (2012) also found a positive correlation (r=0.31) between rhizobium nodulation competiveness and plant performance across 19 studies. Robinson (1969) also found that when T. subterraneum was inoculated with both
81 effective and ineffective strains of R. leguminosarum bv. trifolii the effective strain formed a much greater proportion of nodules. Yates et al. (2005) found that R. leguminosarum strain WSM1325 was a superior inoculant and was highly persistent and competitive for the effective symbiosis with
T. purpureum and T. repens even though the soil contained ineffective strains of R. leguminosarum.
Although there appears to be evidence of sanctioning, this process appears to be strain dependent. In contrast to S. meliloti strains RRI128 and LU1M, plants inoculated with dominant Rhizobium sp., genotype ADA, did not produce more (P<0.001) shoot and root dry matter compared with the uninoculated control despite these plants having the highest number of pink nodules, with an average of 35 per plant. There are many other studies that show no evidence of sanctioning. In an experiment with an effective strain of S. meliloti and their ineffective mutants, Amarger (1981) also demonstrated that competitive success in forming nodules was not influenced by the level of effectiveness of the strain.