The vesicular-arbuscular mycorrhizal symbiosis (VAM) is important for sustaining the pasture ecosystem by enhancing uptake of phosphorus (P) in plants. VAM infection generally decreases with increasing P levels (3,5) but few field studies have investigated long-term exposure of VAM fungi to high P levels, where selection for fungi able to colonise roots of plants already provided with high levels of P may occur (2). This study investigated VAM symbioses after 43 years of a wide range of superphosphate applications. Australian data on VAMs are generally lacking in perennial, high-rainfall pastures.

Methods

Eight grazed sites from a 43 year-old fertiliser experiment based on superphosphate applications of 0, 63, 125 and 250 kg/ha per year, were sampled in spring and autumn. The soil was basaltic colluvium overlying lateritized tertiary sediments. Bray P levels in the treatments ranged from 3 to 63 ppm. Native grasses were prominent in the unfertilised pastures while the sown species,Phalaris aquatics, Lohum perenne, Trifolium repens and T. subterraneum dominated the fertilised sites. Infection (%) of VAM fungi inT. repens roots(1) and soil spore counts(4) were determined.

Results and discussion

A significant decrease (P<0.05) in % infection of VAM fungus occurred in roots with increasing superphosphate rate, notably at the highest level and this is in general agreement with other field studies (3,5). No seasonal effects were observed. Spore numbers were significantly higher in the fertilised sites (P<0.001) with the highest fertiliser treatment having highest spore numbers. Higher spore numbers occurred in spring (P<0.01). Relationships between spore numbers and % infection are not always clear (2). Correlations between spore numbers and soil P levels in the present study are positive although Porter et al. (5) found highest numbers in soils with intermediate P levels. VAM presence in these sites is high by world standards but this may not be generally so, particularly in degraded soils.

Table 1. Infection level in white clover roots and spore counts from soil in spring and autumn. Significant differences (1.s.d. P=0.05) between fertiliser treatments are indicated by dissimilar letters.

Superphosphate (kg/ha per year)

References

3. Abbott, L.K. and Robson, A.D. 1981. Aust. J. Agric. Res. 32, 621-630.

4. Abbott, L.K. and Robson, A.D. 1991. Agric. Ecosyst. and Environ. 35, 121-150.

5. Hayman, D.S., Johnson, A.A. and Ruddlesdin, I. 1975. Plant and Soil 43, 489-495.

6. Land, S., Dauck, H. and von Alten, H. 1989. Agric. Ecosyst. and Envir. 29, 217-224.

7. Porter,W.M., Abbott, L.K. and Robson, A. 1978. Aust. J. Exp. Agric. Anim. Husb.18, 573-578.