Development of fungi below ground in association with plants growing in disturbed and undisturbed soils

It is well established that young maize plants take-up more P when they are sown in soil that has been left undisturbed after the removal of the shoots of previously grown maize plants. In a growth chamber experiment we eliminated arbuscular mycorrhizal (AM) fungi by pasteurization of soil and thus prevented any such stimulation of P uptake for plants growing in undisturbed soil. Arbuscules were absent from roots growing in the pasteurized soil. Arbuscular colonization (the % root length colonized by arbuscules) of roots growing in non-pasteurized soil that had been disturbed by breaking-up and mixing by hand was 32%, compared to arbuscular colonization of 51% in the corresponding undisturbed treatment. Following soil disturbance, non-mycorrhizal fungi showed a response that was similar to that of AM fungi. Colonization of roots in the pasteurized soil by parasitic hyphae in the disturbed treatment was one-third of that in the corresponding undisturbed treatment. At the end of the experiment, the total lengths of hyphae stained with trypan-blue (TB) in the non-pasteurized bulk soil around the roots were 42.5 and 28.7 m g(-1) o.d. soil for the undisturbed and disturbed treatments, respectively; corresponding values in the pasteurized soil were almost identical, at 40.1 and 25.0 m g(-1). These results show that the extent of production of both AM and parasitic fungi inside roots, and the net production of TB-staining hyphae in the bulk soil around those roots, are all lower during the period of growth following soil disturbance, as compared to the undisturbed situation. Caution should be taken in the interpretation of the functional status of TB-stained hyphae collected from bulk soil in the root zone.