Early vesicular-arbuscular mycorrhizal colonisation in soil collected from an annual clover-based pasture in a Mediterranean environment: soil temperature and the timing of autumn rains

The results of 2 experiments investigating the early stages of the formation of vesicular- arbuscular (VA) mycorrhizas in response to both soil temperature and the timing of autumn rains are reported for a Mediterranean environment in the south-west of Western Australia. In Expt 1, treatments including an early break, a late break, and a false break followed by a late break were applied to a mixed and sieved field soil collected dry in the summer and placed in pots in a glasshouse. In each break, pots were watered to field capacity and planted with subterranean clover (Trifolium subterraneum) or capeweed (Arctotheca calendula). In early and false breaks, both initiated on the same day in early autumn, the soil temperature was maintained at 30°C, and in the late break, initiated 50 days later in autumn, the soil temperature was maintained at 18°C. In both early and late breaks, pots were watered to field capacity for either 21 or 42 days when plant and mycorrhizal variables were assessed. In a false break, pots were watered to field capacity for 7 days after which the soil was allowed to dry and newly emerged plants died. These pots were then rewatered and replanted at the same time as pots receiving a late break, and subjected to the same soil temperature (18°C). In Expt 2 performed the following year, soil temperature was maintained at 31 or 18°C in both early and late breaks. Pots were planted with clover and watered to field capacity for 21 or 42 days, when plant and mycorrhizal variables were assessed. In Expt 1, VA mycorrhizal colonisation of both clover and capeweed was initially low in an early break compared with levels observed in a late break. Only mycorrhizas formed by Glomus spp. were observed in the early break, whereas mycorrhizas of Glomus, Acaulospora, and Scutellospora spp. and fine endophytes were observed in the late break. Colonisation was decreased by a false break, predominantly because of a decrease in formation of mycorrhizas of Glomus spp. In Expt 2, mycorrhizas of Glomus spp. predominated in warm soil in both early and late breaks and mycorrhizas of Acaulospora spp., Scutellospora spp., and fine endophytes were observed in greater abundance in cool soil in early and late breaks. These experiments indicate that soil temperature at the time of the break will have a large impact on both the overall levels of VA mycorrhizal colonisation of pasture plants and colonisation by different fungi. In addition, fungi that remain quiescent in warm soil may avoid damage in a false break.