Soil fertility drives plant life down-under

Rachel Standish discusses her recent article: Mycorrhizal symbiosis and phosphorus supply determine interactions among plants with contrasting nutrient-acquisition strategies. Find out more about the importance of below-ground mechanisms for understanding factors determining community structure.

The south-west region of Western Australia is a drawcard for plant nerds. Geographic isolation, a stable climate, and ‘quiet’ flat landscape (i.e., no glaciers or volcanoes) have resulted in the evolution of a remarkable and unusual diversity of plant life. Adaptations are evident above-ground – drought-resistant leaves, woody fruits that open after fire, and carnivory, to name a few, are matched by equally wonderful adaptions below-ground.

Cluster roots of West Australian native shrub Hakea ceratophylla (Proteaceae). Photo by Michael W. Shane.

Such diversity offers ecologists the opportunity to understand mechanisms of species co-existence. While drought, fire and herbivory have likely played key roles, soil fertility has been a critical factor too (Cowling et al. 1996). Competing for limited soil nutrients, and especially phosphorus (P), has resulted in plants adopting one of two basic strategies: scavenging or mining (Lambers et al. 2008). Scavengers include mycorrhizal plants using their fungal symbionts to explore a greater volume of soil than is possible with plant roots alone, whereas miners include cluster-root forming plants that release root exudates to mobilize P. Both jostle for space in the jarrah forest that grows on the lateritic nutrient-poor soils of the Darling Scarp in south-western Australia.

Jarrah (Eucalyptus marginata, Myrtaceae) forest, endemic to south-western Australia. Grasstrees (Xanthorrhoea preissii, Xanthorrhoeaceae) in the foreground. Photo by Rachel J. Standish.

We were introduced to cluster roots by Professor Hans Lambers and the late Dr Michael Shane—their curiosity was infectious! Our research on mycorrhizal plants had suggested a diminishing benefit of mycorrhizal associations with increasing P supply. We wondered how plants with these different nutrient-acquisition strategies would interact with variation in P supply. We were motivated by curiosity to test species coexistence theory.

We selected five plant species endemic to the jarrah forest, and tested the effects of mycorrhizal inoculation and P supply on their interactions in a microcosm experiment. Overall, we found evidence to suggest these species can facilitate each other at very low levels of P but compete when P is abundant. For example, our findings suggested that Hakea undulata might be facilitating Bossieae aquifolium by mobilising P in their shared rhizosphere (Muler et al. 2014).

Microcosm experiment. Photo by Rachel J. Standish.

Like many experiments, our findings have prompted more questions than provided answers. Here we have shown that the nature of interactions among plant species depended on nutrient availability, plasticity in plant responses, and soil fungi. Other factors we did not test are likely to be important too. In an era of climate change, the effects of increasing temperatures and CO2 could have drastic impacts on plant interactions and other soil microorganisms such as plant pathogens. We encourage future research to include these factors given global concern about their impacts.

Rachel Standish Murdoch University, Australia

Read the full article online: Mycorrhizal symbiosis and phosphorus supply determine interactions among plants with contrasting nutrient-acquisition strategies

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