Impacts of Soil Origin and Fire on Arbuscular Mycorrhizal Fungi Associated With Hawkweeds and Tall Tussock
New Zealand’s tussock grasslands have faced ongoing plant invasions of the hawkweeds Pilosella officinarum and Hieracium lepidulum. Plant invasions may be facilitated through plant-soil feedbacks, whereby a plants growth can be improved through feedback from the soil, including soil mutualists. Both hawkweed species readily associate with arbuscular mycorrhizal (AM) fungi, which form mutualisms with plant roots and are known to alter the success of invasive plants. Fire activity is also predicted to increase in many areas where hawkweeds are abundant or projected to spread. Fire is believed to have effects on plant and soil communities but also soil carbon through combustion. There is little information on the AM fungal communities and their interactions with native and invasive plants in New Zealand’s tussock grasslands. This study aimed to determine the impact of soil origin and simulated fire on AM fungal communities, invasive hawkweeds and soil carbon levels in tussock grasslands. The study had three main objectives: to assess how AM communities are structured in tussock grassland soil and whether they differ by soil origin (plant species) or simulated fire; to assess the roles of plant-soil feedbacks on hawkweed growth under differing soil origins and simulated fire; and to assess the effects of soil origin and simulated fire on soil carbon levels in tussock grasslands. I collected soil under P. officinarum, H. lepidulum, Chionochloa macra plants in the field, or “soil origin” treatment. I then subjected the soils to heat treatments to simulate fire: at 30°C, 45°C and 60°C. I used high throughput sequencing (18S, Illumina MiSeq) to identify AM fungal taxa by origin and heat in the soil. I conducted a growth chamber study to measure the plant growth responses of hawkweeds under differing soil treatments: soil origin and heat. Soil carbon levels were measured by C:N analysis. 2 My results showed that soil origin was the major driver of AM fungal community structure in terms of composition. The effect of heat on AM fungal richness differed according to soil origin and, surprisingly, had no impact on community composition. In the growth chamber study, hawkweed biomass was influenced by soil origin but not heat. Interestingly, both hawkweed species had the highest plant biomass in P. officinarum soils. Although soil carbon was slightly elevated in C. macra soils compared to the soils of invasive hawkweeds, soil carbon levels did not differ statistically between different soil origins and heat treatments. The results support the idea that soil origin is a major driver of plant-soil relationships in New Zealand’s tussock grasslands. Further, it suggests that P. officinarum invasion may facilitate invasion by H. lepidulum. My results indicated that fire has less of an effect on AM communities, hawkweed growth and soil carbon than soil origin in tussock grasslands.