Long-term phosphite application is detrimental to a low phosphorus banksia woodland community in the southwest of Western Australia

Tectonic quiescence and lack of orogenesis have lead to very nutrient impoverished soils in Southwest of Western Australia (SWWA). The most limiting macronutrient of which is phosphorus (P) both in the form that is easily accessible to plants and occluded phosphorus. To overcome this many plants have developed unique adaptations such as forming cluster-roots that increase surface area as well as exude carboxylates that help liberate the occluded P from the soils. Over millions of years SWWA flora have adapted to such low soil P levels. However, the introduced oomycete pathogen, P. cinnamomi, has been wreaking havoc on the native vegetation of SWWA and most (wetter areas) of Australia. The only viable option of mitigating the effects of this contagious pathogen is through the application of a phosphorus-based fungicide called phosphite. Phosphite, as applied, is neutralised phosphorus acid H3PO3, which is oxidised to PO43- by soil microorganisms. Phosphate is a soil fertiliser and a very important nutrient for growth and development of plants and animals. This fungicide is either aerially sprayed or applied as trunk-injections on P. cinnamomi infested plant communities to help reduce its impact. For the most part it has proven to be quite successful in suppressing the pathogen’s effects on many plant species. However, there are tradeoffs in the form of phytotoxicity by sensitive species as well as potential for fertilisation, weed invasion, changes to plant communities as well as changes to the soil P pool. In this study the effects of long-term P application as phosphite was studied in a Banksia woodland. The study site has been divided into four monitoring sites by DEC (Albany) since the mid 1990s: (i) unsprayed and P. cinnamomi-free plots; (ii) unsprayed and P. cinnamomi infested plots; (iii) phosphite sprayed and P. cinnamomi infested plots; (iv) phosphite sprayed and P. cinnamomi-free plots. The present study found that long-term phosphite application (from 1996-2010) in the study site of Gull Rock National Park region, south-western Australia (Albany), has increased soil P levels. Both plant-available and total P levels were significantly elevated (p=0.014). The total P levels in the target species Adenanthos cuneatus, Banksia attenuata, Banksia coccinea and Jacksonia spinosa mature leaves (p=0.013, p=0.013, p=0.042 and p=0.157, respectively) and senesced leaves (p=0.001, p=0.005, p=0.044, respectively (A. cuneatus did not have senesced leaves)) were also significantly higher from the unsprayed and disease-free plots except in the case of mature J. spinosa leaves. Although the soil P levels are well below critical levels, the total P levels in B. attenuata and B. coccinea leaves were significantly above phytotoxic levels. This is most probably due to long-term phosphite application and its subsequent accumulation. The study site revealed that phosphite sprayed and P. cinnamomi-free plots have formed a unique assemblage of plants but the species overlap with the unsprayed and P. cinnamomi-free “control” plots. However, the unsprayed and P. cinnamomi infested plots showed the most unique plant community due to the high destructive capacity of the pathogen that has lead to only resistant species surviving. Similarly, Shannon diversity index and Evenness index revealed that the synergistic effect of the pathogen and the fungicide have formed the most species diverse community in the phosphite sprayed and P. cinnamomi infested plots that is also very even when compared to the other three treatment plots. Canopy closure analysis was used as a primary-productivity indicator to estimate biomass production based on the canopy cover. The phosphite sprayed and P. cinnamomi-free plots showed a highly significantly (p<0.001) greater canopy closure when compared to the control treatment, unsprayed and P. cinnamomi-free plots. However, dry weight of biomass from 1m2 harvests of five plots within these two sites did not show a significant difference in biomass. Interestingly, B. attenuata had significantly (p<0.001) high photosynthesis rate in the former plots when compared to the control treatment but B. coccinea had no significant (p=0.786) difference. A glasshouse experiment was also performed using white lupins (Lupinus albus) to understand the pattern of phytotoxicity caused by massive influx of P in the form of phosphate through the application of phosphite. Seven treatments: control (untreated), 120 kg/ha, 240 kg/ha, 480 kg/ha, 720 kg/ha and 1440 kg/ha each with nine replicates were sprayed with the indicated concentrations of phosphite and incubated for five weeks in the glasshouse to allow bio-oxidation of phosphite into phosphate by soil microbes. The treatments are approximate to the numbers of years of annual phosphite application, i.e., 120 kg/ha = 5 years of 24 kg/ha annual phosphite application. This study revealed that soil total- and plant-available P levels were in direct relationship with phosphite application rates. Plant shoot growth and biomass production were severely impeded even at the lowest treatment level of 120 kg/ha. The plants had stunted growth, low leaf-, stem-, and root-mass. The plants also had very low germination rates beginning from 240 kg/ha treatments. In contrast, 120 kg/ha had the shortest germination time but similar phytotoxicity symptoms to other treatments. Of interest was the presence of cluster-root primordia rootlets even at the highest phosphite treatment. These were explained due to the ‘P-depletion zone’ phenomenon as the majority of the cluster roots were induced in the control treatment. The Mediterranean climate of SWWA makes it prone to wildfires and the plants in the region have coping mechanisms to withstand and proliferate post-fire. However, this study was designed around the idea of ‘what would happen to phosphite that is accumulating in the plants after being aerially sprayed? ’. The natural process of senescence has been attributed to the high P levels in soil in the field. However, since most of the fungicide is still in planta only small amounts of that phosphite ends up in the soil and is converted to phosphate. This was also the reason for the apparent high biomass levels observed in phosphite sprayed plots. The glasshouse trail acted as a surrogate to wildfires and simulated the impact of wildfires on the phosphite accumulating in planta. Given that phosphite oxidises to phosphate the toxic levels of phosphate influx after only the equivalent of five years of phosphite application caused widespread phytotoxicity symptoms such as root and shoot necrosis; leading to plant death in the white lupin trial. It is clear that phosphite application has benefits in the form of reducing the impact of P. cinnamomi. Long-term application of this fungicide also has the potential to transform it into a fertiliser as well as cause widespread changes to the ecosystem structure if a wildfire, intense enough to burn all the vegetation to ash, occurs.