Phosphorus acquisition mechanisms of major crop plants under elevated carbon dioxide

Submission note: A thesis submitted in total fulfilment of the requirements for the degree of Doctor of Philosophy to the Department of Agricultural Sciences, School of Life Science, Faculty of Science, Technology and Engineering, La Trobe University, Bundoora.

Phosphorus (P) is an essential macro-nutrient for plant growth. With atmospheric CO2 concentrations increasing, the demand for P is expected to increase. However, in many soils the availability of P to plants is low, which may limit the positive response of plant growth to elevated CO2 (eCO2). Understanding the mechanisms of plant P acquisition under eCO2 is important for developing P management strategies to cope with global climate change. In this thesis, firstly, an experiment using a Free-Air CO2 Enrichment facility was conducted to investigate P uptake of chickpea and field pea in response to eCO2 coupled with P application. Increasing P supply increased plant growth and total P uptake, with the increase being greater under eCO2 than under ambient CO2 (aCO2). Total P uptake was correlated with root length. The increase in P uptake under eCO2 resulted from the increased biomass production, rather than from changes in specific root-absorbing capability. The second experiment tested how eCO2 affected P transformation in the rhizosphere of plants grown in soils of varying P status. Elevated CO2 enhanced the accumulation of organic P (Po) in the rhizosphere of wheat and chickpea grown in soils with high P status. The two crop species did not exhibit different eCO2-triggered capabilities to access any P pools. The third experiment was conducted then to elucidate the mechanisms behind the increased Po under eCO2. With 13C labelling, the plant-originated C flow in the plant-soil continuum was quantified. Elevated CO2 increased 13C enrichment in the rhizosphere and the copy number of rDNA from 13C-DNA fraction. Furthermore, the microbial biomass C and respiration in the rhizosphere exhibited the same increase trend with Po under eCO2. The results suggest that eCO2 increases root exudation which in turn stimulates microbial population and activity, and immobilization of P in the rhizosphere. A further experiment examined plant’s ability to utilise P from sparingly soluble sources under eCO2. Among the sparingly soluble P sources, hydroxyapatite resulted in the maximum biomass and total P uptake in wheat and chickpea when urea was applied as the nitrogen source. Elevated CO2 did not influence P concentration in plants, rhizosphere pH or Olsen P. Elevated CO2 did not specifically affect plant access to P from sparingly soluble P sources. The last experiment investigated the effect of P application on field pea response to drought stress in eCO2 environments in the FACE system. Phosphorus application and eCO2 interactively xii enhanced periodic drought tolerance in field pea. This was achieved by decreased stomatal conductance, deeper rooting and high Pi availability for synthesis of assimilates in leaves. In conclusion, eCO2 increased P demand in crops but did not affect specific P-uptake capability of roots or the utilization of P sources. Elevated CO2 stimulated microbial activity which in turn immobilised P in the rhizosphere. Drought tolerance of field pea could be improved under eCO2 in combination with P application. The microbe-related mechanisms of P transformation in the plant-soil-microbe continuum could be focused in further research.