Limited efficacy of a commercial microbial inoculant for improving growth and physiological performance of native plant species

Abstract

Soil microbial inoculants are increasingly being explored as means to improve soil conditions to facilitate ecological restoration. In southwestern Western Australia, highly biodiverse Banksia woodland plant communities are increasingly threatened by various factors including climate change, land development and mining. Banksia woodland restoration is necessary to conserve this plant community. The use of microbial inoculation in Banksia woodland restoration has not yet been investigated. Here, we evaluated the efficacy of a commercial microbial inoculant (GOGO Juice, Neutrog Australia Pty Ltd) for improving the performance of 10 ecologically diverse Banksia woodland plant species in a pot experiment. Plants were subjected to one of two watering regimes (well-watered and drought) in combination with microbial inoculation treatments (non-inoculated and inoculated). Plants were maintained under these two watering treatments for 10 weeks, at which point plants in all treatments were subjected to a final drought period lasting 8 weeks. Plant performance was evaluated by plant biomass and allocation, gas exchange parameters, foliar carbon and nitrogen and stable isotope (δ¹⁵N and δ¹³C) compositions. Plant xylem sap phytohormones were analysed to investigate the effect of microbial inoculation on plant phytohormone profiles and potential relationships with other observed physiological parameters. Across all investigated plant species, inoculation treatments had small effects on plant growth. Further analysis within each species revealed that inoculation treatments did not result in significant biomass gain under well-watered or drought-stressed conditions, and effects on nitrogen nutrition and photosynthesis were variable and minimal. This suggests that the selected commercial microbial inoculant had limited benefits for the tested plant species. Further investigations on the compatibility between the microorganisms (present in the inoculant) and plants, timing of inoculation, viability of the microorganisms and concentration(s) required to achieve effectiveness, under controlled conditions, and field trials are required to test the feasibility and efficacy in actual restoration environments.

Keywords: Banksia woodlands; microbial inoculation; mine site restoration; phytohormones; xylem sap.

Figure 1

Overview of the experimental timeline, with treatment applications indicated on the X-axis representing time in weeks. Y-axis indicates water content as a percentage of pot water-holding capacity in the well-watered and drought treatment groups. Note that the lines representing the water content are not observed averages but an estimated representation of the experimental conditions.

Figure 2

Representative experimental plants of (a) A. pulchella, (b) A. cygnorum, (c) A. fraseriana, (d) A. manglesii, (e) B. attenuata, (f) B. menziesii, (g) E. todtiana, (h) H. subvaginata, (i) J. floribunda and (j) K. glabrescens, subjected to well-watered and drought watering treatments and without (−) and with (+) inoculation treatments. Scale bars denote 10-cm intervals.

Figure 3

Total biomass of plants grown under well-watered and drought conditions, without (−) and with (+) inoculation treatments. Different letters indicate significant differences between treatments at P < 0.05 (from ANOVA with post hoc Tukey’s HSD test). Error bars are standard errors.

Figure 4

Photosynthesis and stomatal conductance of (a) B. attenuata, (b) B. menziesii and (c) E. todtiana grown under well-watered and drought conditions, without (−) and with (+) inoculation treatments. Measurements were taken at the end of the final drought period. Different letters indicate significant differences between treatments at P < 0.05 (from ANOVA with post hoc Tukey’s HSD test). Error bars are standard errors.

Figure 5

Correlogram for the measured plant growth parameters (leaf, stem, root and total biomass) and resource partitioning (leaf and root mass ratios), gas exchange parameters (photosynthesis and stomatal conductance), foliar chemistry, foliar stable isotope composition (δ¹³C and δ¹⁵N) and xylem sap phytohormones of B. attenuata, B. menziesii and E. todtiana, grown under well-watered conditions, with and without microbial inoculation. Circle size is proportional to the correlation coefficient. Positive correlation is indicated by blue, while negative correlation is indicated by red. Blank squares indicate that the correlation was not significant (α = 0.05)