Phosphorus supply affects seedling growth of mycorrhizal but not cluster-root forming jarrah-forest species

Aims Fertiliser is often used to kick-start ecological restoration despite growing evidence of the potentially negative impacts on plant diversity. Jarrah (Eucalyptus marginata) forest species growing on nutrient (especially phosphorus) impoverished soils in southwestern Australia have a suite of adaptations for phosphorus (P) acquisition, including the formation of cluster roots, and associations with mycorrhizal fungi. Here we investigated how escalating P supply, along with a stoichiometric adjustment of nitrogen (N) supply, impacted the growth and nutrition of a wide range of jarrah forest seedlings. Methods In a pot experiment, we measured seedling biomass and nutritional responses of 12 jarrah forest species to a gradient of P supply in relation to N supply, and for the mycorrhizal species, inoculation with arbuscular mycorrhizal fungi. Results Three cluster-root forming species did not respond to increasing P, probably because they were reliant on seed P. Generally, mycorrhizal species showed a positive biomass response to increasing P when N was available. Mycorrhizas benefited seedling growth at low P (9 mg P added per kg of jarrah forest soil) when N was also available, and were parasitic to seedling growth at high P (243 mg P/ kg soil) without additional N. Conclusions These results highlight importance of P and N supply in determining the nature of the symbiosis between plants and mycorrhizal fungi. Since P supply has the potential to reduce plant growth, for a range of species, our results suggest careful consideration of fertiliser amounts for ecological restoration of ecosystems adapted to nutrient poor soils

Microbial inoculation to improve plant performance in mine-waste substrates: A test using pigeon pea (Cajanus cajan)

Mining activities alter soil physicochemical and biological properties that are critical for plant establishment. Revitalisation of soil biological properties via microbial inoculations can potentially be adopted to improve vegetation restoration. Here, we evaluate the feasibility of using beneficial microorganisms in the form of commercially available inoculants to enhance plant performance in a non-toxic and infertile mine-waste substrate, using pigeon pea [Cajanus cajan (L) Millsp.] as a test plant. Six treatments were established to investigate the effects of inoculants (Bradyrhizobium spp., microbial mix and uninoculated controls) and water availability (low and moderate) in a factorial design over 6 months. Plant performance was determined by physiological parameters (leaf gas exchange, leaf carbon, nitrogen and stable isotopes) and growth (height and biomass). Plant xylem sap phytohormones were measured to determine the plants' physiological status and effects of inoculation treatments. Results revealed that water had a greater effect on plant growth than inoculation treatments. Inoculation treatments, however, improved some physiological parameters. This study suggests that physical conditions such as soil moisture and nutrient availability may occlude more subtle (direct or interactive) effects of beneficial soil microbes on plant growth and plant condition. Prior knowledge on the biological and physicochemical properties of the soil to be amended, and on plant species-specific responses, would be needed to customise microbial inoculants for maximum benefits to ecological restoration, to support future adoption of this practice

Effect of plant root symbionts on performance of native woody species in competition with an invasive grass in multispecies microcosms

The majority of terrestrial plants form mutualistic associations with arbuscular mycorrhizal fungi (AMF) and rhizobia (i.e., nitrogen-fixing bacteria). Understanding these associations has important implications for ecological theory and for restoration practice. Here, we tested whether the presence of AMF and rhizobia influences the performance of native woody plants invaded by a non-native grass in experimental microcosms. We planted eight plant species (i.e., Acacia acuminata, A. microbotrya, Eucalyptus loxophleba subsp. loxophleba, E. astringens, Calothamnus quadrifidus, Callistemon phoeniceus, Hakea lissocarpha and H. prostrata) in microcosms of field-conditioned soil with and without addition of AMF and rhizobia in a fully factorial experimental design. After seedling establishment, we seeded half the microcosms with an invasive grass Bromus diandrus. We measured shoot and root biomass of native plants and Bromus, and on roots, the percentage colonization by AMF, number of rhizobia-forming nodules and number of proteaceous root clusters. We found no effect of plant root symbionts or Bromus addition on performance of myrtaceous, and as predicted, proteaceous species as they rely little or not at all on AMF and rhizobia. Soil treatments with AMF and rhizobia had a strong positive effect (i.e., larger biomass) on native legumes (A. microbotrya and A. acuminata). However, the beneficial effect of root symbionts on legumes became negative (i.e., lower biomass and less nodules) if Bromus was present, especially for one legume, i.e., A. acuminata, suggesting a disruptive effect of the invader on the mutualism. We also found a stimulating effect of Bromus on root nodule production in A. microbotrya and AMF colonization in A. acuminata which could be indicative of legumes’ increased resource acquisition requirement, i.e., for nitrogen and phosphorus, respectively, in response to the Bromus addition. We have demonstrated the importance of measuring belowground effects because the aboveground effects gave limited indication of the effects occurring belowground

Mycorrhizal symbiosis and phosphorus supply determine interactions among plants with contrasting nutrient‐acquisition strategies

Highly diverse plant communities growing on nutrient-impoverished soils are test beds for theories on species coexistence. Here, neighbouring mycorrhizal and non-mycorrhizal plants compete for limited phosphorus. The impact of below-ground interactions on community dynamics is underexplored. We used an experimental approach to investigate effects of inoculation with arbuscular mycorrhizal (AM) fungi and a phosphorus supply gradient on competitive and facilitative interactions among mixed assemblages of woody plants in microcosms. The plant species, one cluster root-forming (CR) species and four AM species, are native to jarrah forest that grows on nutrient-impoverished soils in south-western Australia. We measured plant growth in microcosms, with and without inoculation with the AM fungus Rhizophagus irregularis, and across a gradient of P supply: 0, 9, 27 and 243 mg P per kg of soil. Our data show evidence of plant–plant facilitation at low P supply and competition at high P supply. Growth of the CR species, Hakea undulata, was highest in microcosms with 0P and without AM inoculation. One AM species, Bossiaea aquifolium, also performed better at lower P levels, possibly benefitting from P mobilised by H. undulata. The other three AM species, one strongly obligates, performed better at higher P levels. Data for Acacia celastrifolia suggested it was facultatively mycotropic, and because there was no correlation between AM colonisation and the relative inoculum effect, we suggest positive effects of AM inoculation at 9P might be due to benefits other than P acquisition, such as pathogen defence. Benefit of AM inoculation diminished for three of four mycorrhizal species at the highest P level as we had predicted. The fourth species, Eucalyptus marginata (jarrah), had higher growth in microcosms that were not inoculated with AM, perhaps because the species benefits more from ectomycorrhizas. Synthesis. Our experimental data suggest spatial heterogeneity of soil P, coupled with a diversity of nutrient-acquisition strategies, and plasticity among plant–plant and plant–AM fungi interactions, contributes to plant species coexistence in the nutrient-impoverished jarrah forest. Our research highlights the importance of below-ground mechanisms for understanding factors determining community structure including a potential role of AM fungi in plant pathogen defence

Nitrogen and phosphorus fertilizer regime affect jarrah forest restoration after bauxite mining in Western Australia

Question Removal of vegetation and disturbance of the soil profile during mining reduce nutrient pools available for plants during mine site restoration. Thus, fertilizer is needed to replace the loss of nutrients available to support vegetation establishment. Yet the application of fertilizer can promote the establishment of competitively dominant species to the exclusion of others, particularly in low‐fertility ecosystems. Here, we test effects of fertilizer application rates on the establishment of jarrah forest. Location Five restored mine pits in the jarrah forest that grows on low‐fertility soils in southwest Western Australia. Methods Nitrogen (0 and 20 kg elemental N·ha−1) and phosphorus (0, 10, 20 and 40 kg elemental P·ha−1) were applied factorially to 20 m × 20 m field plots at five sites restored after bauxite mining with seeds and seedlings of jarrah forest species. Results Two and a half years after the experiment was established, there were no effects of N application rates on species richness, percentage vegetation cover or community composition of the restored jarrah forest. In contrast, P application had a number of effects on the restored jarrah forest. Species richness was highest at intermediate P application rates (10–20 kg·ha−1) and lowest at the two extreme treatments (0 and 40 kg·ha−1). Higher rates of P application resulted in a higher seedling density (2.5 vs 5.0 m−2 for 0 and 40 kg P·ha−1, respectively) and increased plant cover. Fertilizer P also significantly affected community composition, favouring ephemerals and weeds at high application rates and re‐sprouting species at lower application rates. Community composition was more similar to the restoration target (unmined jarrah forest) at 0 kg·P·ha−1 (similarity value of 36%) than at higher P application rates. Conclusions The data suggest that, in terms of maximizing species richness and percentage cover, a P application rate of between 10 kg·P·ha−1and 20 kg·P·ha−1is optimal for jarrah forest restoration after bauxite mining. Additionally, the data suggest no benefit of N application at a rate of 20 kg·N·ha−1

Microbial inoculation to improve plant performance in mine‐waste substrates: A test using pigeon pea (Cajanus cajan)

Mining activities alter soil physicochemical and biological properties that are critical for plant establishment. Revitalisation of soil biological properties via microbial inoculations can potentially be adopted to improve vegetation restoration. Here, we evaluate the feasibility of using beneficial microorganisms in the form of commercially available inoculants to enhance plant performance in a non-toxic and infertile mine-waste substrate, using pigeon pea [Cajanus cajan (L) Millsp.] as a test plant. Six treatments were established to investigate the effects of inoculants (Bradyrhizobium spp., microbial mix and uninoculated controls) and water availability (low and moderate) in a factorial design over 6 months. Plant performance was determined by physiological parameters (leaf gas exchange, leaf carbon, nitrogen and stable isotopes) and growth (height and biomass). Plant xylem sap phytohormones were measured to determine the plants' physiological status and effects of inoculation treatments. Results revealed that water had a greater effect on plant growth than inoculation treatments. Inoculation treatments, however, improved some physiological parameters. This study suggests that physical conditions such as soil moisture and nutrient availability may occlude more subtle (direct or interactive) effects of beneficial soil microbes on plant growth and plant condition. Prior knowledge on the biological and physicochemical properties of the soil to be amended, and on plant species-specific responses, would be needed to customise microbial inoculants for maximum benefits to ecological restoration, to support future adoption of this practice

Nutrient enrichment diminishes plant diversity and density, and alters long-term ecological trajectories, in a biodiverse forest restoration

Nutrient enrichment can negatively affect natural plant communities and result in the loss of species diversity and productivity. Despite this, fertiliser (especially phosphorus) is typically applied to restore highly biodiverse communities. Long-term effects of nutrient addition to restored plant communities, particularly those adapted to inherently low nutrient soils, have received little attention. We report results of a large-scale 20-year field experiment established in West Australian jarrah forest restored after bauxite mining Three P-application rates were applied (0, 80 and 120 kg ha−1) once at the beginning of the experiment, and plant communities monitored after 1, 6, 13 and 20 years. One year after the onset of restoration, native plant species richness and plant density was highest at 80 and 120 kg P ha−1. Subsequently, native species richness, plant density, and the richness and density of seeder and slow-growing resprouter species were highest without fertilisation, establishing the negative impact of P enrichment on plant community and ecosystem development in P impoverished soils. Total plant cover was similar for all P treatments across the chronosequence which, when combined with higher stem densities at zero P, suggests zero P favoured smaller, slower growing species. Applied-P initially favoured weeds and ephemerals and, while these species declined over time, other species were lost from these plots. The similarity of the restored communities to unmined reference jarrah forest increased over time and was consistently highest at in the absence of P fertiliser. Jarrah forest restoration is assumed to follow the initial floristic model of plant succession. However, we question this assumption and instead suggest that successional outcomes are contingent on P fertilisation rather than initial floristics per se. Applied P retarded recruitment of resprouter species that were present at zero P, debunking the assumption under IFM that these species do not disperse to restored areas. Consequently, based on the most comprehensive long-term study of P-fertilisation in the context of restoration of P-impoverished ecosystems yet reported, we propose that P limitation is important for the recreation of species diversity in inherently P impoverished forests. These results highlight the necessity of long-term experiments for understanding forest successional dynamics and implications for restoration practices

The Ridgefield Multiple Ecosystem Services Experiment: Can restoration of former agricultural land achieve multiple outcomes?

The ability of restoration approaches to provide valued ecosystem services needs to be assessed. The emerging carbon market provides an incentive to afforest agricultural landscapes and could potentially achieve multiple outcomes. However, planting monocultures for carbon sequestration may preclude effective delivery of other ecosystem services. Here, we describe the rationale behind the Ridgefield Multiple Ecosystem Services Experiment, a long-term investigation into trade-offs that might prevent the simultaneous provision of high levels of multiple services in the agricultural landscape of south-western Australia. We compare a monoculture treatment of a fast growing native species (Eucalyptus loxophleba) with mixtures of 2, 4 or 8 native plant species. We selected species based on different traits that we predict will relate to the provision of different levels of multiple ecosystem services. We ask: Can restoration of multiple ecosystem services be achieved along with carbon sequestration? Nested treatments of nitrogen deposition and weed removal test whether ecosystem service outcomes respond to environmental changes. We show that historical land-use at the site has the potential to affect service delivery via different soil characteristics and survival of planted species. Ridgefield tests the possibility of restoring and managing agricultural landscapes for multiple ecosystem services, providing a much needed experimental investigation of trade-offs among ecosystem functions