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Understanding the combined effects of soil amendments and inoculation of mycorrhizal fungi on the response of different plant species during the phytostabilization process of trace elements contaminated soils is a challenge. This task is more difficult, but more realistic, when the study is carried out under field conditions.

A research team of the IRNAS-CSIC, in collaboration with researchers from EEZ-CSIC, has assessed the combined effects of two amendment doses (high and low doses) and mycorrhizal inoculation on the response of saplings of wild olive and stone pine, planted in a contaminated field site. The amendments were a mix of sugarbeet lime and biosolid compost. The inoculation treatments were made with a commercial inoculum of arbuscular mycorrhizal fungi for wild olive, while we used ectomycorrhizal fungi for stone pine. The experimental plot was located at the Guadiamar Green Corridor (Sevilla).

Results showed a weak or null effect of the mycorrhizal inoculation on plant growth, survival and trace element accumulation. There was only a significant increase on P nutrition for stone pine, growing on non-amended conditions. In general, the positive effect of mycorrhizal colonization on P nutrition is more evident under stress conditions and it can disappear under more favourable soil conditions.

The soil amendments were very effective reducing trace elements availability and their accumulation in both plant species, especially in roots. However, the effects on plant biomass were species-dependent and contrasted; low-dose amendments (30 T ha-1) increased the biomass of wild olive by 33.3%, but reduced by 28% that of pine. On the other hand, the high doses of amendments (60 T ha-1) produced some negative effects on plant growth and nutrition, probably related to the increase of soil salinity.

Both plant species, stone pine and wild olive, have been proved to be adequate for phytostabilization of contaminated soils under Mediterranean climate, due to their tolerance to drought and metals, and the low transfer of trace elements from root to shoot, thus reducing toxicity for the food web.

As a conclusion of this study, we highlight the importance of implementing microbial-assisted phytoremediation approaches, for which a better understanding of the diversity and ecology of plant-associated microorganisms is needed. The use of indigenous mycorrhizal fungi, locally adapted and tolerant to contamination, will favour to reach the phytostabilization purposes.

This study has been published in the Science of the Total Environment:

Madejón, P., Navarro-Fernández, C.M., Madejón, E., López-García, A., Marañón, T. (2021). Plant response to mycorrhizal inoculation and amendments on a contaminated soil, Science of the Total Environment, vol. 789, art. 147943.

Interactions between plants and soil are responsible for providing numerous ecosystem services from forests, including carbon sequestration, protection from erosion, improved soil fertility and stabilization of contaminants.

In the XXV World Congress of the IUFRO (International Union of Forest Research Organizations), held at Curitiba, Brasil from September 29th to October 5th, a scientific session (number E8,d,g) was devoted to the topic “Plant-soil interactions in forests”.

1) Different tree species affect differently to soil properties. For example, soil pH underneath pines (Pinus pinea) and holm oaks (Quercus ilex) was lower than under other five tree species in the same study area. These soil pH differences have relevant consequences for the mobility of trace elements and their uptake by trees (Madejón et al., 2018, Catena 166: 34-43).

2) Soil mycobiota differed in diversity and composition under wild olive (Olea europaea), stone pine (Pinus pinea) and white poplar (Populus alba), and with adjacent treeless sites. The afforestation represented 100% increase of soil fungal diversity, compared with the remediated zones (only with pastures). A total of 6535 OTUs (operational taxonomic units) were estimated as the gamma diversity of soil fungi in the study area (Gil-Martínez et al., in preparation).

The functional diversity of soil fungi (according to the FUNGuild data base) was represented by three main functional groups: saprotrophs (68% of OTUs), pathogens/parasites (18%) and mycorrhizal fungi (12%). Within the mycorrhizal fungi group, arbuscular fungi were associated to soil samples under wild olive and treeless pastures, while ectomycorrhizal fungi were abundant in soil under poplar and pine.

3) The case of mycorrhizal fungi is relevant because of their symbiotic relations with trees. Ectomycorrhizal fungi communities were studied by sampling root tips of 40 holm oaks (Q. ilex) distributed along four sites of the Guadiamar Green Corridor. A total of 55 OTUs were identified, with average of 3.8 fungal species per tree. Soil properties influencing primarily the composition of mycorrhizal fungal communities were total soil carbon, and the concentration of Ca, Cu, Ni and Zn (López-García et al., 2018, Soil Biol Biochem 121: 202-211).

In turn, the composition of mycorrhizal fungal communities affected significantly the accumulation of P in leaves, and the transfer rate of Zn from soil to root (Gil-Martínez et al., 2018, Front Plant Sci : 1682).

4) Soil fungi have a relevant role degrading organic matter and cycling nutrients, by the action of extracellular enzymes. We have found that beta-glucosidase activity (enzyme contributing to cellulose degradation) was much higher in the soil under poplar and pines than in soil under wild olive or in pastures. A feedback process can be inferred, by which tree species influence composition and abundance of soil fungi, and the rate of enzyme activity, which in turn promote organic matter decomposition and release of nutrients, which are up-taken by trees (Gil-Martínez et al., 2018, Proceed. Mine Closure 2018, Leipzig, pp. 636-647).

In summary, there are complex interactions between the aboveground and belowground subsystems of the forest ecosystem. Several examples of tree-microbiota-soil feedback processes have been studied in the Guadiamar Green Corridor. Metal uptake by trees is influenced by soil pH, which in turn can be modified by litter decomposition and root exudates. The composition and diversity of soil fungi are affected by tree species identity; on the other hand, mycorrhizal fungal communities influence some tree processes, like leaf P concentration, and Zn transfer from soil to root. Trees influence on soil enzyme activities, which in turn affect nutrient cycling and their uptake by trees.

A pdf copy of the presentation slides can be consulted in Digital CSIC.