Interaction between lombricompost and AMF communities: consequences on metals accumulation and bioaccessibility in urban africulture crops
Résumé
Context & objective. The quality of sites allocated for urban agriculture (UA) has become a major concern, as pollution rates have been increasing since the industrial revolution. Persistent trace metals (TM), such as cadmium (Cd) and lead (Pb) are common pollutants within urban areas, while antimony (Sb) is an emerging anthropic contaminant (Pierart et al., 2015). Two-thirds of the population leaves currently in urban areas, and this trend is increasing. The recent economic crisis drove many citizen to pursue a will of self-growing food therein (Galt et al., 2014). Food quality is therefore of concern in term of TM rate. In order to promote soil fertility and plant yield, gardeners can use biofertilizers and organic amendments (Kangwankraiphaisan et al., 2013), usually leading to a high carbon rate (Schwartz, 2013). These products can have different physico-chemical properties and reactivity in soil, affecting therefore soil microorganisms and TM geochemical cycles (Jarrah 2014). Leek (Allium porrum L.) are widely cultivated in urban gardens; but as leafy vegetable, they present an important contamination risk. Their relationship with organic matter (OM) is known (Shahid et al., 2012, 2016); but knowledge are still scattered and worse when focusing on the interaction with plant symbiotic fungi (Pierart, 2016). Hence, we focused on the effect of lombricompost on (i) arbuscular mycorrhizal fungi (AMF) communities in plant roots and (ii) on TM phytoaccumulation and human bioaccessibility (Xiong et al., 2016). Methods. Organic leeks were grown in pot up under greenhouse conditions. Two TM sources were compared: geogenic (NTE) and anthropogenic (BZC) at similar Pb concentration (~450ppm) but different Sb and Cd concentrations (~3 vs 15ppm_Sb and 2 vs 0.5ppm_Cd). Lombricompost was brought in half of the pots to increase soil % OM by 1%. Plants and soils were harvested after 5 months, and analyzed to study TM accumulation and bioaccessibility in edible parts. The latter was measured in plants using the adapted Unified Barge Method (UBM, Denys et al., 2012). AMF communities in symbiosis with plants were identified on leek fragments through Illumina MiSeq sequencing. Results. TM phytoaccumulation varied both with soil and TM type: Lombricompost did not change Sb accumulation. In BZC soil, it decreased Cd but increased Pb in plants, while no effect was observed in NTE soil. Pb accumulation was generally low compared to soil contents (450 ppm). The bioaccessibility in leek leaves was not affected by OM addition for all studied TM. Lombricompost induced important changes in root fungal community: in both soils Rhizophagus irregularis increased (+25% & 40% in BZC and NTE respectively). This change of community composition was coupled with a decrease of other Rhizophagus species and Glomeraceae species. In NTE soil, Funneliformis mosseae also disappeared with OM addition. Conclusion. When gardening in urban areas facing TM contamination, using lombricompost can lead to complex effects, probably due to thin interactions and multiple effects between ecotoxicology, plant metabolism, soil properties and element competition. TM bioaccessibility was higher for the anthropic contaminated soil compared with the geogenic soil. This difference was not observed in edible organs suggesting differences in TM species absorbed. TM phytoavailability was modified by lombricompost addition, but the human bioaccessibility was no affected, either in soil or plant. The important shift of symbiotic fungal communities needs to be carefully studied as the main impacted species are known to influence TM mobility and phytoaccumulation. Field trials will be needed, for further investigation to enhance hazard recommendations for gardeners.
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