Bulk soil and maize rhizosphere resistance genes, mobile genetic elements and microbial communities are differently impacted by organic and inorganic fertilization

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Organic soil fertilizers, such as livestock manure and biogas digestate, frequently contain bacteria carrying resistance genes (RGs) to antimicrobial substances and mobile genetic elements (MGEs). The effects of different fertilizers (inorganic, manure, digestate) on RG and MGE abundance and microbial community composition were investigated in a field plot experiment. The relative abundances of RGs [sul1, sul2, tet(A), tet(M), tet(Q), tet(W), qacEΔ1/qacE] and MGEs [intI1, intI2, IncP-1, IncP-1ε and LowGC plasmids] in total community (TC)-DNA from organic fertilizers, bulk soil and maize rhizosphere were quantified by qPCR before/after fertilization and prior to maize harvest. Microbial communities were analyzed via Illumina sequencing of 16S rRNA gene fragments amplified from TC-DNA. Compared to inorganic fertilization, manure treatments increased relative abundances of all RGs analyzed, integrons and few genera affiliated to Bacteroidetes and Firmicutes in bulk soil, while digestate increased sul2, tet(W) and intI2. At harvest, treatment effects vanished in bulk soil. However, organic fertilizer effects were still detectable in the rhizosphere for RGs [manure: intI1, sul1; digestate: tet(W)] and Clostridium related sequences (digestates) with increased relative abundance. Our data indicated transient organic fertilizer effects on RGs, MGEs and microbial community composition in bulk soil with long-term history of digestate or manure application.

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