Immediate vs short-term responses in soil bacterial community structure as induced by amendment with digestate in two contrasting agricultural soils

Use of digestate is increasing attention due to the potential benefits and it can be used as a soil conditioner. Since the impact of soil incorporation of digestate is still poorly investigated, our research aimed at following the lasting effects on soil microbial activity and bacterial community structure, assessing whether the microbial responses were primarily due to the addition of digestate-derived organic matter or to digestate-borne microorganisms, and finally enlightening the role of soil type. To these aims, two contrasting Mediterranean agricultural soils (Figure 1) were amended with solid digestate (30 t ha-1). Selected chemical and microbial soil variables were surface (0-15 cm) monitored at different time scales after the amendment: immediate (days) versus short-term (weeks) sampling. Soil DNA was amplified with tagged primers targeting the V3-V4 hyper-variable region of 16S rDNA and sequenced with Illumina MiSeq platform. The C-related variables (i.e. Corg, MBC, Rbas) showed significant time-dependent responses, although their magnitude was markedly affected by the soil type. The taxonomic assignment evidenced that after the organic amendment marked time-dependent fluctuations were rapidly stimulated in Proteobacteria (especially γ-Proteobacteria), with no difference between soil types at any time. Conversely, site specific responses were observed over the short-term in the acid soil of the olive tree site, where Actinobacteria significantly increased, and in the slightly alkaline citrus orchard soil, with Firmicutes becoming the dominating phyla. We hypothesise that the observed changes in bacterial community composition and activity were primarily due to the addition of easily decomposable organic substrates rather than to digestate-borne microorganisms, and the soil type was the key factor regulating the bacterial responses over time. Ongoing research is aimed at elucidating links between microbial diversity and environmental variables.