The short-term response of soil microbial communities to digestate application depends on the characteristics of the digestate and soil type

Anaerobic digestion of organic waste is a key process to produce renewable energy and meet the growing demand for sustainable energy. The residues of anaerobic digestion - called digestates - can be used as soil amendments to improve crop yields. However, the effect of digestates on the soil biota, especially on microorganisms, needs to be better documented before a large scale use of digestates in agriculture. In addition, how the quality and composition of the digestate may affect soil microbial communities has not been properly addressed yet. We designed a microcosm experiment under controlled experimental conditions to compare effects (42 days) of four digestates produced from varying intakes (cattle manure and/or energy crop and/or food residues and/or slurry) on soil microbial communities; a control microcosm made of undigested cattle manure was also used. Each digestate was applied on three contrasting soils representing contrasted pedo-climatic conditions (especially soil type and climate). These three soils presented different prokaryotic and fungal communities structures. The effect of digestate inputs on the soil microbial biomass and diversity was assessed using molecular DNAbased tools (quantification of extracted soil DNA and high-throughput sequencing, respectively) in comparison to the untreated cattle manure control condition. Our results show that 42 days after digestate application, significant differences of soil microbial communities were observed according to the digestate characteristics; these differences were soil-dependent. Thus, in the silty clay loam soil, no effect of digestates was observed on soil microbial biomass or diversity (P > 0.05), as compared to the undigested cattle manure. In the two other soil types (loam and sandy loam), soil microbial biomass decreased (around -40 %, P < 0.001) when digestates having a low total organic carbon content (from 0.61 to 3.3 g.100 g(-1)) were applied. None of the digestates affected the soil prokaryotic diversity whatever the soil type (P > 0.05). Digestate application resulted in higher fungal diversity (around +35 %; P < 0.001) in soils with low C/N ratio (9.14 in average). The microbial community structure of coarse-textured soil appeared more impacted by organic inputs than fine-textured soils. To conclude, our results show that different soil types, harboring distinct microbial community structures, responded differently to different digestates application. This response was also digestate-dependent.