A safe and functional application of digestate in agriculture requires an in-depth scientific knowledge of its nature in order to speculate about possible impacts on the soil-plant system, as well as on the surrounding environment. As all soil amendments, digestate should consist of stabilized organic matter (OM) in order to avoid phenomena including phytotoxicity, greenhouse gases and odorous emissions, and nutrient losses. To date, the stability of digestate has been assessed using several (often state-of-the-art) techniques and approaches, although most of the studies have been generally carried out on the final product of the anaerobic digestion (AD) and/or in batch experiments. Here, we collected digestate samples from the two primary digesters, the post-digester and the storage tank of a real, full-scale AD plant processing maize silage. Digestates were characterized from the chemical, spectroscopic and thermal points of view in order to assess the evolution of OM at plant scale. Spectroscopic and thermogravimetric data clearly show that changes in OM features mainly occurred in the two primary digesters, resulting in a more recalcitrant (i.e., richer in ligno-cellulosic compounds) and thermally stable OM. On the opposite, no relevant differences were observed in the post-digester and in the storage tank. Moreover, Fourier transform Infrared (FTIR) and fluorescence spectroscopies underlined how, at plant scale, the OM degradation may evolve differently in the two primary digesters, thus suggesting that stochastic microbial community assembly are potentially able to affect the AD process; this phenomenon resulted in digestates having different proportion of aromatic vs. polysaccharide compounds. In conclusion, this study demonstrated that FTIR, molecular fluorescence and thermogravimetric analysis could be used as simple and potentially routine techniques to monitor the digestate during the whole AD process, and that, in full-scale plants like that under investigation, such a monitoring could be limited to the primary digesters.
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