| Summary: | The carbon (C) and nitrogen (N) mineralisation rates of five digestates were studied and compared with pig slurry, compost, and a solid fraction of digestate in aerobic incubation experiments. The objective was to identify the most relevant drivers of C and N mineralisation based on the physicochemical properties of the products. Net organic nitrogen mineralisation of digestates (N<sub>min,net</sub>) was on average 30%, although the range was relatively wide, with digestate from pig manure (39%) reaching double the value of digestate from sewage sludge (21%). The total carbon to total nitrogen (TC:TN) (r = −0.83, <i>p</i> < 0.05) and ammonium nitrogen to total nitrogen (NH<sub>4</sub><sup>+</sup>-N:TN) (r = 0.83, <i>p</i> < 0.05) ratios of the products were strongly correlated with N<sub>min,net</sub>, adequately mirroring the expected fertilising potential of the products. The digestates had C sequestration values between 50 and 81% of applied total organic carbon (TOC), showcasing their potential to contribute to C build-up in agricultural soils. The carbon use efficiency of the amended soils was negatively correlated with dissolved organic carbon (DOC) (r = −0.75, <i>p</i> < 0.05) suggesting that catabolic activities were promoted proportionately to the DOC present in these products. Ratios of DOC:TOC (r = −0.88, <i>p</i> < 0.01) and TC:TN (r = 0.92, <i>p</i> < 0.01) were reliable predictors of the fraction of C that would remain one year after its incorporation and thus could be used as simple quality parameters to denote the C sequestration potential of digestates prior to their use in the field.
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