Anaerobic digestate influences the carbon distribution in soil organic matter pools after six months from its application

The decline of soil organic matter (SOM) and micro and macronutrients is among the most serious threats that many agricultural soils in the world are facing. In many countries, soil amendments from a wide range of organic wastes are increasingly used as a win-win strategy to mitigate global warming while enhancing soil fertility and, in turn, food security. Digestate, a by-product of the anaerobic digestion, is often used as a fertilizer and an amendment. However, an in-depth understanding of its impacts on native SOM and organic carbon (OC) stabilization mechanisms is still needed. The aim of this work was to investigate the influence of solid vs. liquid digestates on both SOM accrual and distribution in particulate organic matter (POM) and mineral-associated organic matter (MAOM) pools, after 0, 3 and 6 months from their application. None of the amendments had significant effects on soil OC and total nitrogen concentrations and stocks. However, after 6 months, a clear difference in OC distribution between SOM pools was observed as a function of the digestate phase applied to soil. In particular, the MAOM to POM ratio decreased in the soil amended with solid digestate with respect to the unamended control soil, while this ratio did not differ following liquid digestate amendment. Thermal analysis suggested that the interaction between SOM and mineral surfaces, rather than the biochemical recalcitrance, is the major SOM stabilization mechanism in both unamended and amended soils. This study highlighted that a more in-depth evaluation of OC accrual through digestate application, especially in the short term, can be achieved by investigating functionally defined fractions rather than bulk SOM.