Carbon Isotope Study of Soil Amendment with Maize Fermentation Digestate

Abstract

Maize digestate was applied to follow its sequestration in arable soil profiles with accent on changes in soil organic carbon (SOC) content and the production of dissolved organic matter (DOM). Two control sites with cultivated Cambisols (rye-grass, kohlrabi) were amended by the addition of digestate from maize silage fermentation. Liquid digestate was applied once in the spring in the quantity of 18 g C/ m2. The process of amendment was checked after 4 and 12 months by the use of natural labelling of the amendment by the difference in the carbon isotope composition of the SOC (C3 vegetation) and applied digestate (C4 maize). The amendment was evaluated from the SOC content, dissolved organic carbon (DOC) and their δ13C isotope composition by sampling across the soil profiles (0–90 cm). An increase of the carbon content by the amendment was measurable in the upper soil layers (0–20 cm) only. The deep soil layers are poor in carbon content (less than 0.2% of SOC) but rich in clay micro particles, which efficiently adsorbed incoming DOC with infiltrated precipitation. Change of the δ13C of soil carbon was measurable down to -80 cm.  Applied digestate increased the carbon content of top soil layer only. We did not observe significant migration of C4 amendment from top soil to the deeper soil layers. Digestate addition increased DOC production at both sites for the upper soil layers only. DOC released from the deep soil layers originates from the upper parts of the soil profile and not from the native carbon in the layer.