Stable C and N isotope variation during anaerobic digestate composting and in the compost-amended soil-plant system

Although the use of composts derived from anaerobic digestates as soil amendments is likely to increase in the future, there is little information concerning the fate of their C and N compounds after their incorporation into soil. This work assesses C and N concentrations and the associated changes in δ¹⁵N and δ¹³C during the composting processes of cattle and pig slurry anaerobic digestates. In addition, the compost effect on C and N fractions and plant uptake were studied during a six-month pot experiment with rosemary plants. The results did not show δ¹³C and δ¹⁵N isotopic discrimination during composting, indicating a previous stabilization of cattle manure and pig slurry during the anaerobic digestion. This fact was also confirmed by the low C losses during the composting processes (1.2-fold and 1.05-fold for the composting piles with cattle and pig slurry anaerobic digestates, respectively). After soil addition, the composts augmented N values (from 0.41 g kg^-1 to around 0.56 g kg^-1 in low dose and 0.68 g kg^-1 in high dose compost amended soils) and δ¹⁵N soil values (increases in the range of 50%-156%), but showed only slight differences in C and δ¹³C values compared to unfertilised control and inorganic fertilized soils. Moreover, the rosemary leaves of the plants grown on the compost amended soils presented higher N and δ¹⁵N abundance than control and inorganic fertilized plants. We conclude that δ¹⁵N abundance of anaerobic digestate composts is useful to discern its N uptake and could thus be a useful tool to detect whether organic or mineral fertiliser types were used for agricultural production.