Carbon routes from decomposing plant residues and living roots into soil food webs assessed with 13C labelling

This field experiment investigated how C from fresh organic amendments and from a growing leek crop was allocated into different soil microbial and faunal groups in an arable field. A ¹³C-enriched red clover green manure was incorporated in one treatment, while the growing leek crop was pulse labelled with ¹³CO₂ in another. Incorporation of ¹³C into microbial fatty acids, micro- and macroarthropods, enchytraeids and earthworms was determined on several occasions during the growing season in order to determine whether different groups or species of microorganisms and fauna were specialised on either the decomposing green manure material or root-derived C. Compound-specific stable isotope ratio analysis showed fatty acid markers of actinomycetes and Gram-positive bacteria to be more strongly linked to C originating from the decomposing green manure material, whereas the marker for arbuscular mycorrhizal fungi was more linked to C from the growing leek crop. In contrast, several markers for Gram-negative bacteria were the most ¹³C-enriched and had incorporated more ¹³C than the other phospholipid fatty acids in both treatments, indicating a general dominance irrespective of C source. Most soil fauna seemed to derive their C directly or indirectly from the decomposing plant material, while C from the growing crop appeared to be of secondary importance in this agroecosystem.