Molecular fate of root and shoot litter on incorporation and decomposition in earthworm casts

Roots and shoots have contrasting molecular composition and decomposition rate in soils and the mechanisms regulating these rates are still debated. Earthworms have a strong influence on litter decomposition and their feces (casts) are complex and dynamic organo-mineral structures. This study aimed at monitoring the incorporation and decomposition of root and shoot litter in earthworm casts at the molecular scale. Casts were collected from a mesocosm experiment, 2, 4, 8, 24 and 54 weeks after the addition of root or shoot litter to the soil surface. Molecular characterization was achieved for litter and casts using pyrolysis gas chromatography-mass spectrometry after in situ tetramethylammonium hydroxide derivatization (TMAH-Py-GC-MS). To maximize understanding of the processes occurring in casts, the study used principal components analysis and specific molecular dynamics monitoring. The fate of root litter in casts was reflected principally in aromatic compound dynamics, while aliphatic compound evolution enabled monitoring of shoot litter fate in casts. Earthworms rapidly incorporated both roots and shoots into casts, resulting in a phase dominated by litter incorporation up to two months of the experiment. After two months, the decomposition of shoot and root (to a lesser extent) litter incorporated into casts was the dominant process. Two pools of lignin-derived compounds were identified, one originating from undecomposed litter (mainly non-oxidized) and one resulting from litter decomposition (more oxidized). Moreover, the significant contribution of aliphatic compounds, such as omega-hydroxyalkanoic and n-alkanedioic acids, by the end of the experiment and in control samples suggested that compounds with a greater degree of oxidation could be preferentially preserved in earthworms casts on a longer term. (C) 2016 Elsevier Ltd. All rights reserved.