Soluble phenols in litter are reduced during passage through the soil macrofauna gut due to the formation of insoluble complexes with proteins: A case study with isopods and Diptera larvae

Abstract

Plant litter represents a major source of soil organic matter and hence understanding the pathway of its decomposition and stabilisation is crucial for understanding the soil carbon dynamic. Importantly, more than half of annual litter production is consumed by soil fauna globally. Here we test the hypothesis that soluble phenols can be removed from the litter during gut passage and bound on nitrogen rich compounds, which results in formation of insoluble complexes in the faeces produced by soil fauna. Two invertebrate species, Diptera larvae Bibio marci and terrestrial isopod Armadilidium vulgare, and litter of two tree species, Quercus robur and Alnus glutinosa, were used. Both species of invertebrates were fed by both litter types in full factorial manner. The litter contained significantly more free phenols than the faeces. In opposite, the content of bound phenols was significantly higher in faeces than in the litter. The loss of total nitrogen associated with removal of bound phenolic compounds from faeces was higher than that of the litter. Thus, soil fauna contributes to the stabilisation of soil organic matter by the conversion of soluble phenols to insoluble ones together with nitrogen immobilization. In conclusion, this process of stabilisation likely plays an important role in mature soils where mineral surfaces have reached carbon saturation or in soils with higher organic horizons.