Soil fauna precipitate the convergence of organic matter quality during decomposition

Plant litter constitutes the dominant resource to soil food webs, which gradually decompose litter and transform it into soil organic matter. A central paradigm of this transformation posits that differences in quality between distinct litter types disappear during decomposition, as litter types converge towards similar physicochemical characteristics. Yet, this paradigm is debated and not based on clear metrics. It is also largely derived from microbial decomposition studies, while the effect of litter-feeding soil animals, by transforming large quantities of litter into faeces, remains poorly documented. We addressed this knowledge gap by quantifying the variability in physicochemical characteristics amongst leaf litter of six tree species of contrasting quality, before and after conversion into faeces by six soil animal species. We found that litter conversion into faeces by diverse soil animals largely reduced the variability in physical and chemical characteristics between contrasting litter types. We also evaluated the consequences of this animal-driven convergence on further microbial-driven convergence during decomposition, by decomposing intact litter and soil animal faeces and comparing the chemical characteristics of decomposed materials. Chemical variability amongst uningested litter and amongst soil animal faeces converged at similar rates. This indicates that animal- and microbial-driven convergence are additive, and that soil animals precipitate organic matter quality convergence during decomposition. We propose here a new framework and an associated metric to study changes in organic matter quality variability during decomposition, which we argue are essential for an improved understanding and modelling of litter decomposition and soil organic matter formation.