Recalcitrance: An Inherent Relative Attribute of Plant Litter Describing Its Potential Decomposability

While the term “litter quality” is vague and can always be associated to a specific litter property (e.g., C:N, %lignin, decay rate), and thus should be replaced with the term that describes that property (e.g., stoichiometry, C-chemistry, recalcitrance), we defend the use of the term “recalcitrance” to describe the potential decomposability of litter, or its components. Recalcitrance can be quantified in laboratory incubations, by measuring the rate of production of CO₂ from litter, under optimal environmental conditions. “Recalcitrance” is an inherent relative attribute of litter resulting from the synergistic interactions of several specific physic-chemical properties (e.g., stoichiometry, chemistry, energetic, physical structure) of that litter which in absence of environmental constraints (i.e., microbial limitation, physical aggregation, mineral-bonding) determine its potential rate of CO₂ production. Because these environmental constraints often cannot be excluded in studies of soil organic matter (SOM), the term “recalcitrance” is not appropriate for SOM, and “persistence” is preferred. Recalcitrance is, of course, relative and requests a time scale of reference, since any litter eventually decomposes. We will illustrate this rational with examples from our latest laboratory incubations using: 1) a variety of plant litter types, 2) litter with differential isotopic enrichment of the metabolic and structural components, 3) soils containing isotopic enriched litter-derived organic matter.