How does the nature of living and dead roots affect the residence time of carbon in the root litter continuum?

Root litter transformation is an important determinant of the carbon cycle in grassland ecosystems. Litter quality and rhizosphere activity are species-dependent factors which depend on the attributes of the dead and living roots respectively. These factors were tested, using non-disturbed soil monoliths ofDactylis glomerata L. andLolium perenne L. monocultures.¹³C-labelled root litter from these monoliths was obtained from a first stand of each crop, cultivated under veryδ ¹³C-depleted atmospheric CO₂ (S1). In a factorial design,¹³C-labelled root litter of each species was submitted to a second, non¹³C-labelled, living stand of each species (S2). Carbon derived from S1 and from S2 was measured during an 18-month incubation in the root phytomass and in three particulate organic matter fractions (POM). The decay rate of each particle size fraction was fitted to the experimental data in a mechanistic model of litter transformation, whose outputs were mineralisation and stabilisation of the litter-C. Few differences were found between species, in the amount and biochemical composition of the initial root litter, butDactylis roots showed a greater C:N ratio, a lower mean root diameter and a greater specific root length compared toLolium. A transient accumulation of litter residues arose successively in POM fractions of decreasing particle size. The litter-continuum hypothesis was validated, i.e. that the attributes of the compartments (C:N, chemical composition and residence time) depended mainly on their particle size. The S1 species influenced the rate of litter decay while the S2 species controlled the efficiency of litter-C stabilisation versus mineralisation:Dactylis litter decomposed faster andLolium rhizosphere allowed a greater proportion of litter C stabilisation. Discussions focus on the processes responsible of species strategy in relation with the morphological root traits, and the implication of strategy diversity for rich grassland communities.