, Volume 267, Issue 1-2, pp 129-141

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

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Abstract

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.