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

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 of Dactylis glomerata L. and Lolium perenne L. monocultures. 13 C-labelled root litter from these monoliths was obtained from a first stand of each crop, cultivated under very δ 13 C-depleted atmospheric CO 2 (S1). In a factorial design, 13 C-labelled root litter of each species was submitted to a second, non 13 C-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, but Dactylis roots showed a greater C:N ratio, a lower mean root diameter and a greater specific root length compared to Lolium. 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 and Lolium 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.

The Effect of Re-Planting Trees on Soil Microbial Communities in a Wildfire-Induced Subalpine Grassland

“…Ingham et al [65] stated that grasslands more strongly dominated by bacteria than a coniferous forest, which supports our results. In grasslands, fast-growing plant species, especially those with highly branched fine root systems, supply large quantities of exudates [66], which are favored by bacteria.…”

Long-term N fertilization and conservation tillage practices conserve surface but not profile SOC stocks under semi-arid irrigated corn

“…Residues with a greater N content transition more quickly into labile particulate organic matter (POM) fraction, which is comprised of partially decomposed plant material and fungal hyphae (Stewart et al, 2016). However, under nutrient stress, plants increase root exudation stimulating SOM decomposition (Personeni and Loiseau, 2004;Shahzad et al, 2015) for nutrient acquisition (i.e., Craine et al, 2007). Decomposition of SOC increases with greater inherent soil N status .…”

Long-term steady state <sup>13</sup>C labelling to investigate soil carbon turnover in grasslands

“…(1−e −k .t ) (abbreviations and coefficients of regression lines see Table 1). Loiseau (2004) reported more depleted δ 13 C values for particulate organic matter fractions (POM) of Lolium perenne compared to Dactylis glomerata. Possible reasons for these differences in isotopic signatures in grasslands exposed to contrasted disturbance levels are: i) contrasted decomposition rates of distinct biochemical components of plant litter (Melillo et al, 1989;Agren et al, 1996) and ii) the contribution and δ 13 C values of different particle sizes to soil OM (Balesdent et al, 1988;Personeni and Loiseau, 2004).…”

“…Loiseau and Soussana, 1999;Personeni et al, 2004;Trueman and Gonzales-Meler, 2005;Dijkstra et al, 2004). However, this method cannot be used to investigate C cycling under ambient CO 2 .…”