[Show abstract][Hide abstract] ABSTRACT: Roots strongly contribute to soil organic carbon accrual, but the rate of soil carbon input via root litter decomposition is still uncertain. Root systems are built up of roots with a variety of different diameter size classes, ranging from very fine to very coarse roots. Since fine roots have low C:N ratios and coarse roots have high C:N ratios, root systems are heterogeneous in quality, spanning a range of different C:N ratios. Litter decomposition rates are generally well predicted by litter C:N ratios, thus decomposition of roots may be controlled by the relative abundance of fine versus coarse roots. With this study we asked how root architecture (i.e. the relative abundance of fine versus coarse roots) affects the decomposition of roots systems in the biofuels crop switchgrass (Panicum virgatum L.). To understand how root architecture affects root decomposition rates, we collected roots from eight switchgrass cultivars (Alamo, Kanlow, Carthage, Cave-in-Rock, Forestburg, Southlow, Sunburst, Blackwell), grown at FermiLab (IL), by taking 4.8-cm diameter soil cores from on top of the crown and directly next to the crown of individual plants. Roots were carefully excised from the cores by washing and analyzed for root diameter size class distribution using WinRhizo. Subsequently, root systems of each of the plants (4 replicates per cultivar) were separated in 'fine' (0-0.5 mm), 'medium' (0.5-1 mm) and 'coarse' roots (1-2.5 mm), dried, cut into 0.5 cm (medium and coarse roots) and 2 mm pieces (fine roots), and incubated for 90 days. For each of the cultivars we established five root-treatments: 20g of soil was amended with 0.2g of (1) fine roots, (2) medium roots, (3) coarse roots, (4) a 1:1:1 mixture of fine, medium and coarse roots, and (5) a mixture combining fine, medium and coarse roots in realistic proportions. We measured CO2 respiration at days 1, 3, 7, 15, 30, 60 and 90 during the experiment. The 13C signature of the soil was -260/00, and the 13C signature of plants was -120/00, enabling us to differentiate between root-derived C and native SOM-C respiration. We found that the relative abundance of fine, medium and coarse roots were significantly different among cultivars. Root systems of Alamo, Kanlow and Cave-in-Rock were characterized by a large abundance of coarse-, relative to fine roots, whereas Carthage, Forestburg and Blackwell had a large abundance of fine, relative to coarse roots. Fine roots had a 28% lower C:N ratio than medium and coarse roots. These differences led to different root decomposition rates. We conclude that root architecture should be taken into account when predicting root decomposition rates; enhanced understanding of the mechanisms of root decomposition will improve model predictions of C input to soil organic matter.
[Show abstract][Hide abstract] ABSTRACT: Rising atmospheric CO2 concentrations can alter litter decomposition processes directly, via changes in litter chemistry, and indirectly, via changes in plant species compositions. These interactions may be particularly important belowground where the roots of different species intermingle and are in direct contact with the soil. To tease apart how elevated [CO2] may directly and indirectly alter root decomposition, we initiated a 120 day incubation with roots from tree plant species (Trifolium repens, Lespedeza cuneata, and Festuca pratense) grown under long-term elevated [CO2], and soil that had been exposed to elevated [CO2] for 5 years. The roots were added to the soil both individually and in mix. Our experiment resulted in 3 main results: 1) Elevated CO2 significantly reduced decomposition of T. repens and L. cuneata roots, whereas decomposition of F. pratense remained unchanged; 2) mixing the roots of the three species produced under ambient CO2 significantly enhanced decomposition rates compared to the average decomposition rates of individual roots; 3) when roots produced under elevated CO2 were mixed, decomposition was not significantly enhanced. These data suggest that if elevated CO2 reduces the quality of the most labile roots in a plant species community, it may decrease overall rates of root decomposition.
[Show abstract][Hide abstract] ABSTRACT: The effect of prolonged elevated atmospheric CO2 on soil C and N cycling remains a widely debated topic. Elevated atmospheric CO2 may alter decomposition rates through changes in plant residue quality and through its impact on soil microbial activity. This study examines whether plant residues produced under elevated CO2 decompose differently than residues produced under ambient CO2. Moreover, a long-term experiment offered a unique opportunity to evaluate assumptions about C-cycling under elevated CO2 made in coupled climate-SOM models. Trifolium repens and Lolium perenne residues, produced under elevated and ambient CO2, at two levels of N fertilizer were incubated in soil for 90 days. Soils and residues used for the incubation had been exposed to ambient and elevated CO2 under Free Air Carbon Enrichment (FACE)-conditions and had received 15N-N-fertilizer for 9 years. The rate of decomposition of L. perenne and T. repens residues was unaffected by elevated atmospheric CO2 and rate of N fertilization; soil respiration and recovery of 15N in the soils was independent whether the residues were grown under ambient or elevated CO2. Any changes in litter C:N ratio due to elevated CO2, did not affect residue decomposition. If under prolonged elevated CO2 changes in soil microbial dynamics had occurred, it was not reflected in the rate of residue decomposition. Only respiration of L. perenne soil, following low N fertilization was enhanced after exposure to elevated CO2, irrespective of the addition of ambient or elevated residues. This increase in respiration was not reflected in an increase in the microbial biomass of the L. perenne soil. The contribution of old and newly sequestered C to soil respiration, as revealed by the 13C-CO2 signature, reflected the turnover times of SOM-C pools as described by multi-pool SOM models. The results appear not to confirm the assumption of a negative feedback induced in the C-cycle by elevated CO2 used in coupled climate-SOM models following an increase in elevated CO2. Moreover, this study showed no evidence for a positive feedback in the C-cycle following additional N fertilization.