Marie-Anne de Graaff

Variation in Root Architecture Among Switchgrass Cultivars Impacts Root Decomposition Rates

Marie-Anne de Graaff et al. — Fri, 22 Feb 2013 14:30:25 PST

Roots regulate soil carbon (C) input, but fine root decomposition rates and root impacts on soil organic C turnover (SOC) are uncertain. This uncertainty is, partly, caused by the heterogeneity of root systems, which vary in diameter distributions and tissue chemistry. Here, we evaluated how root diameter distributions affect root and SOC decomposition. Roots from eight Panicum virgatum (switchgrass) cultivars were analyzed for root diameter size-class distribution and C:N ratio. Roots from each cultivar were mixed with C₃ soil according to five root diameter treatments: (1) 0–0.5 mm, (2) 0.5–1 mm, (3) 1–2.5 mm, (4) a 1:1:1 mixture of roots from each diameter size class, and (5) a mixture combining diameter classes in proportions representing measured size distributions for each cultivar. All treatments were incubated for 90 days under laboratory conditions. Respired CO₂ was measured throughout and the microbial community structure was measured at termination of the experiment. Carbon-13 isotope techniques were used to partition respiration into root-derived C versus native SOC-derived C. Results indicated: (1) specific root length differed among the cultivars, (2) root decomposition rates within the three size classes varied by cultivar, but were not correlated with cultivar differences in root C:N ratios, (3) root diameter size class affected root and SOC decomposition, and (4) mixing roots of different diameters did not lead to synergistic increases in decomposition. We conclude that intraspecific variation in root architecture is significant and that fine root diameter size class distribution is an important trait for shaping decomposition processes.

Elevated CO₂ and Plant Species Diversity Interact to Slow Root Decomposition

Marie-Anne de Graaff et al. — Fri, 22 Feb 2013 14:30:22 PST

Changes in plant species diversity can result in synergistic increases in decomposition rates, while elevated atmospheric CO₂ can slow the decomposition rates; yet it remains unclear how diversity and changes in atmospheric CO₂ may interact to alter root decomposition. To investigate how elevated CO₂ interacts with changes in root-litter diversity to alter decomposition rates, we conducted a 120-day laboratory incubation. Roots from three species (Trifolium repens, Lespedeza cuneata, and Festuca pratense) grown under ambient or elevated CO₂ were incubated individually or in combination in soils that were exposed to ambient or elevated CO₂ for five years. Our experiment resulted in two main findings: (1) Roots from T. repens and L. cuneata, both nitrogen (N) fixers, grown under elevated CO₂ treatments had significantly slower decomposition rates than similar roots grown under ambient CO₂ treatments; but the decomposition rate of F. pratense roots (a non-N-fixing species) was similar regardless of CO₂ treatment. (2) Roots of the three species grown under ambient CO₂ and decomposed in combination with each other had faster decomposition rates than when they were decomposed as single species. However, roots of the three species grown under elevated CO₂ had similar decomposition rates when they were incubated alone or in combination with other species. These data suggest that if elevated CO₂ reduces the root decomposition rate of even a few species in the community, it may slow root decomposition of the entire plant community.

Labile Soil Carbon Inputs Mediate the Soil Microbial Community Composition and Plant Residue Decomposition Rates

Marie-Anne de Graaff et al. — Fri, 07 Jan 2011 13:17:28 PST

• Root carbon (C) inputs may regulate decomposition rates in soil, and in this study we ask: how do labile C inputs regulate decomposition of plant residues, and soil microbial communities? • In a 14 d laboratory incubation, we added C compounds often found in root exudates in seven different concentrations (0, 0.7, 1.4, 3.6, 7.2, 14.4 and 21.7 mg C g soil) to soils amended with and without ¹³C-labeled plant residue. We measured CO₂ respiration and shifts in relative fungal and bacterial rRNA gene copy numbers using quantitative polymerase chain reaction (qPCR). • Increased labile C input enhanced total C respiration, but only addition of C at low concentrations (0.7 mg C g^-1) stimulated plant residue decomposition (+2%). Intermediate concentrations (1.4, 3.6 mg C g^-1) had no impact on plant residue decomposition, while greater concentrations of C (> 7.2 mg C g^-1) reduced decomposition -50%). Concurrently, high exudate concentrations (> 3.6 mg C g^-1) increased fungal and bacterial gene copy numbers, whereas low exudate concentrations (< 3.6 mg C g^-1) increased metabolic activity rather than gene copy numbers. • These results underscore that labile soil C inputs can regulate decomposition of more recalcitrant soil C by controlling the activity and relative abundance of fungi and bacteria.

Interactions Among Elevated CO2, Root Litter Diversity and Decomposition

M. de Graaff et al. — Tue, 03 Aug 2010 10:51:20 PDT

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.

At the Root of Soil Nutrient Cycling

Marie-Anne de Graaff — Tue, 03 Aug 2010 10:47:04 PDT

Linking Root Exudation to Microbial Community Dynamics

Marie-Anne de Graaff — Tue, 03 Aug 2010 10:45:38 PDT