M. de Graaff

Boise State University, Boise, Idaho, United States

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Publications (8)25.27 Total impact

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    ABSTRACT: Soil microorganisms regulate multiple input and loss pathways of soil carbon (C); hence, changes in microbial communities are expected to affect soil organic matter (SOM) cycling and storage. Despite this, very little is known about how microbes respond to changes in soil structure and vegetation with land use and land cover change. This study aimed to identify relationships between microbial community composition and the distribution of SOM among soil aggregate fractions to answer the following research questions: (1) Are different microbial groups associated with different SOM pools? and (2) How do these relationships differ with changes in vegetation during tropical forest succession? We measured microbial composition via phospholipid fatty acid (PLFA) analysis and C and nitrogen (N) concentrations on physically separated aggregate fractions of soils from pastures, secondary forests (40 and 90 years old) naturally regrowing on abandoned pastures, and reference or primary forests in Puerto Rico. We found different microbial communities associated with different soil aggregate fractions. Fungal to bacterial ratios decreased and gram-positive to gram-negative bacterial ratios increased with decreasing physical fraction size (from the macroaggregates to the silt and clay fractions). Microbial composition also varied with land cover type and forest successional stage, with consistent trends among soil fractions. These results show that the soil matrix and soil microsite properties play an important role in the spatial distribution of fungal and bacterial-dominated communities. The similarities in land cover effects on microbial communities at different spatial scales suggest similar controls may be influencing microbial composition with potential implications for SOM storage and turnover. In addition, the majority of C and N (relative to total soil C and fraction mass) was isolated in the macroaggregate-occluded silt and clay-sized fractions, suggesting that association with mineral surfaces, and not occlusion of particulate organic matter within aggregates, is the dominant stabilization mechanism for SOM in these highly-weathered, fine-textured soils. These results highlight the importance of soil aggregation in C storage but through mechanisms different than those reported for temperate grassland soils.
    Soil Biology and Biochemistry 10/2014; 77:292-303. DOI:10.1016/j.soilbio.2014.05.030 · 4.41 Impact Factor
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    Soil Biology and Biochemistry 11/2011; DOI:10.1016/j.soilbio.2011.07.006 · 4.41 Impact Factor
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    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.
    AGU Fall Meeting Abstracts; 12/2010
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    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.
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    ABSTRACT: Soil is the largest reservoir of organic carbon (C) in the terrestrial biosphere and soil C has a relatively long mean residence time. Rising atmospheric carbon dioxide (CO(2)) concentrations generally increase plant growth and C input to soil, suggesting that soil might help mitigate atmospheric CO(2) rise and global warming. But to what extent mitigation will occur is unclear. The large size of the soil C pool not only makes it a potential buffer against rising atmospheric CO(2), but also makes it difficult to measure changes amid the existing background. Meta-analysis is one tool that can overcome the limited power of single studies. Four recent meta-analyses addressed this issue but reached somewhat different conclusions about the effect of elevated CO(2) on soil C accumulation, especially regarding the role of nitrogen (N) inputs. Here, we assess the extent of differences between these conclusions and propose a new analysis of the data. The four meta-analyses included different studies, derived different effect size estimates from common studies, used different weighting functions and metrics of effect size, and used different approaches to address nonindependence of effect sizes. Although all factors influenced the mean effect size estimates and subsequent inferences, the approach to independence had the largest influence. We recommend that meta-analysts critically assess and report choices about effect size metrics and weighting functions, and criteria for study selection and independence. Such decisions need to be justified carefully because they affect the basis for inference. Our new analysis, with a combined data set, confirms that the effect of elevated CO(2) on net soil C accumulation increases with the addition of N fertilizers. Although the effect at low N inputs was not significant, statistical power to detect biogeochemically important effect sizes at low N is limited, even with meta-analysis, suggesting the continued need for long-term experiments.
    Global Change Biology 08/2009; 15(8):2020. DOI:10.1111/j.1365-2486.2009.01866.x · 8.22 Impact Factor
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    ABSTRACT: 1365-2486
    Global Change Biology 11/2006; 12:2077. DOI:10.1111/j.1365-2486.2006.01240.x · 8.22 Impact Factor
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    12/2005: pages 373-391;
  • M. de Graaff, D. Harris, H. Blum, C. van Kessel
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    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.

Publication Stats

210 Citations
25.27 Total Impact Points

Institutions

  • 2011–2014
    • Boise State University
      • Department of Biological Sciences
      Boise, Idaho, United States
  • 2009
    • Oak Ridge National Laboratory
      • Biosciences Division
      Oak Ridge, Florida, United States
  • 2005–2006
    • University of California, Davis
      • Department of Plant Sciences
      Davis, CA, United States