Photo credit: J Brooks

Belowground carbon partitioning in historical FACE experiment: on-going research

In summer 2011, we returned to the site of a historical Free-Air CO2-enrichment (FACE) experiment in a sweetgum plantation. We girdled one-half of the trees in each treatment plot with goals of determining the effects of tree carbon inputs on soil carbon and nutrient cycling, and quantifying the size of belowground carbon storage pools.

Collaborators: J Childs, CT Garten, RJ Norby, JM Warren

See news coverage of our excellent student interns on a local TV station.

Photo credit: R. Norby

Carbon partitioning in trees and soils:
on-going research
Our objective was to improve the carbon partitioning routines in existing ecosystem models, and test the models using short-term, comprehensive field measurements of processes related to carbon partitioning from leaves to roots and roots to soil.

Warren JM, Iversen CM, Garten CT, Norby RJ, Childs J, Brice DJ, Evans RM, Gu L, Thornton PE, Weston DJ (2012). Timing and magnitude of carbon partitioning through a young loblolly pine (Pinus taeda L.) stand using 13C labeling and shade treatments. Tree Physiology, DOI: 10.1093/treephys/tpr129. [pdf]

Photo credit: J. Childs.

Belowground harvest: on-going research
We excavated two large soil pits by hand in each treatment ring in late June, 2009. Roots were separated into diameter classes for biomass and nutrient analyses. Subsamples of sieved soil were used in a soil incubation experiment to determine carbon and nitrogen mineralization throughout the soil profile.

Iversen CM, Keller JK, Garten CT, Norby RJ (2012). Soil carbon and nitrogen cycling and storage throughout the soil profile in a sweetgum plantation after 11 years of CO2-enrichment. Global Change Biology 18: 1684-1697. [pdf]

Plant and soil samples have been archived and are available to the scientific community upon request.

Root decomposition and carbon storage in soil organic matter: on-going research
We combined root decomposition techniques with soil fractionation techniques to quantify the transfer of carbon and nitrogen from decomposing fine-root litter to relatively long-lived SOM using the unique depleted C-13 signature of organic material in plants and soils enriched with elevated [CO2]. We hypothesized that increased root quantity and decreased root quality under elevated [CO2] would increase carbon and nitrogen storage in soil organic matter. This research was funded by a dissertation improvement grant from the National Science Foundation.

Collaborators: JD Jastrow, RJ Norby

Nitrogen cycling throughout the soil profile under elevated [CO2]
We used isotope pool dilution to measure potential gross nitrogen cycling rates throughout the soil profile. We found that nitrogen mineralization at depth in the soil, combined with increased root exploration of the soil volume under elevated [CO2], may be more important than changes in potential gross nitrogen cycling rates in sustaining forest responses to rising atmospheric CO2.

Iversen CM, Hooker TD, Classen AT, Norby RJ (2011). Net mineralization of N at deeper soil depths as a potential mechanism for sustained forest production under elevated [CO2]. Global Change Biology 17: 1130-1139. [pdf]

Digging deeper: Rooting distributions in CO2- enriched forests
Experimental evidence from a diverse set of forested ecosystems indicates that CO2-enrichment may lead to deeper rooting distributions. Altered rooting distributions are expected to affect important ecosystem processes such as root physiology and soil nutrient cycling. However, the causes of greater root production at deeper soil depths under elevated [CO2] require further investigation. Progress in understanding and modeling the interface between deeper rooting distributions and soil nutrient cycling will be critical in projecting the sustainability of forest responses to rising atmospheric [CO2].

Iversen CM (2010). Digging deeper: Fine root responses to rising atmospheric [CO2] in forested ecosystems. New Phytologist 186: 346-357. This paper was a finalist in the New Phytologist Tansley Medal competition. [pdf]

Missing links in the root-SOM continuum
Our goal was to synthesize root- and soil-centric studies into an integrated understanding of belowground ecosystem processes in an organized oral session at an annual ESA meeting. Speakers emphasized the importance of the rhizosphere and soil environment for the transformation of root-derived carbon to long-lived SOM. Integration of observations made along the root-SOM continuum can lead to a more holistic view of belowground ecology.

Iversen CM, O’Brien SL (2010). Organized Oral Session 3. Missing links in the root–soil organic matter continuum. Bulletin of the Ecological Society of America 91: 54-64. [pdf]

O ’Brien SL, Iversen CM (2009). Missing links in the root-soil organic matter continuum. New Phytologist 184: 513-516. [pdf]

Root-derived carbon and nitrogen input to the soil
We combined a long-term minirhizotron data set with continuous, root-specific measurements to assess carbon and nitrogen input from root mortality. We found that the flux of carbon and nitrogen into the soil nearly doubled under elevated [CO2] due to stimulated root production and mortality. Moreover, much of the carbon and nitrogen input occurred relatively deep in the soil profile where decomposition dynamics are likely to be different from what is commonly observed and modeled in the upper soil.

Iversen CM, Ledford J, Norby RJ (2008). CO2 enrichment increases carbon and nitrogen input from fine roots in a deciduous forest. New Phytologist: 179: 837-847. [pdf]

Nitrogen limitation controls carbon partitioning under elevated atmospheric [CO2]
In a nitrogen fertilization experiment in an adjacent sweetgum stand, we found that the increased production of ephemeral roots under elevated [CO2] was most likely a mechanism for greater nitrogen acquisition in response to nitrogen limitation within the stand. In turn, the increased production of fine roots with a relatively large nitrogen concentration necessitated increased nitrogen uptake from the soil. 

Iversen CM, Norby RJ (2008). Nitrogen limitation in a sweetgum plantation: Implications for carbon allocation and storage. Canadian Journal of Forest Research 38: 1021-1032. [pdf]

Photo credit: J. Childs

Fine-root dynamics in a constructed old-field
We combined a long-term minirhizotron data set with continuous, root-specific measurements to assess root biomass production and mortality in a constructed old-field ecosystem exposed to elevated [CO2], warming, and drought conditions. We asked whether multiple atmospheric and climatic factors would interact to influence root production and turnover throughout the soil profile.

Collaborators: J Childs, AT Classen, RJ Norby

Photo credit: S. Bridgham

Scaling plant nutrient use in peatland ecosystems
We fertilized a natural gradient of nutrient-limited peatland ecosystems in Michigan, USA, with nitrogen, phosphorus, or a combination of both nutrients. Our objectives were to determine how changes in carbon and nitrogen partitioning within a plant, and changes in community composition, would affect plant nitrogen-use efficiency. Plant nitrogen-use efficiency and its components differed from the leaf- to community level, and depended on nitrogen or phosphorus limitation.

Iversen CM, Bridgham SD, Kellogg LE (2010). Scaling plant nitrogen-use and uptake efficiencies in response to nutrient addition in peatlands. Ecology 91: 693-707. [pdf]