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Plant Carbon
   Belowground

Belowground deposition of carbon in root materials
is an important component of the carbon cycle.

Project Goals
Significant quantities of carbon are deposited below the surface of the soil as root material and exudates. In many respects, this carbon has the potential for a greater contribution to long-term soil carbon sequestration because it is deposited below the soil surface and is less easily oxidized than surface plant residue. Much of total ecosystem respiration (CO2 loss) is comprised of daily root respiration and soil and crop residue decomposition. Our goal is to quantify total belowground carbon deposition in these corn-based systems as well as the proportion of soil CO2 respiration attributed to decomposition or autotrophic (plant) respiration. This information is very important to assembling the carbon balance of these managed systems and understanding the contribution of belowground carbon to total ecosystem respiration and soil carbon sequestration.

Soil carbon (C) sequestration is the result of a complex set of soil processes that govern the
transfer of energy and material below the soil surface. Enhancing soil carbon sequestration
is important to the sustainability of our food supply and in the reduction of greenhouse gases
in the atmosphere. Understanding the processes that govern the production and fate of
belowground C is the goal of our work.

The objectives of our research include:

- Quantifying the spatial and temporal root biomass production of corn and soybean under rainfed and irrigated conditions.

- Separation of the sources of soil respiration (CO2) into that attributed to autotrophic root respiration and heterotrophic decomposition of soil and crop residues.

Project Description
Standing root biomass is measured at four times within the growing season by extracting replicate soil core transects within each of the intensive measurement zones (IMZs). Transects are designed to allow for estimation of spatial distribution of roots. Cores are carefully washed, stained and cleaned of non-living tissue before scanning for size class and weighing for biomass. Spatially weighted field means and standard errors are then calculated from the individual IMZ measurements.

Field sampling of corn root material.

After washing soil from root cores, live roots are carefully sorted
from dead material for accurate biomass determination.

Sorted roots are scanned by computer to determine
length, surface area and size class distribution.

Separation of heterotrophic and autotrophic components of soil respiration employs the analysis of the stable isotope fraction of 13C in soil CO2 respiration. Analysis is done of the 13C signature of respiration in root excluded soil and non-root excluded soil.

Photograph of the apparatus used to sample soil respiration for 13C isotope
composition. This analysis allows for the separation of C derived from
soybean or corn respiration and soil or residue decomposition.

Progress and Results

13C signature of soil respiration from root-excluded and non-root-excluded soybean
sites as a function of row position. A more negative number on the y-axis
indicates a greater proportion of soil respiration is derived from soybean.
Analysis of this data indicated that 65% of soil respiration was derived from
soybean root respiration on the data of measurement.

Typical seasonal variation of carbon input to the root system of corn.
Total carbon input includes standing root biomass as well as the exudation
of organic materials, sloughing of cells and root death. Total (non-respired)
root carbon input is estimated at 30 g C/plant/year.

Staff
Dan Walters Department of Agronomy and Horticulture

Madhavan Soundararajan Senior Lecturer, Department of Biochemistry

Brigid Amos Postdoctoral Research Associate, Department of Agronomy and Horticulture

Tim Arkebauer is a professor in the Department of Agronomy and Horticulture. In addition to leaf and soil gas exchange and carbon sequestration he has interests in plant growth and development, plant water relations and evapotranspiration.


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