1. FROM PRAIRIES TO CORN FIELDS FOR FUEL: A TALE OF LOST CARBON
Glenna M. Malcolm, Plant Sciences Dept., Penn State University
CASE LEARNING GOALS:
To integrate knowledge of energy flows, nutrient cycling, and decomposition
To use eco-’systems’ thinking
To model (on paper) how and why the carbon cycle changes when prairie grasslands are plowed and planted to corn
To consider how carbon loss associated with plowing prairie grasslands might be mitigated
PART I: INTRODUCTION
Since 2010, farmers in Nebraska and the Dakotas have plowed millions of prairie grassland acres to plant corn for ethanol. One farmer said, “It’s not hard to do the math there as to what’s profitable to have. Farmers can make about $500 an acre planting corn. I think an ethanol plant is a farmer’s friend. 1” How much is prairie grassland conservation worth?
The rapid expansion of corn ethanol production started in 2007 with incentives put in place by the U.S. government to mix millions of gallons of renewable biofuels in with petroleum diesel by 2022.
As a result of prairies being plowed to plant corn for ethanol production, scientists estimate an average of 48% organic carbon has been lost that was stored in the thick sod layer beneath a diverse canopy of perennial and annual plant species.2
Why was the carbon lost? Where did that carbon go? Can we do anything to mitigate more carbon being lost from prairie grasslands in the future?
TEACHER NOTES SUMMARY:
Students could work through Parts II and III independently or with an interrupted case format, where teaching is integrated in with learning.
This case builds to Part IV, where students will be required to integrate their knowledge from the rest of the case.
Due to expected misconceptions regarding carbon cycling, teachers may want to assess student’s understanding of the prairie grassland carbon cycle before having students model and explain the altered corn carbon cycle.
PART II: PLANT C POOL
Explore how aboveground net primary productivity differs in a prairie grassland system compared to a corn field (Ex. Table 1).
PART IV: CARBON CYCLING MODELING
Draw fluxes of carbon between C pools in a prairie grassland.
Indicate forms carbon fluxes are in (ex. CO2 or organic-C).
IMPLICATIONS
How might soil carbon loss from prairies being plows to corn fields for ethanol be mitigated?
Students can investigate:
incentives offered by a U.S. government program, called Conservation Reserve Program (CRP), for farmers to set land aside rather than farm it.
different options farmers might use other than plowing to manage their fields to try to store carbon rather than release it.
Table 1: Average ANPP for corn at Kanawha Experimental Site under different fertilizer rates.3
 Corn ANPP (Mg ha-1 yr-1)
Fertilizer Application Rate
Experimental Site
0 kg (N/ha)
90 kg (N/ha)
180 kg (N/ha)
270 kg (N/ha)
Kanawha
8.0
10.3
11.5
11.3
Atmospheric C Pool
Model (draw) how carbon pool and flux sizes change when prairie grassland is plowed and eventually planted to corn.
Explain why you made those changes to the carbon cycle referring to other parts of the case.
Example Flux: CO2
Prairie Plant
C Pool
PART III: SOIL C POOL
Compare soil profiles (Fig. 1) and root systems (Fig. 2) below annual and perennial plants.2
Fig. 2
Soil
C Pool
REFERENCES
1Brokaw & Gillum Green energy push replaces native prairies with corn as ethanol industry turns profitable. Associated Press.
2DeLuca & Zabinski Prairie ecosystems and the carbon problem. Frontiers in Ecology and the Environment 9:407–413.
3Russell, A. E., et al Nitrogen fertilizer effects on soil carbon balances in Midwestern U.S. agricultural systems. Ecological Applications 19:
Annual Plant Roots
Perrenial Plant Roots
Fig. 1: Soil profiles in (a) prairie grasslands (b) plowed fields.