FROM PRAIRIES TO CORN FIELDS FOR FUEL: A TALE OF LOST CARBONGlenna M. Malcolm, Plant Sciences Dept., Penn State University

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

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?

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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 POOLExplore how aboveground net primary productivity differs in a prairie grassland system compared to a corn field (Ex. Table 1).

PART III: SOIL C POOLCompare soil profiles (Fig. 1) and root systems (Fig. 2) below annual and perennial plants.2

Fig. 2

Annual Plant Roots

Perrenial Plant Roots

Fig. 1: Soil profiles in (a) prairie grasslands (b) plowed fields.

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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).

Atmospheric C Pool

Prairie PlantC Pool

SoilC 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.

IMPLICATIONSHow might soil carbon loss from prairies being plows to corn fields for ethanol be mitigated?

Students can investigate: 1) incentives offered by a

U.S. government program, called Conservation Reserve Program (CRP), for farmers to set land aside rather than farm it.

2) different options farmers might use other than plowing to manage their fields to try to store carbon rather than release it.

REFERENCES1Brokaw & Gillum. 2013. Green energy push replaces native prairies with corn as ethanol industry turns profitable. Associated Press. 2DeLuca & Zabinski. 2011. Prairie ecosystems and the carbon problem. Frontiers in Ecology and the Environment 9:407–413.3Russell, A. E., et al. 2009. Nitrogen fertilizer effects on soil carbon balances in Midwestern U.S. agricultural systems. Ecological Applications 19:1102-1113.

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