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Fertilizing biofuels may cause release of greenhouse gasses

Data provided by Leilei Ruan. Written by Sandy Erwin, Anne Royer, and Liz Schultheis

Background Information:

One way to reduce the amount of greenhouse gases we release into the atmosphere could be to grow our fuel instead of drilling for it. Unlike fossil fuels that can only release CO2, biofuels remove CO2 from the atmosphere as they grow and photosynthesize, balancing the CO2 released when they are burned for fuel.

However, the plants we grow for biofuels don’t necessarily absorb all greenhouse gas that is released during the process of growing them on farms and converting them into fuels – they’re generally not “carbon neutral”, meaning that if we release 10 units of CO2 to produce the biofuel, the plants during their lifetime will absorb perhaps 8 units. One reason that biofuels are not carbon neutral is that they must be fertilized, which results in the soil releasing N2O, a gas with global warming potential 298 times higher than CO2! The fraction of fertilizer that is released as N2O may change depending on how much fertilizer is applied to the field

Scientists at Michigan State University decided to test this prediction by applying different amounts of fertilizer to fields of biofuels and measuring how much N2O the soil in each treatment produces over several years. In the table below we have two years of their data.

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Scientists adding fertilizer to a field of switchgrass, a plant grown for biofuel production.

(A) DATA: Look at the data below. See if you can identify some potential patterns. Then, proceed to (B).

|Amount of Nitrogen fertilizer |N2O emissions in biofuel fields |N2O emissions in biofuel fields |

|applied |during 2009 |during 2010 |

|(kg N ha-1 ) |(kg N2O-N ha-1) |(kg N2O-N ha-1) |

|0 |0.30 |0.26 |

|28 |0.50 |0.55 |

|56 |0.64 |0.68 |

|84 |0.85 |0.98 |

|112 |1.11 |1.29 |

|140 |1.26 |1.59 |

|168 |1.41 |1.90 |

|196 |1.70 |2.91 |

(B) Use the data given above to answer the following questions:

Scientific Question: What is the relationship between N2O emissions and applied nitrogen fertilizer? How do the results differ between 2009 and 2010? Why do you think these differences occurred?

Evidence: You will use the data in the table to answer the scientific question by making a claim. The data is the evidence that you will base your claim on, just like a lawyer would in a court case. Drawing a graph or making a simpler table can help you and others to visualize the data.

Use the data given above to graph the relationship between applied nitrogen fertilizer and total N2O emissions in 2009 and 2010 (Draw your graph below).

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Claim: Write a conclusion that answers the questions above.

There is a positive relationship between the amount of fertilizer applied to a biofuel field and the release of N20 as a greenhouse gas from that field. The positive relationship remains consistent over different years, however the slope of the line changes from 2009 to 2010. Different amounts of N20 are released depending on the year.

Reasoning: How does this evidence support your claim? Draw arrows and/or circle the specific parts of the graph that you used as evidence.

a) What is the relationship between N2O emissions and applied nitrogen fertilizer?

The more nitrogen fertilizer applied, the more emissions of the greenhouse gas. The evidence for this pattern comes from the fact that the lines have a positive slope.

a) How do the results differ between 2009 and 2010? Why do you think these differences occurred?

More N20 was released in 2010 compared to 2009. There are many reasons that this could have occurred – students can brainstorm responses. Perhaps there was more N stored in the soil in 2010 compared to 2009 because there had been more years of fertilizer application. Perhaps the weather was different that growing season. Soil temperature or moisture may affect release of N20.

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