Experimenting

Nitrogen (N), required by all living organisms, can limit crop yield, but too much can cause pollution. Use this simulation model to track nitrogen as it cycles from the atmosphere (or fertilizer) to soil to crops, and out to streams or back to the atmosphere.

Here’s the challenge: Can you manage the agricultural system for crop production without negative impacts for nitrate run-off to streams and greenhouse gas production?

By the Law of Conservation of Mass, the total N mass in the system, summed across all the stocks, is constant. However, because N cycles, the amount within each individual stock can change. We can use this principle to experiment with how to maximize the desirable stocks (Yield, Soil Organic N), and minimize the undesirable stocks (nitrous oxide, ~20% of total denitrification, and N loss to Streams).  The user can design experiments to test hypotheses about the effects of cropping system, management, and climate change on N cycling by manipulating the variables described above. Over the course of multiple model runs, the user can collect data from the model output to evaluate their hypotheses about effects of climate change and management on N cycling in an agricultural setting.

Hypothesis testing about management factors.

Use the model in ‘Lab’ mode, to test hypotheses by comparing output from a baseline run with a run in which you have changed only one parameter, e.g., fertilizer addition or management.

Follow these steps:

  1. Identify the parameters for the baseline run for your experiment (i.e., Cropping System, Fertilizer, Management).  
  2. Identify the parameter you need to change in the second run to be able to test the specific hypothesis.
  3. What output data on stocks do you need to record to make comparisons that will allow you to evaluate the hypothesis?
  4. What other stocks should you look at to determine other, perhaps unintended effects of the change made in the parameter?

Collate the data into a Table:

 

Parameter Settings

Final N Stocks (All in kg/ha)

 

Cropping system

Fertilizer

Management

Atmosphere

Soil Inorganic N

Yield

Soil Organic N

Stream (10-yr total)

Buffer Strip

Hypothesis #

Run #1: Baseline

Run #1: Baseline

Run #1: Baseline

 

 

 

 

 

 

Run #2:

1 of these 3 parameters changed

 

 

 

 

 

 

Examples of Hypotheses to test:

    Note: In all cases, the single changed parameter is highlighted in yellow, the response variable in green.

H1:  Planting continuous corn with N fertilizer at 270 in spring increases Yield.

     Results: 

 

Parameter Settings

Final N Stocks (All in kg/ha)

 

Cropping system

Fertilizer

Management

Atmosphere

Soil Inorganic N

Yield

Soil Organic N

Stream (10-yr total)

Buffer Strip

H1

Contin. Corn

None

Tilled

8000

31

502

5820

72

0

 

Contin. Corn

270 in spring

Tilled

8096

232

1519

6611

458

0

 

H2:  Planting perennial Alfalfa increases Soil Organic N stocks.

    Results:

 

Parameter Settings

Final N Stocks (All in kg/ha)

 

Cropping system

Fertilizer

Management

Atmosphere

Soil Inorganic N

Yield

Soil Organic N

Stream (10-yr total)

Buffer Strip

H2

Contin. Corn

None

Tilled

8000

31

502

5820

72

0

 

Alfalfa

None

Tilled

4127

221

1775

7821

138

0

                                                                                                                                                                                                             

H3: Planting a corn-soybean rotation (N fertilizer = 180 in spring) with management of ‘Cover Crop’ reduces N stocks in ‘N Stream.’  What is the significance of this?

   Results:

 

Parameter Settings

Final N Stocks after soybean (All in kg/ha)

 

Cropping system

Fertilizer

Management

Atmosphere

Soil Inorganic N

Yield

Soil Organic N

Stream (10-yr total)

Buffer Strip

H3

Corn-soy

180 in spring

Tilled

8008

298

1255

5329

280

0

 

Corn-soy

180 in spring

Cover crop

7361

128

1131

5544

210

0