Question

comparing carbon accumulation in tilled and no - till agricultural soils
measuring soil carbon content is one way to determine the amount of organic matter in a soil. in theory, the practice of no - till agriculture on a farm should result in more soil carbon compared to a similar farm that practices conventional tilled agriculture. a native grassland that has not been farmed should be relatively undisturbed and therefore contain more organic matter and a greater carbon content than a plot of land tilled conventionally or a no - till plot.
a study in nebraska measured the carbon content in soil on three adjacent areas of land: native grassland, tilled farmland, and no - till farmland. the investigators reported the findings for the first 5 cm of soil and then for the horizon from a 5 cm to a 20 cm depth. the study produced the following findings:

treatment	soil depth	carbon content (g c m⁻²)	nitrogen content (g n m⁻²)
native grassland	5–20 cm	2,653	265
tilled farmland	0–5 cm	699	82
tilled farmland	5–20 cm	2,208	230
no - till farmland	0–5 cm	1,129	108
no - till farmland	5–20 cm	2,299	366

data from: j. six et al. soil sci. soc. am. j. 62 (1998): 1367–1377.

1. add the 0–5 cm and 5–20 cm horizon for each treatment and compare the results of the three treatments.

the carbon content in g c m⁻² for the 0–20 cm depth can be calculated by adding the 0–5 cm and 5–20 cm horizons:
native grassland: 1,437 g c m⁻² + 2,653 g c m⁻² = 4,090 g c m⁻²
tilled farmland: 699 g c m⁻² + 2,208 g c m⁻² = 2,907 g c m⁻²
no - till farmland: 1,129 g c m⁻² + 2,299 g c m⁻² = 3,428 g c m⁻²

1. describe the two treatments as a percentage of the native grassland carbon content.

tilled farmland as a percentage of native grassland = (2,907 g c m⁻²) ÷ (4,090 g c m⁻²) = 0.7108×100% = 71.08%
no - till farmland as a percentage of native grassland = (3,428 g c m⁻²) ÷ (4,090 g c m⁻²) = 0.8381×100% = 83.81%
the no - till farmland contains 83.81 percent of the carbon contained in the native grassland while the tilled farmland contains only 71.08 percent of the native grassland carbon content.

1. draw a conclusion about the effects of tilled agriculture versus no - till agriculture compared to native grassland.

this study supports the idea that there is more carbon retained in a no - till agricultural farm then in a conventionally tilled farm, relative to native grassland.
your turn
conduct a similar analysis for nitrogen.

1. add the 0–5 cm and 5–20 cm horizon for each treatment and compare the results of the three treatments.
2. describe the two treatments as a percentage of the native grassland nitrogen content.
3. draw a conclusion about the effects of tilled agriculture versus no - till agriculture compared to native grassland.

Explanation:

Part 1: Add 0–5 cm and 5–20 cm nitrogen content for each treatment

Step 1: Native grassland nitrogen content

Add the nitrogen content of 0–5 cm and 5–20 cm horizons.
$131\ \text{g N m}^{-2} + 265\ \text{g N m}^{-2} = 396\ \text{g N m}^{-2}$

Step 2: Tilled farmland nitrogen content

Add the nitrogen content of 0–5 cm and 5–20 cm horizons.
$82\ \text{g N m}^{-2} + 230\ \text{g N m}^{-2} = 312\ \text{g N m}^{-2}$

Step 3: No - till farmland nitrogen content

Add the nitrogen content of 0–5 cm and 5–20 cm horizons.
$108\ \text{g N m}^{-2} + 366\ \text{g N m}^{-2} = 474\ \text{g N m}^{-2}$

Part 2: Describe treatments as a percentage of native grassland nitrogen content

Step 1: Tilled farmland percentage

Use the formula $\text{Percentage}=\frac{\text{Tilled farmland nitrogen content}}{\text{Native grassland nitrogen content}}\times100\%$
$\frac{312}{396}\times 100\% \approx 78.79\%$

Step 2: No - till farmland percentage

Use the formula $\text{Percentage}=\frac{\text{No - till farmland nitrogen content}}{\text{Native grassland nitrogen content}}\times100\%$
$\frac{474}{396}\times 100\% \approx 119.69\%$

Part 3: Conclusion about effects

For nitrogen content, tilled farmland has about 78.79% of native grassland's nitrogen, while no - till farmland has about 119.69%. This suggests that no - till agriculture retains more nitrogen (even more than native grassland in this case) compared to tilled agriculture, which retains less nitrogen than native grassland.

Answer:

s:

1. Native grassland: $396\ \text{g N m}^{-2}$, Tilled farmland: $312\ \text{g N m}^{-2}$, No - till farmland: $474\ \text{g N m}^{-2}$
2. Tilled farmland: ~78.79%, No - till farmland: ~119.69%
3. No - till agriculture retains more nitrogen (even more than native grassland) compared to tilled agriculture which retains less nitrogen than native grassland.