Question 148·200 Super-Hard SAT Reading Questions·Information and Ideas
Researchers studying salt-marsh soil expected nearly all nitrate removal during a 14-day experiment to occur through either plant-root uptake or denitrification, a process performed by microbes under the low-oxygen conditions produced by flooding. Denitrifying microbes preferentially consume nitrate containing nitrogen-14, so denitrification lowers nitrate concentration while increasing the nitrogen-15 enrichment index of the remaining nitrate. By contrast, the plants used in the experiment absorb nitrogen-14 and nitrogen-15 at nearly equal rates, so root uptake lowers nitrate concentration with little change in the enrichment index. All soil groups initially had a nitrate concentration of 12.0 milligrams per liter and an enrichment index of 4.0.
Mean Results after 14 Days
| Water condition | Plants present | Final nitrate concentration (milligrams per liter) | Final nitrogen-15 enrichment index |
|---|---|---|---|
| Drained | No | 11.5 | 4.1 |
| Drained | Yes | 7.1 | 4.3 |
| Flooded | No | 6.8 | 8.7 |
| Flooded | Yes | 6.6 | 8.9 |
The researchers concluded that most nitrate loss in the flooded soil resulted from denitrification rather than plant-root uptake. Which choice identifies the finding that most directly supports this conclusion?
For a data-based command-of-evidence question, first translate the passage's explanation into predicted data patterns. Then identify which comparisons isolate the relevant factor. Prefer the choice that addresses every part of the conclusion—including the proposed cause, the competing explanation, and the measurements that distinguish them—rather than one that reports only a related difference.
Hints
Translate each process into a pattern
Determine how nitrate concentration and the enrichment index should change under each removal process described in the text.
Control for plant presence
Compare the two flooded rows, which share the same water condition but differ in whether plants are present.
Look for complete evidence
The strongest choice should address both the amount of nitrate lost and the change in the indicator associated with the proposed process.
Step-by-step Explanation
Identify the signatures of the two processes
According to the text, denitrification lowers nitrate and substantially raises the nitrogen-15 enrichment index. Root uptake lowers nitrate but causes little change in the index.
Examine the flooded groups
In the flooded group without plants, nitrate decreased from 12.0 to 6.8 while the index increased from 4.0 to 8.7. In the flooded group with plants, nitrate decreased to 6.6 while the index increased to 8.9.
Connect the data to the conclusion
The flooded groups had nearly the same nitrate loss and enrichment increase even though only one contained plants. Because the denitrification pattern occurred without plants and changed little when plants were added, denitrification best accounts for most of the nitrate loss.