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Verbal·Information and Ideas·Command of Evidence
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Average DOC and TDN Concentrations in Rainwater Samples
Precipitation eventRainwater typeAverage DOC concentration (mg/L)Average TDN concentration (mg/L)
6throughfall7.670.30
9stemflow98.703.22
12stemflow51.841.50
15throughfall21.820.63
To investigate how trees influence the chemistry of rainwater, a student consulted data collected during several precipitation events at sites in a temperate forest in the upper Midwest of the United States. The student considered three kinds of rainwater samples: regular (falling directly into a container), throughfall (passing through the tree canopy), and stemflow (running down branches and trunks). Noting that regular samples had average concentrations of 0.90 mg/L of dissolved organic carbon (DOC) and 0.19 mg/L of total dissolved nitrogen (TDN) across events, the student concluded that contact with trees changes the chemical composition of rainwater before it reaches the ground.
Which choice best explains how well the data in the table support the student’s conclusion?
A
The differences between the average concentrations of DOC and TDN in the stemflow and throughfall samples and those reported for regular samples vary greatly across precipitation events, which calls the conclusion into question.
B
The average concentrations of DOC and TDN in the stemflow samples are higher than those in the throughfall samples, supporting the conclusion for only one kind of tree contact.
C
The average concentrations of DOC and TDN are higher in both the stemflow and throughfall samples than in the regular samples, which strongly supports the conclusion.
D
The average DOC concentrations are higher in the stemflow and throughfall samples than in the regular samples, whereas the average TDN concentrations are slightly lower than in the regular samples, which weakly supports the conclusion.