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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)
5throughfall7.670.3
7stemflow44.611.46
11throughfall13.820.24
14stemflow106.172.41
To investigate how the chemical composition of rainwater is affected by trees, a student consulted data collected during different precipitation events at sites in a deciduous forest in the northeastern United States. The student considered three types of rainwater samples: regular (falling directly into a container), throughfall (passing through the tree canopy), and stemflow (flowing down tree 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 alters the chemical composition of rainwater before it reaches the forest floor.
Which choice best describes the extent to which the student’s conclusion is supported by data from the table?
A
The average concentrations of DOC and TDN are higher in the stemflow samples and throughfall samples than those reported for the regular samples, which strongly supports the conclusion.
B
The average concentrations of DOC and TDN in the stemflow samples are higher than those in the throughfall samples, which supports the conclusion for only one type of contact with trees.
C
The average concentrations of DOC are higher in the stemflow samples and throughfall samples than those reported for the regular samples, while the average concentrations of TDN are slightly lower than those reported for the regular samples, which weakly supports the conclusion.
D
The degree of difference between the average concentrations of DOC and TDN in stemflow samples and throughfall samples and those reported for regular samples varies widely across precipitation events, which challenges the conclusion.