When 2 types of bacteria found in the soil of a wetland

When 2 types of bacteria found in the soil of a wetland (land having a high water table) break down organic matter, gases are generated. Aerobic bacteria, which require O₂, generate CO₂. Anaerobic bacteria, which require little or no O₂, generate CH₄.

Study

At the beginning of a particular summer, 3 soil sections, each 1.5 m long, 1 m wide, and 0.6 m deep, were removed intact from the surface of each of 2 wetlands — a bog and a fen — after all live plants had been removed from the sections. Each section was placed in a separate 100 L tank having sides and a lid made entirely of glass. An instrument to measure gas emissions was mounted on the underside of the lid, above the soil. All the tanks were placed at an outdoor site near the wetlands.

Different amounts of water were added to the 3 tanks containing bog soil sections to produce a water table (WT) 1 cm above the surface (+1 cm) of the first soil section, a WT 10 cm below the surface (-10 cm) of the second soil section, and a WT 20 cm below the surface (-20 cm) of the third soil section. This procedure was repeated for the 3 tanks containing fen soil sections. All the lids were then closed.

Over the next 3 months, gas emissions from each soil section were measured, in moles of carbon per square meter (mol C/m²). Throughout this period, the temperature inside the tanks was kept the same as the outdoor temperature. Figure 1 shows the total emission of CO₂ and the total emission of CH₄ from each bog soil section due to bacterial activity; Figure 2 does the same for each fen soil section.

14. The researchers who conducted the study most likely chose to conduct it during the summer rather than during the winter because organic matter in soil is broken down:

F. in the summer by aerobic bacteria only and in the winter by anaerobic bacteria only.
G. in the summer by anaerobic bacteria only and in the winter by aerobic bacteria only.
H. by both aerobic and anaerobic bacteria more rapidly in the summer than in the winter.
J. by both aerobic and anaerobic bacteria less rapidly in the summer than in the winter.

15. According to the results of the study, as the water table for the bog soil sections and the fen soil sections became progressively lower, did the total CO₂ emission increase or decrease, and did the total CH₄ emission increase or decrease?

16. One of the reasons that the lid on each tank was kept closed for the 3-month period was to:

F. minimize the amount of emitted gas that exited the tank.
G. maximize the amount of atmospheric gas that entered the tank.
H. prevent bacteria from leaving the tank.
J. prevent sunlight from entering the tank.

17. The 2 types of wetland investigated in this study — bogs and fens — have different levels of the nutrientsthat sustain bacterial growth. Do the results of the study suggest that the levels of the nutrients that sustain aerobic bacteria are higher in bogs or in fens, and do the results of the study suggest that the levels of the nutrients that sustain anaerobic bacteria are higher in bogs or in fens?

18. Based on the results of the study for the 2 soil sections that were completely submerged in water, were aerobic bacteria present in those sections?

F. Yes; CO₂ was emitted from those sections.
G. Yes; CH₄ was emitted from those sections.
H. No; only CO₂ was emitted from those sections.
J. No; only CH₄ was emitted from those sections.

19. Consider the total CO₂ emission from the fen soil section having a WT of -10 cm. Based on that result, over the 3 months, the average CO₂ emission from that soil section per month would have been closest to which of the following?

A. 10 mol C/m²
B. 13 mol C/m²
C. 16 mol C/m²
D. 19 mol C/m²

20. The study was conducted at an outdoor site near the wetlands to ensure that the tanks would be nearly identical to the wetlands with respect to which of the following conditions?

F. Amount of precipitation
G. Types of plants present
H. Volume of soil
J. Hours of daylight

Answers

14. The best answer is H. F and G are incorrect; both CH4 and CO2 were produced indicating that the organic matter was being broken down by both aerobic and anaerobic bacteria.

15. The best answer is A. According to Figures 1 and 2, for both bog soil and fen soil, as the water table levels decreased from +1 cm to -20 cm, the total CO₂ emissions increased. The figures also show that for both bog soil and fen soil, as the water table levels decreased, the total CH₄ emissions also decreased.

16. The best answer is F. According to the passage, gas emissions for each soil section were measured. By keeping a lid on each tank, the scientists would have been able to collect and measure all the gases emitted by each section.

17. The best answer is D. According to Figures 1 and 2, the amount of CO₂ emitted from the fen soil at each WT level was greater than the amount emitted from the bog soil. The amount of CH₄ emitted from the fen soil at each WT level was also greater than the amount emitted from the bog soil. One may conclude that the levels of nutrients that sustain both types of bacteria are greater in the fen soil than in the bog soil.

18. The best answer is F. According to the passage, aerobic bacteria generate CO₂. The results illustrated in Figures 1 and 2 show that CO2 was produced in both types of soil at all WT levels, indicating that aerobic bacteria were present in the soil sections that were completely submerged in water.

19. The best answer is C. According to Figure 2, the fen soil section with a WT of -10 cm had a total CO₂ emission of 48 mol C/m² over the 3-month period. The average CO₂ emission per month is 48/3 = 16 mol C/m².

20. The best answer is J. F is incorrect; because the tanks were covered, no precipitation would fall on the samples.