The nitrogen cycle
Grade 9 Science Worksheet - Challenge Me
Circle the best answer.
Zee reads the graph. What was the bean plot's average gain per week from week 0 to week 8?
- 2 mg per kg per week
- 1 mg per kg per week
- 4 mg per kg per week
- 8 mg per kg per week
Luz reads the graph. Which statement explains why the two lines moved apart?
Use the picture in question 1.
- Bean leaves took N₂ gas from the air, but grass leaves could not
- Decomposers destroyed nitrogen in the grass plot, but not in the bean plot
- Rhizobium in the bean roots added fixed nitrogen, while grass took nitrate up
- Nitrogen taken up by grass was used up, and beans gained by chance
In a model bean plot, Rhizobium bacteria are missing from the root nodules. Bo predicts what happens to the nitrogen the beans can use.
- It falls, because nothing in the nodules fixes N₂
- It stays the same, because soil nitrogen does not run out
- It rises, because the bacteria were taking nitrogen from the beans
- It stays the same, because bean leaves take N₂ from the air
In a model soil sample, denitrifying bacteria become more active. Luz predicts what changes in the soil.
- More N₂ becomes usable nitrogen, so soil nitrate rises
- Nitrate stays equal, since decomposers destroy the extra nitrogen
- More nitrate becomes N₂ gas, so soil nitrate falls
- More ammonium becomes nitrite, so soil nitrite falls
A pile of leaf remains and wastes lies in a model plot, and decomposers start work. Noah predicts what rises in the soil first.
- Nothing, since decomposers destroy the nitrogen in remains
- Ammonium, made by ammonification
- N₂ gas, released by the decomposers
- Nitrate, made straight from the remains
Read this first
Use this
Flow diagram (model, not to scale), soil and air. Boxes: nitrate; N₂ gas in air. Arrows: from nitrate to N₂ gas in air: process denitrification, by denitrifying bacteria such as Pseudomonas and Clostridium. Notebook, 3 lines.
Ren reads a notebook entry about denitrification. Which line has a flaw?
- Line 3: plants cannot use N₂, and fixation makes usable forms
- No line has a flaw, since denitrification and fixation are one step
- Line 2: the nitrogen is used up, so nothing returns to the air
- Line 1: nitrifying bacteria, not denitrifying ones, change nitrate to N₂
Use the table
Card (model data). Soil nitrate at the start and at week 8.
plot crop start (mg per kg) week 8 (mg per kg) 1 grass 15 11 2 beans 15 23 3 peas 12 18 Notebook, 3 lines. Line 1: Grass roots took up nitrate, so the grass plot fell from 15 to 11 mg per kg. Line 2: So the grass plot lost nitrate. Line 3: The bean plot went from 15 to 23, so it gained 23 mg per kg.
Tala reads a notebook entry about two plots. Which line has a flaw?
- No line has a flaw, since the bean plot gained the full 23
- Line 2: decomposers destroyed the nitrate the grass lost
- Line 3: the bean plot gained 23 − 15 = 8 mg per kg
- Line 1: grass makes nitrate from the N₂ in the air
Ari's friend says plants use up the nitrogen in soil, so soil always runs short. Which reply is best?
- Agree: the nitrogen a plant takes up leaves the cycle for good
- Disagree: plants take N₂ from the air, so soil nitrogen does not matter
- Disagree: nitrogen in wastes and remains returns to the soil as ammonium
- Agree: decomposers destroy nitrogen in remains, so less returns each year
Kira's friend says bacteria in soil only harm plants, so soil would be better without them. Which reply is best?
Use the picture in question 8.
- Disagree: soil bacteria turn nitrate into N₂ gas, which plants use
- Disagree: plants need no bacteria, since leaves take N₂ from the air
- Disagree: soil bacteria fix N₂ and make nitrate that roots take up
- Agree: bacteria compete with roots for the nitrogen in the soil
Esme's friend says fixation and denitrification are the same process, since both change N₂. Which reply is best?
Use the picture in question 8.
- Disagree: fixation makes N₂ usable; denitrification turns nitrate into N₂
- Disagree: fixation makes ammonium from remains, and denitrification makes nitrate
- Agree: both processes turn N₂ into forms plants can use
- Disagree: fixation returns N₂ to the air, and denitrification makes it usable