| Date | May 2012 | Marks available | 2 | Reference code | 12M.3.SL.TZ1.7 |
| Level | Standard level | Paper | Paper 3 | Time zone | Time zone 1 |
| Command term | Describe | Question number | 7 | Adapted from | N/A |
Question
In a long-term experiment in Harvard Forest, Massachusetts in northeastern USA, nitrogen was added to the soil in two different areas of the forest containing either hardwood or softwood trees and the effects on release of CO2 from the soil were measured.
Estimate the difference between the lowest and highest rates of release of CO2 from the soil in the hardwood area, giving the units.
Suggest one process occurring in tree roots that could cause the release of CO2 from the soil.
Describe the relationship between rates of nitrogen addition and release of CO2 from soil in the hardwood area.
Suggest a reason for this relationship.
Compare the effects of the nitrogen addition treatments on the hardwood and softwood areas of the Harvard Forest.
Markscheme
(255–184/183 =) 71/72 mg CO2 m–2 hr–1 (units required)
respiration (in tree root cells/mycorrhizal fungi/bacteria/other microorganisms)
as nitrogen addition increases, release of CO2 decreases;
greater decrease from zero to 50 kg ha–1 yr–1 than from 50–150 kg ha–1 yr–1 / from zero to 50 kg ha–1 yr–1 decrease is 43 mg CO2 m–2 hr–1, from 50–150 kg ha–1 yr–1 decrease is 28 mg CO2 m–2 hr–1;
increased nitrogen addition leads to less root/fungal/bacterial growth/alters pH/osmotic potential of soil so affecting/decreasing respiration of organisms resulting in less CO2 production
as nitrogen addition treatment increases, release of CO2 from soil decreases in the hardwood area whereas there is no significant change in the softwood area;
release of CO2 from soil is higher in hardwood area than in softwood area at all nitrogen addition treatments;
at nitrogen addition treatment of 50 kg ha–1 yr–1 hardwood shows (large) decrease in CO2 release from zero treatment whereas softwood shows (slight) increase / other valid numerical comparison;
Examiners report
This was mainly answered correctly with the correct units being given.
Most candidates stated the process of respiration.
Many gained a mark for saying that release of carbon dioxide decreased as nitrogen addition increased, but few quoted any data from the graph to support their statement, so did not gain a second mark.
There were few correct answers here, but stronger candidates did suggest that there was decreased respiration of organisms resulting in less carbon dioxide production.
Many answers did not compare the two areas in the forest, and those that gained one mark did not utilise the data accurately to gain a second mark.
