At a disused coastal shrimp pond, a student records changes after young mangrove trees are planted. The salty water contains sodium chloride, and the trees ...
Question 1 Report
At a disused coastal shrimp pond, a student records changes after young mangrove trees are planted. The salty water contains sodium chloride, and the trees use energy from sunlight to make carbon compounds. Their roots also trap fallen leaves in wet mud. Fig. 1 summarises some transfers of carbon at the site. The field team wants to know whether restoring the mangroves can increase the mass of carbon stored in the sediment.
Table 1 gives annual measurements from one 0.35 hectare part of the restored site.
| Measurement | Mass of carbon / kg per year |
|---|---|
| Carbon taken in by mangrove trees | 840 |
| Carbon released by respiration and decay | 290 |
(a) Name the process shown by arrow X. [1]
(b) Describe how the process shown by arrow Y transfers carbon from the trees to the atmosphere. [2]
(c) Use Fig. 1 to explain why waterlogged sediment can act as a carbon store. [3]
(d) Calculate the net mass of carbon stored per hectare each year in this part of the site. Show your working. [2]
(e) Explain how an increase in water temperature could change the rate at which carbon is released from the sediment. [2]
Answer Details
(a) Photosynthesis [1] is the process shown by arrow X, transferring carbon dioxide from the air into mangrove trees.
(b) [2] Animals eat mangrove material containing carbon compounds. They then respire and release carbon dioxide to the atmosphere.
(c) [3] Mangrove trees remove carbon from atmospheric carbon dioxide by photosynthesis. Leaves and other plant material enter the sediment. Waterlogged conditions reduce decomposition because oxygen cannot easily reach many decomposers, so carbon remains stored.
(d) [2] Net carbon stored in the measured area is:
\[840-290=550\text{ kg carbon per year}\]
This area is 0.35 hectares, so per hectare:
\[\frac{550}{0.35}=1571.4\ldots\text{ kg carbon per hectare per year}\]
Therefore the net storage is 1570 kg carbon per hectare per year, or \(1.57\times10^3\text{ kg carbon per hectare per year}\).
(e) [2] Higher temperature increases enzyme-controlled reactions and decomposer activity. Decomposition and respiration increase, releasing more carbon dioxide from the sediment.
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