Plants In The Carbon Cycle

In nature's processes, carbon is exchanged continually between the biosphere and the atmosphere; the oceans are by far the largest global carbon sink. On a global basis the annual net uptake of carbon in vegetation is about 13 000 million tons of carbon. This is approximately double all emissions from human society. The difference, however, is in the process of being almost annulled by the current deforestation of tropical rainforests.

Plants contain carbohydrates in the form of sugar, starch and cellulose. These are the most important nutritional and accumulative substances in plant organisms. Sugar is formed in the green parts of the plant by carbon dioxide CO2 from airand water H2O subjected to sunlight.

During this reaction, oxygen is released. The plant later transforms sugars to starch and cellulose. Cellulose builds up the cells and the starch is stored. When the plant dies, it degrades back to carbon dioxide, water and ash. Oxygen is a necessary ingredient for this process. If there is very little or no oxygen, the plant becomes peat, and much of the carbon will be transformed to methane (CH4).

At night, partofthe carbon taken up by the plants in daytime is released, but the overall carbon uptake increases as the plant grows, more so during the early years of growth (Table 10.6). After a certain age, which is about 60 years

Table 10.6 Characteristics of different tree types for carbon storage

Type

Annual storage (tCO2/ha/year)

Average fixation (tCO2/ha)

Oak/beech

5-6.9

154-535

Spruce

9.7-13

229-510

Poplar, 15 years

13.1

95

Willow, 1 year

15-20

50

Apart from the tree species, their age, the land quality and the climate conditions play an Important role. The annual storage can differ 50 per cent in much better or worse conditions. (Source: Gielen, 2000)

Apart from the tree species, their age, the land quality and the climate conditions play an Important role. The annual storage can differ 50 per cent in much better or worse conditions. (Source: Gielen, 2000)

forspruce trees and over 100 for pine, the plants no longer absorb more carbon by day than they release at night.

Harvested plants are largely composed of carbon that has been bound during the plant's growth.The carbon will remain intact until the product is burned or composted. The carbon content of dry plant matter is normally about 50%. In mixed materials the carbon content may be lower, forexample around 40% in paper products with mineral fillers added. Each kilogram of bound carbon corresponds to binding 3.6 kilograms of carbon dioxide.

When used plant products are burned the CO2 is returned to the atmosphere, in the same quantities as were originally bound in the plant. Bioenergy is thus considered to be carbon neutral. On the other hand, if plant material is dumped and buried at waste tips it will rot and produce methane gas (CH4) due to the lack of oxygen. Methane gas is a much more potent greenhouse gas than CO2, but the product will still be in balance seen from the emissions point of view. The decomposition of wooden materials takes up to 150 years, and the half life emission time for the CH4 emissions is about 11 years (Petersen etal, 2002).

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