Plants and the carbon cycle

Carbon is the common currency of life. The major biological molecules are all constructed from a framework of carbon, and so living organisms need this element in especially large quantities if they are to grow and maintain their tissues. Carbon-containing molecules also serve as a store of energy for cells to work; the bonds within the molecules are broken and energy is released. For these two purposes building bodies and fueling them—organisms are always grabbing carbon from one another, or in the case of plants, directly out of the atmosphere by photosynthesis. While many other important elements, like calcium and sulfur, are transferred too, carbon is needed in the greatest quantities and generally the most urgently.

Carbon atoms are shuttled from one molecule to another within the cells and tissues of an organism, and from one organism to another (by prédation, parasitism, herbivory, decay and all the other ways in which organisms interact), until they are eventually "oxidized"—in effect, burnt—to give carbon dioxide, which goes into the atmosphere. Each C02 molecule may later be taken up by plants in photosynthesis, perhaps in a completely diiferent part of the world because molecules carried by the wind can travel hundreds of miles in a few days. Having been taken up by a plant, the carbon atom can go shooting down a food chain once again. It may end up in soil, and perhaps only released by decay after many decades. Or the C02 molecule may drift out across an ocean, dissolve in its waters and stay there for thousands of years before it eventually wanders back out again. It might even end up in sediment on the sea floor, be buried, folded down into the earth and only released millions of years later when it gets spat out of a volcano. The global shuttling process of carbon atoms between organisms, atmosphere, oceans, rocks and soil is together known as the "carbon cycle" (Figure 7.1). Carbon has been rccyclcd between these different compartments sincc the beginning of life on earth, billions of years ago. For sure, there are carbon atoms in each of us that once formed part of the DNA of dinosaurs.

Many people new to ecology imagine that the carbon cycle is a benevolent set-up. with organisms all helping one another out in an interlinked and cosy network.

Carbon dioxide ?25 Methane 3

Curbon monoxide 0 2

ATMOSPHERE

l>ciorcsiatioa i 1-2

Phoios>nihcsis Respiration 120 60

Air Sea Exchange

Photosynthesis i>exbornc material

Valcanba

Hrmtun A

0 41 I01 A

Marine Biota 3

5 particulate flux

OCEAN

Dissolved Inorganic Carbon 37.900

Dissolved Organic Carbon 1000

»'articúlale Carbon 30

t i>exbornc material

BIOSPHERE

Phoios>nihcsis Respiration 120 60

Hiarnavs 560

I jltcrfail 6U

Lillcr 60 Hurauv 1500 Peal 165

ETt^r CONTINENTAL CRUST

Coal, oil 5,000-10.000

Limestone and Marble 20 million

Organic carbon in rocks 5 million

Sedimentation

Figure 7.1. Some basic components of the carbon cycle. From: Schwartzman. (Note: the arrows indicate fluxes, in billions of tonnes per year. The figures without arrows are reservoirs, in billions of tonnes.)

Certainly, it is interlinked, and all forms of life do depend upon it but not at all cosy. After all, no organism in its right mind wants to get eaten outright. The carbon cycle is largely the product of a dog-eat-dog world (or, at least, a carnivore-eat-herbivore world), with organisms acting out of pure selfishness wherever there is a chance to make a living. Most of the carbon that gets transferred along a food chain is a result of misfortune for some organism or other, and not willingly given at all.

As well as providing the manufacturing base for everything else in the living world, plants excel at storing carbon. Well over 99% of the carbon in living organisms on earth is held within plants, most of this being in trees. Of the still larger "dead" store of carbon in soils, most is derived directly from breakdown of uneaten plant tissues such as fallen leaves and wood. In some ways it is a mystery why so much living green plant material manages to sit uneaten when there are hungry herbivorous insects just about everywhere. It may be that most plant tissues are just too poisonous. too poor in nutrients or too indigestible to be worth eating. Much of the world may look lush and green, but this does not mean that it is edible.

Because it is not eaten, most plant material in land ecosystems ends up falling to the ground and decaying. A lot of the early work in this is done by fungi and bacteria which can work slowly but relentlessly on the tough low-nutrient material of dead leaves and wood. Eventually, all that is left is a dark, soft material consisting of the most unreactive molecules: generally, mostly six-membered phenolic rings of carbon atoms, each ring linked to the others around it. This is what farmers and gardeners call "humus", and it is one of the major stores of organic carbon on earth. The amount of humus and other organic material that builds up in a soil depends on various factors: partly, how active the vegetation above it is sending down a rain of dead leaves, twigs and suchlike. In a moister forest climate, more carbon is likely to build up because the organisms that break it down can barely keep pace with the rate of supply. Where ground is waterlogged, oxygen is in short supply and without oxygen microbes are barely able to break down the tough, unreactive cell walls of plants. Only partially consumed, the residue of dead plant material builds up as peat, sometimes to great thicknesses. Much of Siberia and Canada arc blanketed in peat that builds up in small lakes scooped out by glaciers, on low-lying river floodplains, or over the top of permafrost layers where water cannot drain away, so the ground is always sodden when it thaws in summer. Other large areas of peat have formed in the low-lying forested floodplains of rivers in central Africa and South-East Asia. The world's peats contain perhaps 500 billion tonnes of carbon, rivaling the amount of carbon in C02 in the atmosphere. Despite its vast bulk, the peat of an interglacial stage such as the present usually only survives a few thousand years before the climate changes once again to conditions dry enough to allow it to decay, or too cold for the plants to grow and produce the raw materials for peat. During dry, cold glacial phases (such as the one that ended 11.000 years ago) it seems that the world generally has much less peat, and nearly all the present world's vast peat deposits have built up since the onset of the warmer, moister interglacial. Sometimes peat can survive much longer, if it is buried by other sediments before it can decay. Peats that were laid down in sinking river deltas many millions of years ago have become buried, compressed and heated to form coal, one of the major fossil fuel reservoirs which we are now burning to top up the atmosphere with C02 (see below).

Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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