Connections between the water and carbon cycles

Temperate areas.

[Previous Substacks have examined the connections between the Water cycle and the Carbon cycle, all in the context of the rainforest. [See A Level of Geography Substack links - by David Redfern for the links to the posts.] This post looks at these links in the context of temperate areas, such as the UK.]

Globally, there is three times as much carbon stored in soils as there is in the atmosphere. Many terrestrial environments act as carbon sinks, which means that they are accumulating more carbon over time. However, some terrestrial environments also act as sources of carbon. Carbon can be lost in a variety of different ways, through different pathways and processes. The movement of carbon from one store to another is known as a carbon flux. Some carbon is lost from the soil to the atmosphere through respiration, and some is lost to the fluvial (river) system.

In a temperate area, such as the UK, the main type of carbon which enters the river system from the soil is organic carbon - derived from living organisms. This organic carbon is either in particulate form or dissolved form.

Particulate organic carbon (POC)

Particulate organic carbon (POC) refers to solid particles of soil which contain carbon compounds. POC is defined as particles larger than 0.45 μm. When such particles become detached from the soil and are transported over a hillslope into the river system, there is POC loss from the land.

Water plays an important role in these processes. For example:

·         when rainfall intensities are high, soil particles can be detached by raindrops which hit the ground at high speeds. This breaks up the soil into smaller units that are easier to transport. They may start to move some of these particles closer to the stream channel.

·         during cold conditions, moist soil can be weathered by freeze–thaw processes. Some soils undergo desiccation when it is dry. Both processes detach soil particles and make them available for transport into the river system.

·         once particles have been detached, they can be transported into the river system by water which travels across the surface of the ground as overland flow. This includes sheetwash processes, and water travelling through rills or gullies. The force of this running water may also detach more particles from the soil surface, which may then be transported in the flowing water.

Concentrations of POC tend to be higher under dry conditions. Under these conditions weathered soil particles can accumulate on the soil surface without being washed away. When a rainfall event happens after a dry period, concentrations of POC in the river system can become very high.

Soil organic-carbon concentrations tend to be high in areas where organic matter can accumulate due to low rates of decomposition. This happens where it is wet and cold, and decomposition processes are inhibited. Peat soils, for example, have slow rates of decomposition and a high organic carbon content. Some peats are almost 50% organic carbon by mass. In areas dominated by soils with a high organic content, losses of particulate carbon to the river system will be high.

This also depends on rates of erosion which allow soil particles to enter the river system. Erosion rates are high on steep slopes, in areas where rainfall is intense, in unvegetated areas, and in areas which are subject to human interference, such as ploughing.

Dissolved organic carbon (DOC)

Dissolved organic carbon (DOC), held in the soil water, is produced as a byproduct of decomposition of organic matter and enters the pore spaces around individual soil particles. DOC is defined as organic material with pore size of 0.45 μm or less. DOC enters the stream system following rainfall, as water from soil pores is ‘washed out’ of the soil by rainwater that is travelling to the stream channel through the soil. This sub-surface flow is throughflow.

Like POC, high concentrations of DOC build up in when it is dry. This is not only because the DOC-rich pore water does not get washed out of the soils, but also because rates of decomposition tend to be higher when soils are drier. Organisms that decompose organic matter need oxygen to function, and when the pore spaces in the soil are full of water, conditions become anaerobic (lacking oxygen). Under anaerobic conditions, decomposers are inhibited so rates of decomposition, and therefore DOC production, are reduced.

Another important factor is temperature. Rates of decomposition are higher when temperatures are warmer.

Overbank deposition

When river discharge is high, rivers can overtop their banks and water spills out on to the floodplain. If this river water is carrying a high load of carbon, it will be deposited on the floodplain. This is most common when the river is carrying a high load of particulate carbon (POC). This particulate carbon may be assimilated back into the floodplain soils, adding to the terrestrial carbon store. However, if there is heavy rain before this can occur, the particulate carbon may be re-mobilised and washed back into the river system as the ground is saturated.

In-stream processing

Once it is in the river, instead of just being transported by the water, carbon may be processed from one form to another. In most cases, this involves the larger forms of carbon being broken down into smaller ones, so that the carbon is in gaseous form.

The processing of carbon can occur in different ways:

·         Photodegradation involves carbon being broken down by light, so rates are highest when incoming light (irradiation) is high.

·         Biodegradation involves carbon being broken down by micro-organisms, such as bacteria and fungi, which use the organic material for energy. Rates of biodegradation tend to increase with temperature.

If there is oxygen in the water, the carbon may become oxidised to form carbon dioxide (CO₂). If the oxygen is limited, methane (CH₄) may be formed instead. If a lot of these gases is produced in the river, so that the concentrations in the water are higher than those in the atmosphere, the gases will diffuse out of the water into the atmosphere. This is known as evasion. It increases the amount of carbon in the atmospheric store.

Carbon in lakes and reservoirs

The above processes can also occur in lakes and reservoirs, so that they too release gaseous carbon to the atmosphere. However, in many lakes and reservoirs, carbon can also be stored in bed sediments. As water from the river or stream enters a lake or reservoir, its flow slows down. This slower-flowing water no longer has enough energy to carry sediments in suspension, and they are deposited on the bed.

In lakes, a lot of carbon can accumulate in sediments over many years, whereas the carbon in reservoirs tends to be more recent (because reservoirs are newer), and may be temporary, as the level of water in reservoirs changes depending on water-resource needs.

This storage of carbon in lakes and reservoirs can be a significant carbon sink, as they store a lot of carbon relative to their size. This is particularly true for smaller lakes. There is often relatively little oxygen in lakes, so methane can be produced, and then released to the atmosphere by evasion.

Another consequence of sediment settling out on to the bed of the lake is that the water is clearer (less turbid), so more sunlight can reach aquatic plants and algae. These plants use sunlight to photosynthesise, removing carbon from the atmospheric store and using it to build tissue. This is termed primary production, and rates are relatively high in lakes due to the increased light availability. However, these plants also respire, releasing some carbon dioxide back into the atmosphere.