Primary productivity

Primary productivity uses the process of photosynthesis or chemosynthesis to make organic compounds from carbon dioxide and water. Life on earth would not be possible without primary production as all animals are either directly or indirectly reliant on the organisms at the base of the food chain. In terrestrial ecosystems plants are the primary producers whereas algae and phytoplankton fill the role in the aquatic systems.

Primary productivity will be either gross or net and is usually measured for a whole trophic level. Gross primary productivity (GPP) is all the biomass produced by primary producers in a given amount of time (before any of it is used for respiration). This is very difficult to measure because the products of photosynthesis are used in respiration and repair. What we actually see of a plant is the net primary productivity (NPP). NPP takes into account respiratory losses (R) and is shown as the following equation:

NPP = GPP – R

NPP represents the amount of usable biomass in an ecosystem some of this will be used for growth and some will be consumed by herbivores. NPP and GPP are both expressed as g m-2 yr-1.

NPP is not all that efficient. If you assume a plant receives 100 units of solar energy, 50 will be unsuitable as it is the wrong wavelength (green) so it is scattered. 5 will go through the leaf (transmitted) and 5 will be reflected. That leaves 40 units going in to the leaf, 30 of which will be used for photosynthesis. The remaining 10 units are used to make new leaf material or lost in respiration.

Variation in NPP

Not all biomes are created equal and there is a significant difference in the amount of NPP. In terrestrial ecosystems temperature, water availability and solar radiation influence the rate of primary productivity. In aquatic ecosystems water is not a limiting factor – though salinity is. Temperatures vary less in the ocean than on land, water has a high specific capacity, which buffers changes. A major limiting factor in aquatic ecosystems is light availability – once you go below 200 meters there is not enough light for photosynthesis.

This map shows that maximum NPP on land-based ecosystems are around the equator where temperatures and sunlight levels are higher whilst maximum NPP in the oceans is in the cooler waters.

It is interesting to note that primary productivity is about three times greater in the oceans than an area of land at the same latitude.

From the diagram, we can create the following table.

We can use the data in the table to show NSP and GSP.

GSP = 200 – 100

GSP = 100

Therefore,

NSP = 100 – 67

NSP = 33

Note that NSP in the grasshopper diagram is growth - which is correct because NSP is what is left after other life processes have taken place. It works for us too. A adult woman requires about 1,800 calories per day with average activity levels. If that woman eats 1,800 calories per day and then sits about doing nothing then her basic body functions will burn off approximately 1,200 of those calories leaving 600 spare and seeing as an adult female is no longer growing up the option is growing out! Calories surplus to basic needs will be stored as NSP or body mass.

Try for yourself!

Using the food chain diagram above to calculate:

1. The consumption by herbivores?
2. The respiratory losses?

Matter cycles

Energy flows but matter cycles. There may be contradictions in some literature but if you look up flow in a dictionary you will get something about moving steadily and continuously in one direction. That is what energy does, it flows through ecosystems in one direction – from high quality, low entropy sunlight to low quality high entropy heat. Cycle on the other hand is defined as something repeating itself and that is what matter does it goes in one end of the food chain and passes along the various trophic levels and then is broken down to start again. The data below applies to all the planetary matter and energy.

There are three major cycles that you must know:

• The hydrological cycle which is closely linked to and covered in Topic 4 Water and aquatic food production systems and societies.
• The carbon cycle which is covered in the next section and has very close links with climate change and energy production.
• The nitrogen cycle is discussed after that and is closely tied to Soil systems and terrestrial food production systems and societies.

In all these cycles matter flows between stores (also called sinks), the flows are either transformations or transfers. In diagrams the stores are shown as boxes and the flows as arrows pointing from where the matter starts in the direction of where it goes. In some diagrams the boxes and arrows are drawn in proportion to the magnitude of the flows and stores.

Stores

The major stores in the carbon cycle are shown in Table 1. Some of the stores are organic such as plants and animals whereas the rest are inorganic.

* amounts shown are very rough approximations.

Theory of Knowledge

If the figures in Table 1 are rough approximations how can we expect models to usefully predict the behaviour of the atmosphere?

Flows

The flows between the stores are shown in the carbon cycle diagram above, some are transfers and some are transformations. Most of the flows in the carbon cycle are relatively fast, that is they take 0 to 100 years to move carbon between stores. A few of the flows are very slow and take thousands to millions of years to move the carbon into or out of the store such as deep circulation, rock formation and fossilization.

Respiration and photosynthesis move carbon into and out of the atmospheric store. Respiration releases energy from carbohydrates and CO2 is the waste product that is exhaled when animals breathe. Photosynthesis absorbs atmospheric CO2 and transforms it into carbohydrates.

Decay by the decomposers and detritivores releases the nutrients locked up in dead organic matter along with CO2 into the atmosphere.

Combustion breaks the long-chain hydrocarbons found in fossil fuels and wood and releases CO2 into the atmosphere. This is an area in which human activity is influencing the carbon cycle. Burning fossil fuels releases carbon that has been stored for millions of year and it becomes an active part of the cycle.

At the surface of the ocean CO2 is exchanged between the water and the atmosphere. CO2 simply dissolves into the surface waters of oceans and some diffuses back out. Once in the ocean the dissolved CO2 is used in photosynthesis by marine plants (turned into organic matter). Marine animals also use this dissolved carbon dioxide to make calcium carbonate to build skeletons and shells. The dissolved CO2 is quickly used up so more can be added to the ocean form the atmosphere. In this way the ocean acts as a carbon sink holding as much as 60 times more carbon than the atmosphere.

As plant and animal life dies in the ocean it has two fates – it is eaten and the carbon stays within the food chain and close enough to the surface to remain an active part of the carbon cycle. Alternatively it sinks to the bottom to circulate in the deep ocean currents or forms sediments on the ocean floor. These flows remove carbon from the active part of the cycle for a long time, maybe hundreds of years.

'Feast in the deep'

Monterey Bay Aquarium Research Institute (MBARI)

This movement of carbon from the atmosphere through the marine food web into the deep oceans is referred to as the biological pump. Techniques to manipulate the biological pump are currently being investigated as a way of lowering atmospheric levels of carbon dioxide.

Sediments can enter the ocean as described in the previous paragraph or they can be washed in with the rivers as they bring weathered rock from the land. As they sink to the ocean floor they go through the process of sedimentation to form rocks. Calcium carbonate (carbon) within the shells and the fossilized skeletons of animals are then trapped for millions of years. In very specific circumstances gas and oil will form.

Examiner Tip

You may be given information about the carbon stores and flows in an ecosystem. From the information you could be asked to produce a diagram to represent that information - you may have to do a sketch or an accurate diagram with the boxes and arrows to scale so read the question carefully.

Example question:

In a mid latitude, mixed woodland ecosystem the atmospheric store of carbon is 755 GtC yr-1. The plants store 560 GtC yr-1, they absorb 120 GtC yr-1 through photosynthesis but returns 60 GtC yr-1 in plant respiration. The soil carbon stores is the largest stores with 2,330 GtC yr-1 but 60 GtC yr-1 moves from the soil into the atmosphere through decomposition.

GtC yr-1 = gigatons of carbon per year

This is the sort of information you may be given and then be required to draw an accurate diagram as below:

Carbon cycle and human interaction

Humans depend on the carbon cycle in ways that we may not be aware of but we are altering it in many ways – reducing some stores, increasing others, and speeding up and adding to the magnitude of some of the flows. Each year humans harvest food, fuel wood and fibre from the land, that amounts to approximately 25% of all plant biomass.

The carbon cycle regulates climate through the amount of CO2 in the atmosphere. That store has been increasing steadily since 1760 when it was 280 ppm to early 2015 when it broke the 400 ppm mark.

Most of this increase is blamed on the industrial revolution and our subsequent reliance on fossil fuels for...well for everything really. If you stop and think about your lifestyle you will quickly realise that just about everything in your life relies on fossil fuels on some way or another. Anything that needs transportation almost definitely involves the combustion of fossil fuels at some point, packaging, food etc.

It is not just burning fossil fuels that is the problem, we are also changing land use on a massive scale. Deforestation for agriculture and urban growth releases carbon from the soil and plant biomass. To make matters worse it also means fewer trees are available to remove CO2 from the atmospheric store. The counter-argument to this is that as awareness of global climate change has grown we have started reforestation. That coupled with increasing cropland efficiencies there is an overall increase of land based photosynthesis CO2 uptake. The figures are still not good; according to the Millennium Ecosystems Assessment anthropogenic (human based) emissions were around 9 GtC yr-1 in 2009. Increased photosynthesis absorbed 5 GtC yr-1 of that leaving an additional 4 GtC yr-1 in the atmosphere.

As you would expect, the nitrogen cycle describes the way nitrogen moves around the biosphere. Nitrogen is essential for all life on earth as it is a major component of amino acids - the building blocks for protein and nucleic acids (DNA and RNA) – the building blocks of life. Just as humans are changing the carbon cycle, so we are changing the nitrogen cycle too.

Stores

The major nitrogen stores are shown in Table 1 below. Note the amounts of nitrogen are far less than those of carbon. We can see that the biggest nitrogen store is the atmosphere; in fact nitrogen makes up 78% of the atmospheric gases. The problem is that N2 gas is all but useless to the ecosystem, because plants can only take in nitrogen in solid form – ammonium and nitrate. So despite an abundance of nitrogen in the system its availability is often a limiting factor in net primary productivity. Soil has a significant store of nitrogen in the form of N2 gas, nitrates (NO3-) and ammonium (NH4+). Plants hold nitrogen in their biomass as a micro-nutrient and ocean water contains nitrogen in solution. The diagram below it shows the nitrogen stores drawn in proportion to each other, the atmospheric store is too big to be shown.

Transformations

As we already learned plants can not use atmospheric nitrogen so how does it become available to plants and the rest of the food chain? It is transformed between its various chemical forms; the major ones are ammonium (NH4+), nitrate (NO3-) and nitrogen gas (N2). Transformations occur through biological (performed by microbes) or physical processes.

The majority of the nitrogen in terrestrial ecosystems comes from the atmosphere through biological fixation where bacteria transform nitrogen into ammonium (toxic to plants in large quantities) or through physical fixation in the lightning process.

Lightning has enough energy to transform atmospheric nitrogen into nitrogen oxides, which then combine with moisture in the air to form nitrates that can enter the soil in rainwater. Lightning strikes in the soil will have the same impact on any N2 present in soil pore spaces. This processes moves around about 30 million tonnes of nitrogen into the soil every year.

Lightning fixation only accounts for a small proportion of the transformations from N2 gas to useable compounds. Specialised microorganisms such as bacteria and cyanobacteria do most of the work. This biological nitrogen fixation transforms atmospheric nitrogen into ammonium. Plants can use ammonium but in large quantities it becomes toxic so another transformation is needed. That transformation processes is nitrification in which other bacteria transform ammonium into nitrates. The nitrates are absorbed by the plant, and then assimilated into the biomass to form amino acids, nucleic acids and chlorophyll.

Many of these microorganisms have mutualistic relationships with plants. Legumes (beans and peas) are particularly well known examples of this type of mutualism. The nitrogen-fixing bacteria live in root nodules where they benefit from a moist safe environment with a good supply of carbohydrates from the plants. In return the plant receives nitrogen in useable form. It is estimated that between 140-200 million tonnes of nitrogen move from the atmosphere into the ecosystem (soil) every year.

Nitrogen fixation accounts for about 10% of all plant nitrogen needs, the rest comes from the ammonification of organic material. Decomposers such as fungi and bacteria breakdown dead organic matter and turn the nitrogen into ammonium, ready for nitrification.

Nitrogen will leave the ecosystem in one of three ways:

• Special bacteria remove nitrates from the soil, convert them into nitrous oxides or N2 gas and release them back into the atmosphere. This process is called denitrification. This happens under anaerobic (oxygen depleted) conditions found in wetlands and bogs where waterlogged soils are common.
• Leaching may dissolve the soluble nitrates and wash them out of the soil. They may just be moved down the soil profile into the ground water or into nearby surface water bodies and into the oceans.
• Combustion releases nitrogen locked up in organic material as nitrogen oxide or nitrogen dioxide.

Detailed knowledge of the role of bacteria in is not required.

Be Aware

You do not need to know the details of how bacteria perform nitrogen fixation, nitrification or ammonification but you do need to have some knowledge of how nitrogen moves around the cycle and how it is transformed.

Nitrogen cycle and human activity

Humans have some major impacts on the nitrogen cycle:

• Fossil fuel combustion and forest fires increases nitrogen oxides, an urban air pollutant, that also contribute to acid rain and photochemical smog.
• With the increase in human population we are draining wetlands to allow for expansion of urban areas, agricultural expansion. Since denitrification takes place in wetland areas denitrification is reduced and less nitrogen enters the atmosphere.
• To ensure high net primary productivity in agricultural crops farmers use inorganic fertilizers. This has two impacts on the nitrogen cycle:
  • Firstly in the production of fertilizers ammonia (raw material of inorganic fertilizers) is produced using the Haber-Bosch process. This takes nitrogen from the atmosphere thus supplementing the natural fixation rates. Currently this removes about 90 million tonnes of nitrogen from the atmosphere each year.
  • Secondly the application of the inorganic fertilizers increases denitrification and leaching. Increased leaching can lead to eutrophication in water bodies.
• Farmers choosing to reduce dependency on the use of inorganic fertilisers may use leguminous crops to improve the nitrogen status of the soil. This practice increase the biological nitrogen fixation rates so the amount of nitrogen taken out of the atmosphere and moved in to the soil increases.
• Pastoral (livestock ranching) agricultural practices impact the nitrogen cycle by increasing the amount of ammonia that enters the soil. Livestock releases large amounts of ammonia in their waste and that enters the soil and potentially nearby aquatic ecosystems.

All agricultural references will be further explained in subtopic 5.2 and 5.3 so you only need an overview at this stage.