Material cycles and decay

Many different materials cycle through the abiotic and biotic components of an ecosystem. All materials in the living world are recycled to provide the building blocks for future organisms — the carbon atoms in you were in something else before, and will be again.

The carbon cycle returns carbon from organisms to the atmosphere as carbon dioxide, to be used by plants in photosynthesis.

The water cycle provides fresh water for plants and animals on land before draining into the seas. Water is continuously evaporated and precipitated.

You are not expected to study the nitrogen cycle.

The water cycle has four processes to name, and a question showing you a diagram will ask you to explain them rather than list them.

• Evaporation — energy from the Sun turns liquid water at the surface of the seas into water vapour. Plants add to this by transpiration, losing water vapour through their stomata. • Condensation — the vapour rises, cools, and condenses back into tiny droplets, forming clouds. • Precipitation — the droplets join until they are heavy enough to fall as rain, snow or hail, much of it onto land. • Runoff and collection — the water drains through the soil and along rivers, back into the seas, and the cycle repeats.

Why it matters to living organisms is the part most often left out: the cycle provides fresh water for plants and animals on land. Sea water is salty and unusable for most organisms, and evaporation leaves the salt behind — so the cycle is what turns an ocean into drinkable water, continuously, and delivers it inland.

The role of microorganisms is the part most often left out of an answer. Decomposers break down dead plants and animals and waste, returning carbon to the atmosphere as carbon dioxide through their own respiration, and returning mineral ions to the soil where plants can absorb them again. Without them the nutrients would stay locked in dead material and the cycle would stop.

Plants need nitrogen to make amino acids and therefore proteins. The air is almost four-fifths nitrogen gas, and a plant can use none of it: the molecule is too unreactive. What a plant can absorb is nitrates, dissolved in water, taken up through the roots. Getting nitrogen out of the air and into a nitrate is the work of bacteria, and four groups do it — knowing which does what is most of this topic.

Lightning also fixes a small amount of nitrogen directly, and the whole cycle closes when animals eat plants, take in the plant protein, and return nitrogen to the soil in their waste and when they die.

Farmers work with the cycle in three ways. Fertilisers add nitrates directly, which is quick but expensive and washes into waterways. Crop rotation grows a legume such as clover or peas every few years, so the nitrogen-fixing bacteria in its root nodules restore the soil for free — and ploughing the crop back in adds the nitrogen it holds. Draining and ploughing land keeps the soil aerated, which favours the nitrifying bacteria that make nitrates and works against the denitrifying bacteria that destroy them.

Three factors affect the rate of decay of biological material, and each acts on the microorganisms doing the decaying:

• Temperature — warmth speeds up the enzymes in the decomposers, up to an optimum; too hot and those enzymes denature, so decay slows again. • Water — decomposers need moisture to survive and to secrete their enzymes; dry material barely decays, which is why food is preserved by drying. • Availability of oxygen — most decomposers respire aerobically, so more oxygen means faster decay.

Gardeners and farmers try to provide optimum conditions for rapid decay of waste biological material. The compost produced is used as a natural fertiliser for growing garden plants or crops — which is why a compost heap is kept moist, turned to let air in, and works faster in summer.

Anaerobic decay produces methane gas. Biogas generators can be used to produce methane as a fuel — decay without oxygen, deliberately, so the gas can be collected and burned.

Handling decay data is examined alongside the biology. Calculate rate changes as 1 ÷ time, then translate the information between numerical and graphical form: temperature on the x-axis, rate on the y-axis, a scale for the axes that fills at least half the grid, and a smooth curve rather than joined dots. Reading back the other way, quote the rate at a stated temperature with its units, and describe the curve as rising to an optimum and then falling — the same enzyme shape as every other rate graph in the course.

Some species survive only within a narrow range of conditions. Because of that, finding them — or failing to find them — measures the conditions without any equipment at all. A species used this way is called an indicator species.

In water, the measure is usually dissolved oxygen, which falls when organic pollution such as sewage or fertiliser runoff feeds a bloom of decomposers. Bloodworms and sludgeworms tolerate very low oxygen levels, so finding them in numbers indicates polluted water. Stonefly nymphs and freshwater shrimp need high oxygen levels, so finding them indicates clean water.

In air, the measure is usually sulfur dioxide. Lichens are sensitive to it, so a wide variety of lichens on trees and walls indicates clean air, and bare bark indicates polluted air. Blackspot fungus on rose leaves behaves the same way — it disappears where the air is polluted.

Evaluating the method is what the higher-tier marks are usually for, so hold both sides ready. It is cheap, needs no equipment and no expertise beyond identification, and because the organisms have lived there for a long time it reflects conditions over months rather than at one instant.

Against that, it is qualitative: it tells you the water is polluted, not by how much and not with what. Other factors affect which species are present — temperature, acidity, the type of riverbed, whether a species ever reached the site at all — so an absence has more than one possible cause. A sensor or a chemical test gives a number and names the pollutant, but only for the moment it is taken.