Concepts: Productivity and decomposition

2.1 Productivity

Every ecosystem needs a steady input of solar energy. Primary production is the amount of biomass or organic matter that plants produce per unit area over a period of time during photosynthesis. It is measured in terms of weight, such as grams per square metre, or energy, such as kilocalories per square metre. The rate of that production is productivity, written as gram per square metre per year or kilocalorie per square metre per year, and the rate is what allows comparisons between ecosystems.

Productivity has two parts. Gross primary productivity, or GPP, is the rate at which organic matter is produced during photosynthesis. Plants use a considerable share of that for respiration. Subtract the respiration loss R from GPP and the remainder is net primary productivity, NPP: GPP - R = NPP. NPP is the biomass available to heterotrophs, meaning herbivores and decomposers. Secondary productivity is the rate at which consumers build new organic matter.

Primary productivity depends on which plant species occupy the area, on environmental factors, on nutrient supply and on the photosynthetic capacity of the plants, so it differs from one ecosystem to another. The whole biosphere produces about 170 billion tons of organic matter a year on a dry weight basis. The oceans cover about 70 per cent of the surface but contribute only 55 billion tons; the rest comes from land. Why so low? Sunlight penetrates only the top layer, and open water often runs short of nutrients such as nitrogen and phosphorus.

2.2 Decomposition

Fallen leaves, bark, flowers and the dead bodies and droppings of animals together form detritus. This material is the raw material for decomposition, the process that breaks complex organic matter into carbon dioxide, water and inorganic nutrients.

Five steps are named in the textbook. Earthworms and similar detritivores break detritus into smaller pieces, which is fragmentation. Leaching carries water-soluble inorganic nutrients down into the soil, where they precipitate as salts that are not immediately available. Bacterial and fungal enzymes then degrade detritus into simpler inorganic substances, which is catabolism. Humification produces humus, a dark amorphous substance that resists microbial attack and breaks down very slowly; being colloidal, it acts as a store of nutrients. Mineralisation is the further breakdown of humus by microbes, releasing inorganic nutrients.

The steps do not happen one after another in a neat line. Fragmentation, leaching and catabolism operate on the detritus at the same time, and humification and mineralisation go on in the soil alongside them.

2.3 What controls the rate

Decomposition needs oxygen. Its rate depends on the chemical composition of the detritus and on climatic factors. Under the same climate, detritus rich in lignin and chitin breaks down slowly, while detritus rich in nitrogen and in soluble substances such as sugars breaks down quickly. The two climatic controls that matter most are temperature and soil moisture, since both set how active the soil microbes are. Warm and moist conditions favour decomposition; low temperature and anaerobic conditions slow it and allow organic material to build up, which is how peat and similar deposits form.

2.4 Reading the productivity figures

The gap between gross and net primary productivity is not a small correction. A forest uses a large share of its gross production in respiration, so net production can be less than half the gross figure. The energy that finally reaches herbivores is smaller again, because they take only part of the plant material and lose much of it in their own respiration. This cascade is why a single number for productivity should always be labelled as gross or net. In an exam, an unlabelled figure is treated as an error even when the arithmetic is right.

2.5 Why decomposition matters to the next crop

Without decomposition the nutrients locked in dead matter would stay unavailable. Every step in the process moves material closer to a form a plant can absorb again. Humus is the slow stage, holding nutrients in the soil for a long time, while mineralisation is the release valve that frees them for uptake. A soil rich in humus therefore keeps a reserve that a crop can draw on, which is why organic matter in soil is treated as an asset in farming rather than only a stage in a cycle. The same soil also holds moisture better, so the benefit extends to dry spells as well.

2.6 Detritus in the field

Anyone who has lifted a leaf pile after rain has seen decomposition at work. The leaves at the bottom have turned dark and crumbly, which is humus, while the fresh ones on top are still intact. The same process runs in forest litter, in farmyard manure and in the mud of a pond. What changes between these places is the moisture and temperature, and therefore the speed. In a cold mountain forest the same leaf layer may take years to break down, while in a warm moist lowland it disappears within months.