Decomposition is an essential ecological process. All living organisms use chemical elements such as carbon, nitrogen and phosphorus as building blocks to grow and build tissue. Once used, these essential elements are locked-away in the biomass of organisms and cannot be used by others. Once an organism dies, their bodies (their biomass) is being recycled so the building blocks can be reused. The break-down and recycling of dead organic material is called decomposition, and carbon is one of the key elements being recycled during decomposition.
The speed with which dead plant litter is decomposed, and how much of the carbon is released and stored is determined by three factors. 1) the presence of decomposers, soil organisms such as bacteria, fungi, soil mites or worms, 2) the chemical composition of the plant litter determines whether it is easy or difficult to break-down, and 3) the local conditions, such as the temperature and the availability of water and nutrients, determine how active the decomposers are.
The global carbon cycle describes how carbon atoms move from one compartment to the next. The biological carbon cycle starts with plants and other organisms that photosynthesize. Plants use light to capture CO2 from the air and convert it to sugars, in a process called photosynthesis. Just like solar panels, plants store the energy of sunlight. Plants use the captured energy and carbon to grow and, for example, make leaves. When the leaves fall to the ground, soil organisms eat the dead leaves. Some of the carbon stored in the leaves is used by the decomposers to grow, some is released as CO2 or CH4 gas and returns to the air, the rest of the carbon is stabilised and stored in the soil. Peatlands are carbon-storage champions. Decomposition in peatland ecosystems is slow because of the natural conditions. As a result, more carbon is captures by the plants than is recycled by the decomposers. This means that peatlands are a natural carbon sink. Over thousands of year, all the organic material that is not broken down is piled-up and forms thick layers of peat. About 30% of all the carbon in soils is stored in peatlands. If natural conditions change, decomposers may break-down all that stored carbon. This already happens when peatlands are used for forestry, or to collect turf used for potting soil and fuel. Because of their capacity to store carbon, peatlands are our natural friend in fighting climate change. This makes protecting natural peatlands, and restoring degraded peatlands extra important.
The role of soil properties & climate in decomposition
Climate
The soil organisms responsible for decomposition, the decomposers, are most active when conditions are not too cold, not too warm, not too dry, not too wet. Climatic conditions give us an indication of the conditions the decomposers experience. How much rainfall is there, how often does it rain? How warm is it during the summer, are there large temperature differences from day-to-day or between night and day? National meteorological institutes have measurement stations across each country to record the weather conditions. All this information is stored in databases. Researchers can use these databases to estimate the average climate conditions for most locations on Earth and observe whether the climate changes over time.
Soil Bulk Density
The bulk density of a soil tells us how loose the soil is. It is measured as the weight of dry soil per volume, gram /cm3. For most soils, half of the soil is made-up of solid particles (organic matter and mineral particles) and the other half contains water and air. The density of soils determines how easy water and air can move around, and how easy roots and other soil organisms can find their way in soils. A soil is less dense if it contains more space for water and air, and if it contains more organic matter (dead plant remains). Some soils, like peatlands, are very rich in organic matter and have a very low bulk density. In other soils, like clay or the soil where lots of people or machines move around, all the solid particles are packed together in a small space. These soils have a high bulk density.
Soil pH
The pH of a soil refers to how acidic or alkaline the soil is; pH indicates the amount of H+ ions that are available for (bio)chemical reactions. This is important for both plants and soil organisms because the pH determines how easy or difficult some chemical reactions take place that they rely on. For example, some nutrients are bound to soil particles at a low pH but are available to be taken up at a higher pH.
Soil pH is influenced by the type of soil, the vegetation and environmental pollution. Plants with roots leak acids into the soil on purpose in a process called exudation. Acid exudates chemically fee nutrients from the soil so the plant can use them for growth. Sphagnum mosses, the builders of peat, use a similar trick. Sphagnum mosses produce an acid called sphagnan to release nutrients they need to grow. Plants also produce organic acids as part of their tissues and to build strong cells. When plants die and their dead tissues are decomposed, these acids end-up in the soil, changing the soil pH. This feedback between soil conditions and plant growth complicates the ecological role of soil pH. In fact, soil pH is both the result of ecological processes AND an important influencing factor shaping soil processes.
Soil Moisture
The soil moisture content represents how much water is in the soil. Soil organisms and plants need water to grow. Water surrounds soil particles as a thin film and sits in the spaces between the particles. Many decomposer organisms live in these water films. But plant roots and many soil organisms need air as well as water. Too much water, however, is not good. When soils are water-logged or flooded all the air is pushed out. That is why many soil processes, like decomposition, have a soil moisture optimum: not too dry, not too wet.
Soil Texture
Soil is composed of air, water and solid particles, namely organic matter, sand, silt and clay. Sand, silt and clay are mineral particles of different sizes. The proportion of sand, silt and clay is represented by soil texture, the way the soil ‘feels’. Soil texture gives a lot of information about how plants and decomposers ‘experience’ their environment. It is related to how many nutrients and water a soil can hold, how much air spaces there are (important for breathing), how well water flows away after a heavy rainstorm. Measuring soil texture informs us about soil behaviour and soil health.
