Nutrient Cycling and Decomposition
The source of all essential nutrients is either in the atmosphere or in the form of rocks and minerals around us.
These nutrients make their way through living and inorganic forms through nutrient cycling. As organisms are develop, they take in these nutrients and store them in their tissues and/or use them for life’s processes, effectively locking them up until such a time as they are released back into the global pool of nutrients from which other organisms can draw from through a process known as decomposition.
Decomposition is the breakdown of the chemical bonds which were formed during the construction of plant and animal tissues and is really a complex process made up of leaching, fragmentation (not to be confused with landscape fragmentation), changes in physical and chemical structure, ingestion, and excretion of waste. Decomposers are organisms that feed on dead organic matter or detritus and include the microbial decomposers, made up of fungi and bacteria, and the detritivores, made up of animals which feed primarily on feces. These decomposers can be further broken down into the microflora (aerobic and anaerobic bacteria as well as fungi), microfauna (organisms less than 100um, primarily protozoans and nematodes), mesofauna (100um to 2mm, mostly potworms and springtails), macrofauna (between 2-20mm), and megafauna (any organism larger than 20mm). Organisms that feed on the microbes themselves are known as microbivores, thus ensuring that the nutrients are cycling fully.
Whenever organic matter is transformed from organic into inorganic constituents by decomposers, it is known as mineralization. When these same decomposers take in and use those nutrients for their own life processes, we call this immobilization. Decomposed nutrients are always in a cycle of mineralization and immobilization.
Not all organic matter decomposes at the same rate, and the rate of decay is related to two factors: the quality of the substrate, judged by the amount of lignin which is a series of compounds which yield almost no energy to microbes, and the features of the environment, such as temperature, pH, and precipitation. In warmer, more humid environments, rates of decomposition are very high and the soil organic matter has a residence time of only 1-2 years. However, in drier, colder, regions of the world, organic matter in soil can stay relatively untouched for thousands of years.
All ecosystems have a vertical separation between the zones of primary productivity and the zones of decomposition. Terrestrial and coastal ecosystems are bridged physically by plants, while open-water ecosystems are separated into distinct zones divided by relative temperature: epilimnion (nutrient poor warm surface water), thermocline (middle waters), and hypolimnion (nutrient rich cold deep water). In open-water ecosystems, the changes in surface water temperature over the seasons causes the thermocline to break down and the epilimnion and hypolimnion to mix in a process known as turnover.
Aquatic environments, such as open-water ponds, lakes, and oceans, have living and dead organisms drift between the layers of the water column known as particulate organic matter and dissolved organic matter. Aerobic bacteria break down the material at the top very quickly while anaerobic bacteria break down the matter at the bottom very slowly. However, despite the bacterial process being slower at the bottom, the relatively large sink of nutrients that are at the bottom lead to a relatively higher amount of decomposition taking place at the bottom of open-water regions as opposed to the top.
Terrestrial environments are driven by the soil microbial loop, due to the actions of plants within a region of the soil known as the rhizosphere. Within the rhizosphere, plant roots release carbohydrates, known as root exudates, in order to supplement the microbe communities present in the soil. With an abundant source of carbon provided by the plants, the limiting factor for the microbes becomes other nutrients, and so they begin to break down the organic matter found in the soil. This sudden increase in microbial populations attracts microbivores which then release more nutrients into the system, allowing the plants to happily take in all these newly available nutrients.