Apart from energy flow, ecosystems rely significantly on nutrient cycling as a vital functional aspect. Elements such as carbon, nitrogen, sulfur, oxygen, hydrogen, and phosphorus, among others, circulate through both living (biotic) and non-living (abiotic) components, constituting what is known as biogeochemical cycles. Additionally, water undergoes a cyclic movement recognized as the hydrological cycle. These nutrients progress through the food chain, eventually reaching the detritus compartment, where various microorganisms facilitate decomposition. Within this process, organic nutrients from deceased plants and animals are transformed into inorganic substances through microbial decomposition, becoming accessible for use by plants (primary producers). This cycle perpetuates as plants utilize these nutrients, initiating the cycle anew.
The cycling of nitrogen, a vital nutrient, is illustrated in the below figure. Initially existing in the atmosphere as N2 in a substantial quantity (78%), it undergoes fixation through either the physical occurrence of lightning or biological processes facilitated by certain bacteria and/or cyanobacteria, such as blue-green algae. Plants absorb this nitrogen, employing it in metabolic processes for the synthesis of amino acids, proteins, vitamins, and more, which then move through the food chain. Upon the demise of plants and animals, the organic nitrogen within their deceased tissues undergoes decomposition by various ammonifying and nitrifying bacteria groups. These microorganisms transform the organic matter into ammonia, nitrites, and nitrates, subsequently utilized by plants. Additionally, specific bacteria convert nitrates back into molecular nitrogen (N2), releasing it back into the atmosphere, thus perpetuating the cycle.
At times, human interventions disrupt the natural flow of essential nutrients, leading to imbalances. Consider the well-regulated carbon cycle in nature (refer to the below figure). Carbon, primarily in the form of carbon dioxide, serves as a crucial resource for green plants during photosynthesis, facilitating the creation of various carbohydrates and organic compounds. This carbon traverses through the food chain, eventually returning to the atmosphere through microorganisms’ breakdown of organic carbon in the deceased matter, converting it back to carbon dioxide. While all organisms respire, generating carbon dioxide, plants absorb this gas. However, recent years have witnessed a surge in atmospheric carbon dioxide levels due to activities like fossil fuel combustion, disrupting this natural cycle. Consequently, the world grapples with the pressing issue of global warming, stemming from escalated carbon dioxide emissions.
The phosphorus cycle, depicted in the below figure, stands as a crucial nutrient cycle. Initially housed within rocks and fossils, phosphorus is extracted by humans for fertilizer use. However, this extraction leads to the indiscriminate application of phosphate fertilizers by farmers, resulting in an overflow of phosphates through runoff. This excess contributes to eutrophication in lakes, fostering algal blooms. A significant portion of these phosphates carried by runoff ends up in oceans, settling into deep sediments. Unfortunately, mankind’s excessive exploitation of the limited phosphorus supply in Earth’s phosphate rocks disrupts the natural cycle, with a considerable amount lost to the oceans. This human intervention renders the phosphorus cycle somewhat irregular. Conversely, sea birds play a vital role in phosphorus cycling by consuming phosphorus-rich sea fish and redistributing phosphorus through their droppings on land. Notably, the guano deposits along the coasts of Peru serve as abundant sources of phosphorus.
Sulphur plays a crucial role as a component in specific amino acids and various B-complex vitamins, making it essential for both plants and animals. This element is sourced from the atmosphere in the form of hydrogen sulphide (H2S) and sulphur dioxide (SO2) gases, historically emitted primarily from volcanoes. However, in contemporary times, the burning of fossil fuels has significantly escalated the release of these gases into the atmosphere. In soil, sulphur exists in various forms such as sulphides, sulphates, and organic sulphur. Plants absorb sulphur, which then cycles back to the soil through the food chain. Fungi like Aspergillus and Neurospora break down sulphur from proteins into sulphates through aerobic decomposition, followed by further transformation. Bacteria like Escherichia, in aerobic conditions, produce H2S by decomposing proteins. The current era of widespread industrialization and rapid urban growth has led to a surge in sulphur concentrations in the atmosphere, triggering reactions with moisture that result in acid rain. Consequently, this acidity adversely affects both soil and aquatic ecosystems, posing threats to various organisms. The sulphur cycle is shown below.