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| Nutrient cycling |
Within an ecosystem, nutrients move through biogeochemical cycles. Those cycles involve chemical exchanges of elements among the earth’s atmosphere, water, living organisms, soil, and rocks.
All biogeochemical cycles have a common structure, sharing three basic components: inputs, internal cycling, and outputs.
Input of Nutrients
The input of nutrients to an ecosystem depends on the type of biogeochemical cycle. Nutrients with a gaseous cycle, such as carbon and nitrogen, enter an ecosystem from the atmosphere. For example, carbon enters ecosystems almost solely through photosynthesis, which converts carbon dioxide to organic carbon compounds.
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Nitrogen enters ecosystems through a few pathways, including lightning, nitrogen-fixing bacteria, and atmospheric deposition. In agricultural ecosystems, nitrogen fertilization provides a great amount of nitrogen influx, much larger than by any other influx pathways.
In contrast to carbon and nitrogen with input fromthe atmosphere, the input of nutrients such as calcium and phosphorus depends on the weathering of rocks and minerals.
Soil characteristics and the process of soil formation have a major influence on processes involved in nutrient release to recycling pools. Supplementary soil nutrients come from airborne particles and aerosols, as wet or dry depositions. Such atmospheric deposition can supply more than half of the input of nutrients to some ecosystems.
The major sources of nutrients for aquatic ecosystems are inputs from the surrounding land. These inputs can take the forms of drainage water, detritus and sediment, and precipitation. Flowing aquatic systems are highly dependent on a steady input of detrital material from the watershed through which they flow.
Internal Cycling
Internal cycling of nutrients occurs when nutrients are transformed in ecosystems. Plants take up mineral (mostly inorganic) nutrients from soil through their roots and incorporate them into living tissues. Nutrients in the living tissues occur in various forms of organic compounds and perform different functions in terms of physiology and morphology.
When these living tissues reach senescence, the nutrients are usually returned to the soil in the form of dead organic matter. However, nitrogen can be reabsorbed from senescent leaves and transferred to other living tissues.
Various microbial decomposers transform the organic nutrients into mineral forms through a process called mineralization. The mineralized nutrients are once again available to the plants for uptake and incorporation into new tissues.
This process is repeated, forming the internal cycle of nutrients. Within the internal cycles, the majority of nutrients are stored in organic forms, either in living tissues or dead organic matter, where as mineral nutrients represent a small proportion of the total nutrient pools.
Output of Nutrients
The output of nutrients from an ecosystem represents a loss. Output can occur in various ways, depending on the nature of a specific biogeochemical cycle. Carbon is released from ecosystems to the atmosphere in the form of carbon dioxide via the process of respiration by plants, animals, and microorganisms.
Nitrogen is lost to the atmosphere in gaseous forms of nitrogen, nitrous oxide, and ammonia, mostly as by-products of microbial activities in soil. Nitrogen is also lost through leaching from the soil and carried out of ecosystems by groundwater flow to streams. Leaching also results in export of carbon, phosphorus, and other nutrients out of ecosystems.
Output of nutrients from ecosystems can also occur through surface flow of water and soil erosion. However, loss of nutrients from one ecosystem may represent input to other ecosystems.
Output of organic matter from terrestrial ecosystems constitutes the majority of nutrient input into stream ecosystems. Organic matter can also be transferred between ecosystems by herbivores. For example, moose feeding on aquatic plants can transport nutrients to adjacent terrestrial ecosystems and deposit them in the form of feces.
Considerable quantities of nutrients are lost permanently from ecosystems by harvesting, especially in farming and logging lands, when biomass is directly removed from ecosystems. Fire usually results in the loss of large amounts of nutrients. Fire kills vegetation and converts portions of biomass and organic soil matter to ash.
Fire causes loss of nutrients through volatilization and airborne particulate. After fire,many nutrients become readily available, and nutrients in ash are subject to rapid mineralization. If not taken up by plants during vegetation recovery, nutrients are likely to be lost from ecosystems through leaching and erosion.
The Hubbard Brook Example
Nutrient cycling has been studied in several intact ecosystems. One of the most notable experiments was conducted in the Hubbard Brook experimental forest in New Hampshire. The experimental forest was established initially for forest hydrology research.
Begun in the early 1960’s, one of the longest-running studies of water and nutrient dynamics of forest ecosystems has been on the Hubbard Brook site. Both water and nutrient concentrations in precipitation inputs and stream outputs were regularly monitored, allowing estimations of nutrient balances over the watershed ecosystems.
One of the major findings from the Hubbard Brook study was that undisturbed forests exhibit regularity and predictability in their input-output balances for water and certain chemical elements.
Nitrogen, however, shows a more complex, but still explicable, pattern of stream concentrations. Losses of nitrates from the control water shed are higher in the dormant season, when biological activity is low.
Losses are near zero during the growing season, when biological demand for nitrogen by plants and microbes are high. Removal of vegetation in the Hubbard Brook forest had a marked effect on water and nutrient balances. Summer stream flow during the devegetation experiment was nearly four times higher than in the control watershed.
The increase in stream flow, combined with increases in the concentration of nutrients within the stream, resulted in increases in loss rates of nitrate much higher than those of undisturbed areas. Similarly, loss of potassium used in large quantities by plants showed a great increase.
Nutrient Uptake and Competition
Ecosystem nutrient cycling is critical for plant growth and ecosystem productivity. Plant uptake of essential nutrient elements is related to nutrient availability, root absorption surface, rooting depth, and uptake kinetics of roots. A nutrient-rich site usually supports more plants of different species than a site with fewer available nutrients.
Nutrient competition among plants is usually manifested through physiological, morphological, and ecological traits. Usually grasses and forbs can coexist in one grassland ecosystem, for example, through different rooting depth.
To compete for less soluble nutrients such as phosphorus, plants usually extend their root surfaces using symbiotic relationships with mycorrhizae. Differential seasonality in nutrient uptake and rooting depth become more critical to compete limited nutrients.
