In this article we will discuss about:- 1. Definition of Forest Nutrient Cycling 2. Types of Nutrient Cycling 3. Pathways 4. Gaseous Inputs and Outputs 5. Nutrient Deposition.
Definition of Forest Nutrient Cycling:
Forest nutrient cycling is defined as the processes of nutrient uptake, incorporation of mineral nutrients into biological tissues of plants and trees, litter fall and the decomposition of organic matter with the concomitant release of nutrients to soil by microorganisms.
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To maintain all the biochemical reactions necessary for plant growth, at least 18 essential mineral nutrients are required from the geosphere. These nutrients are extracted from the soil by plant uptake and returned to the soil directly or indirectly as organic matter.
The organic matter in turn is a source of energy for heterotrophs which further oxidize the organic compounds by decomposition processes (i.e., by respiration), simultaneously releasing the mineral nutrients back to the soil. Thus nutrient cycling is essential to the cyclic flow of nutrients in forest ecosystems.
Nutrient Pool in Vegetation:
The quantity of nutrients contained in the living biomass of an ecosystem is termed as nutrient pool in vegetation. The total biomass and nutrient pools vary widely between contrasting forest types. Vegetation pool is typically segregated by individual species within an ecosystem and within a given species into various components- foliage, woody tissue, bark and roots. Several rotations of nutrient removal by forest harvest from low nutrient sites may result in a loss of productivity in subsequent rotations.
Plant Uptake:
The nutrients are taken into a plant by root and foliar uptake. But foliar uptake generally represents a very small proportion of the total nutrient uptake. Mycorrhizae fungi associations with plant roots can enhance the availability and uptake of several essential nutrients (e.g., phosphorus, copper, calcium and iron). Foliar uptake consists of ion uptake from wet deposition across the cell membrane or direct absorption of gases (e.g., SO2 and NO2).
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The annual uptake (or nutrient demand) changes greatly over the course of development of a forest from establishment to maturity. During the period of early establishment, nutrient uptake will increase with increasing gross ecosystem production.
Uptake rates will peak at approximately the time of canopy closure. Following canopy closure, gross production slows due to competition and mortality and more carbon is allocated to woody materials having a lower nutrient content (e.g., stem). Thus, uptake rates will decline from their maximum level at canopy closure and maintain a relatively constant value.
Growth Requirement of Nutrients:
The growth requirement is defined as the total amount of a nutrient that is required each year by vegetation to meet its demands for nutrients associated with the annual increment, litter fall and canopy leaching.
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The requirement is obtained by uptake and re-translocation processes:
Growth Requirement = Uptake + Re-translocation
Growth Requirement = Annual Increment + Litter Fall + Canopy Leaching
Canopy Leaching = Through Fall + Stem Flow – Atmospheric Deposition
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Re-Translocation:
Nutrient removal from plant tissue into the perennial part of the plant prior to senescence is termed as re-translocation. Re-translocation occurs primarily from foliage and fine roots. Re-translocated nutrients are available for reuse by the plant in the next growth cycle.
The amount of re-translocation is dependent on the mobility of the element, the tree species and the nutrient status of the soil. The percentage of nutrients re-translocated is generally greater on nutrient-poor sites reflecting the role of re-translocation as a nutrient conservation mechanism.
Annual Increment:
The net annual elemental addition of a nutrient incorporated into the above-ground and below-ground biomass is called annual increment. In other words, the amount of a nutrient added to storage in vegetation biomass each year. The annual increment is very small compared to growth requirement and uptake. As a forest reaches steady state, the annual increment of a forest ecosystem approaches zero as the growth of biomass is balanced by mortality and litter fall returns.
Litterfall:
The return of nutrients to the soil via senescence of plant tissues is termed as litterfall. The primary components of litter fall are foliage, woody tissues, roots and reproductive tissues. Litter fall nutrient fluxes vary widely between forest types and with the stage of forest development. Litter fall fluxes are greater for deciduous trees compared to coniferous trees growing in the same climatic zone. Litter fall fluxes show an increase with decreasing latitude: tropical > temperature > boreal. This progression in litter fall corresponds with an increase in net primary productivity from boreal to tropical ecosystems.
Throughfall and Stemflow:
Precipitation that passes through a vegetation canopy reaches the soil surface as either canopy throughfall or stemflow. As the water penetrates the canopy and flows down the stems, nutrients may be leached from or taken up by the plant tissue. Thus, nutrient concentrations may either increase or decrease relative to the amount input by atmospheric deposition (e.g., wetfall and dryfall).
Net Canopy Exchange = Total Deposition (Wet + Dry) – (Throughfall + Stemflow)
Herbivory:
The consumption of living tissue by herbivores is termed as herbivory. Herbivory short-circuits the nutrient cycle by consuming living tissues prior to senescence and litterfall. Many of the nutrients may be returned directly to the ecosystem by herbivore defecation and death or the nutrients may be transferred to other ecosystems by herbivore migration. In most forest ecosystems, herbivores consume less than 10 per cent of the annual net primary productivity.
Decomposition:
Decomposition is defined as the interrelated processes by which organic matter is broken down to CO2 and humus with a simultaneous release of nutrients. These processes are a critical link responsible for recycling of nutrients in the intra-system nutrient cycling. The decomposition process often begins with soil macrofauna which physically break down the particle-size of organic matter and predigesting organic materials. Action by microorganisms results in the progressive breakdown of organic matter.
Litter → partially decomposed organic matter → Humus
Factors Affecting Decomposition:
The rate of decomposition is dependent on litter quality, microbial activity and environmental conditions. Litter quality includes nutrient content (e.g., C/N ratio), composition of organic matter especially lignin concentrations (lignin/nitrogen ratio) and concentrations of polyphenols (including tannins). Soil temperature and moisture content are very important factors affecting decomposition rates.
i. Immobilization:
Immobilization is the conversion of an element from an inorganic to organic form by microorganisms.
ii. Mineralization:
Mineralization refers specifically to those decomposition processes that release inorganic compounds from organic matter.
The amount of a nutrient released by mineralization that is available for plant utilization (uptake) is referred to as net mineralization:
Net Mineralization = Gross Mineralization – Immobilization
Types of Nutrient Cycling:
A nutrient cycle is the movement and exchange of organic and inorganic matter back into the production of living matter. The process is regulated by food web pathways that decompose matter into mineral nutrients. Nutrient cycles occur within ecosystems. Ecosystems are interconnected systems where matter and energy flows and is exchanged as organisms feed, digest and migrate.
Minerals and nutrients accumulate in varied densities and uneven configurations across the planet. Ecosystems recycle locally, converting mineral nutrients into the production of biomass, and on a larger scale they participate in a global system of inputs and outputs where matter is exchanged and transported through a larger system of biogeochemical cycles.
Biogeochemical cycle is the movement (or cycling) of matter through a system (atmosphere, lithosphere, hydrosphere, and biosphere). Biogeochemical cycles include carbon cycle, nitrogen cycle, sulfur cycle, phosphorus cycle, iron cycle, water cycle, oxygen cycle and others that continually recycle along with other mineral nutrients into productive ecological nutrition.
There are two types of nutrient cycling explained below:
1. Geochemical nutrient cycling.
2. Biological nutrient cycling.
1. Geochemical Nutrient Cycling:
It is an open system which concerns the import export relationship of nutrients into and out of the ecosystem. It includes nutrient imports from such sources as nitrogen and fertilization. The outputs include leaching and erosion losses in drainage water, volatile losses from fire and desertification and removal in harvests.
2. Biological Nutrient Cycling:
It is the closed system which involves plant-soil exchanges of nutrients. It involves the transfer of nutrients between forest floor soil, associated plants and animal communities. It may also include the internal transfer of nutrients among organs within trees. Biological nutrient cycling deals with the translocation of nutrients within standing tree mass and nutrient transfer between soils and trees.
Pathways in Nutrient Cycling:
Out of 126 naturally occurring chemical elements only 18 have been identified as essential elements (or nutrients) without which plants cannot grow and complete their life cycles. Elements essential for plant growth include- C, H, O, N, P, K, S, Ca, Mg (elements used by plants in relatively large amounts called as macronutrients), Fe, Mn, B, Zn, Cu, CI, Co, Mo, Ni (elements used by plants in small amounts called as micronutrients).
These essential nutrients play various important roles in the structure and metabolism of plants. They are continuously cycled through the atmosphere-soil-plant system according to specific transformation and transport processes.
Some of these processes occur in soils and make elements available to plant roots and soil organisms:
i. Carbon Cycle:
The carbon cycle is a bio-geochemical cycle which exchanges carbon between various reservoirs or storage places within the earth system viz. the land, the oceans and the atmosphere.
ii. Nitrogen Cycle:
It is the biogeochemical cycle that describes the transformations of nitrogen and nitrogen containing compounds in nature. It is the transformation of nitrogen from atmosphere (air) to soil, plants, animal life and returns back to air or soil through decay or denitrification.
iii. Phosphorus Cycle:
The phosphorus cycle describes the movement of phosphorus through the lithosphere, hydrosphere and biosphere. The atmosphere does not play a significant role, because phosphorus and phosphorus-based compounds are usually solids at the typical ranges of temperature and pressure found on Earth.
iv. Sulfur Cycle:
It is the cyclic movement of sulfur in different chemical forms from the environment to organisms and then back to the environment from organisms. It includes mineralization of organic sulfur into inorganic forms such as hydrogen sulfide (H2S), elemental sulfur, as well as sulfide minerals and then oxidation of hydrogen sulfide, sulfide, and elemental sulfur (S) to sulfate (SO42-). Finally reduction of sulfate to sulfide and incorporation sulfide into organic compounds occur.
v. Iron Cycle:
The iron cycle is the biogeochemical cycle of iron through landforms, atmosphere and oceans. The cycling of Iron consist of largely of oxidation-reduction (redox) reactions that reduce ferric ion (Fe3+) to ferrous ion (Fe2+) and similarly oxidizes Fe2+ to Fe3+.
Nutrient cycle in forest ecosystem is a dynamic and complex system of geological, chemical and biological cycling through which the soil organic matter and nutrient supplies are replenished and maintained. It ensures continued productivity of the site.
The processes encompassing nutrient cycling are of critical importance to the functioning and evolution of forest ecosystems. Cycling of nutrients in forest ecosystems is an integrating set of processes involving the transfer of energy and nutrients within an ecosystem, including inter-system and intra-system interactions within and between the atmosphere, biosphere, geosphere, and hydrosphere.
Nutrient cycling involves a series of interrelated processes, such as nitrogen fixation, atmospheric deposition of nutrients, organic matter decomposition and mineralization, rock weathering and nutrient uptake. Since one process functions as the precursor to another, the flow of nutrients is linked in a set of specific interconnected steps that ultimately lead to a series of cyclic pathways.
Gaseous Inputs and Outputs:
i. Nitrogen Fixation:
Microorganisms may contribute significantly to nutrient inputs and losses from an ecosystem through oxidation and reduction reactions. The primary processes of importance to forest nutrient cycling are N2 fixation and de-nitrification. Several genera of non-leguminous shrubs and trees have root nodules with symbiotic microorganisms which may fix atmospheric nitrogen.
Nitrogen fixation is performed by a wide variety of microorganisms including symbiotic microorganisms (Rhizobium) and free-living organisms (organotrophs such as Azotobacter and phototrophs such as Cyanobacteria), Actinomycete (such as Frankia common to Alnus, Ceanothus, Casuarina) and lichens (phototrophic association of algae with fungi). Nitrogen fixation rates vary widely among species and ecosystems with a reported range of 2 to 300 kg/ha/year.
ii. Denitrification:
It is an important process in nitrate-rich ecosystems experiencing periodic anaerobic conditions. Under anaerobic conditions, NO3– is used by certain microorganisms as an electron receptor. The process involves microbial reduction of NO3– to NO2‑ and then to gaseous N2O and N2, which are subsequently lost to the atmosphere. Within the context of the global nitrogen cycle, the amount of biological nitrogen fixation (140 x 1012 g N per year) in terrestrial ecosystems nearly balances the estimated loss of nitrogen from terrestrial ecosystems by denitrification (130 x 1012 g N per year).
Nutrient Deposition:
i. Dry Deposition:
Gravitational sedimentation of particles during periods without precipitation. These particles include: aerosols, sea salts, particulate material and adsorbed/ reacted gases captured by vegetation.
ii. Wet Deposition:
The dissolved constituents in precipitation reaching forest ecosystem is called wet deposition. Wet deposition may contribute relatively large annual inputs of nutrients into forest ecosystems, especially in regions affected by air pollution.