7.3 Nutrient Cycling and Bioavailability in Soils
3 min read•july 24, 2024
Soil nutrient cycles are the lifeblood of ecosystems. Carbon, nitrogen, and phosphorus move through complex pathways, transforming and recycling as they go. Understanding these cycles is key to managing soil health and plant growth.
Nutrient bioavailability depends on soil chemistry, microbes, and environmental factors. Sources like weathering and decomposition add nutrients, while processes like and plant uptake remove them. Balancing these flows is crucial for sustainable agriculture and ecosystems.
Nutrient Cycles in Soil
Nutrient cycles in soil
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- Photosynthesis fixes atmospheric CO₂ into plant biomass forming carbohydrates and other organic compounds
- breaks down dead plant material and releases carbon back into soil
- Soil respiration releases CO₂ as microorganisms and plant roots metabolize organic matter (heterotrophic and autotrophic respiration)
- Carbon sequestration stores carbon long-term in stable (humus)
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- Nitrogen fixation converts atmospheric N₂ to biologically available NH₃ through symbiotic bacteria (rhizobia) or free-living microorganisms
- transforms organic N in decomposing matter to NH₄⁺ through microbial activity
- oxidizes NH₄⁺ to NO₂⁻ then to NO₃⁻ by specialized bacteria (Nitrosomonas, Nitrobacter)
- reduces NO₃⁻ to N₂ gas under anaerobic conditions releasing N to atmosphere
- Leaching and plant uptake of NO₃⁻ moves nitrogen through soil profile or into biomass
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- Weathering of phosphate-containing minerals releases inorganic P into soil solution
- Organic phosphorus decomposition mineralizes P from plant residues and soil organic matter
- Phosphorus adsorption and desorption in soil regulates P availability through interactions with clay minerals and metal oxides
- Plant uptake and return through litter fall cycles P between biomass and soil pools
Concept of nutrient bioavailability
- Bioavailability refers to proportion of nutrients in soil accessible to plants for uptake and utilization
- Factors affecting bioavailability:
- influences solubility and speciation of nutrients (optimum range 6.0-7.5 for most crops)
- affect oxidation state and mobility of elements (Fe, Mn)
- Organic matter content enhances and release through decomposition
- determines cation exchange capacity and nutrient holding ability
- regulates nutrient movement and microbial activity (field capacity ideal)
- impacts chemical reaction rates and biological processes (optimum range varies by crop)
- Nutrient forms:
- Soluble ions in soil solution directly available for plant uptake (NO₃⁻, K⁺, Ca²⁺)
- Exchangeable ions on soil particles rapidly equilibrate with soil solution
- Precipitated minerals slowly release nutrients through dissolution (calcium phosphates)
- Organically bound nutrients gradually become available through mineralization
Soil microorganisms in nutrient cycling
- Decomposition of organic matter releases nutrients and builds soil structure
- Nitrogen transformations:
- Nitrogen fixation by symbiotic (Rhizobium) and free-living bacteria (Azotobacter) converts N₂ to NH₃
- Ammonification by heterotrophic microorganisms breaks down organic N to NH₄⁺
- Nitrification by autotrophic bacteria (Nitrosomonas, Nitrobacter) oxidizes NH₄⁺ to NO₃⁻
- Phosphorus solubilization:
- Production of organic acids by fungi and bacteria dissolves inorganic P compounds
- Phosphatase enzyme activity releases organically bound P
- Mycorrhizal associations:
- Enhanced nutrient uptake especially phosphorus through fungal partner
- Extended root surface area through fungal hyphae increases soil exploration
- Rhizosphere effects:
- Root exudates stimulate microbial activity in immediate root zone
- Nutrient mobilization in the root zone through microbial processes and plant-microbe interactions
Sources and sinks of soil nutrients
- Sources:
- Atmospheric deposition brings nutrients through rainfall (wet) and particulate matter (dry)
- Mineral weathering releases elements from primary and secondary minerals
- Organic matter decomposition mineralizes nutrients from plant and animal residues
- Fertilizer applications add nutrients in readily available forms (chemical fertilizers, manure)
- Biological nitrogen fixation adds N through symbiotic and free-living microorganisms
- Sinks:
- Plant uptake incorporates nutrients into biomass (temporary storage)
- Leaching moves soluble nutrients (NO₃⁻, SO₄²⁻) to groundwater
- Erosion and runoff transport nutrients attached to soil particles
- Volatilization releases gaseous forms of nutrients (NH₃ from urea fertilizer)
- Adsorption to soil particles immobilizes nutrients on clay and organic matter surfaces
- Precipitation as insoluble compounds reduces nutrient availability (calcium phosphates)
- Nutrient pools:
- Soil solution contains immediately available nutrients for plant uptake
- Exchangeable nutrients on soil particles readily equilibrate with soil solution
- Fixed or occluded nutrients slowly released through weathering or desorption
- Organic matter serves as potential nutrient source through mineralization
Key Terms to Review (20)
Agricultural runoff: Agricultural runoff refers to the water that flows over agricultural land and carries with it various contaminants, including fertilizers, pesticides, and sediments, into nearby water bodies. This process can significantly impact ecosystems and water quality, linking agricultural practices to broader environmental issues such as nutrient pollution and biodiversity loss.
Ammonification: Ammonification is the biological process through which organic nitrogen, primarily from dead plants and animals or their waste, is converted into ammonia. This crucial step in the nitrogen cycle enhances the availability of nitrogen to plants and microorganisms, playing a vital role in soil fertility and nutrient cycling.
Bioavailable nitrogen: Bioavailable nitrogen refers to the form of nitrogen in the soil that can be readily absorbed and utilized by plants and microorganisms. This includes forms such as ammonium (NH₄⁺) and nitrate (NO₃⁻), which are critical for plant growth and ecosystem health. Understanding bioavailable nitrogen is essential for grasping how nitrogen cycling works and its influence on soil fertility and productivity.
Carbon cycle: The carbon cycle is the process through which carbon atoms circulate through the Earth's ecosystems, atmosphere, and geosphere, playing a vital role in maintaining life and regulating climate. This cycle involves various biological, geological, and chemical processes that move carbon between the atmosphere, land, oceans, and living organisms, connecting to essential functions like photosynthesis, respiration, and decomposition.
Clay mineralogy: Clay mineralogy is the study of the composition, structure, and properties of clay minerals, which are a group of hydrous aluminum silicates that play a crucial role in soil chemistry and environmental processes. These minerals are significant for their ability to retain water and nutrients, as well as their influence on the sorption and desorption of contaminants in soils. Understanding clay mineralogy helps in assessing soil fertility, nutrient cycling, and the overall environmental impact of various compounds.
Denitrification: Denitrification is the microbial process of converting nitrate ($$NO_3^-$$) and nitrite ($$NO_2^-$$) into nitrogen gas ($$N_2$$) or, to a lesser extent, nitrous oxide ($$N_2O$$), effectively reducing the nitrogen compounds in the soil or water. This process plays a crucial role in the nitrogen cycle, helping to maintain the balance of nitrogen in ecosystems while influencing overall nutrient cycling, including the bioavailability of nutrients in soils and the transformation of contaminants in aquatic environments.
Eutrophication: Eutrophication is the process where water bodies become enriched with nutrients, particularly nitrogen and phosphorus, leading to excessive growth of algae and aquatic plants. This process can result in decreased oxygen levels in the water, harming aquatic life and disrupting ecosystems, ultimately affecting water quality and the health of various species.
Leaching: Leaching is the process by which soluble substances are washed out of soil or other materials, typically as water moves through the soil profile. This process can significantly influence nutrient cycling, as essential elements like nitrogen and phosphorus can be removed from the soil, impacting their availability to plants and microorganisms. Additionally, leaching can lead to the contamination of groundwater if harmful substances are involved.
Nitrification: Nitrification is a microbial process that converts ammonia into nitrate through a two-step oxidation process, involving specific bacteria. This transformation is crucial in the nitrogen cycle as it helps make nitrogen available in forms that plants can use, linking it to broader biogeochemical processes and nutrient cycling in ecosystems.
Nitrogen cycle: The nitrogen cycle is the series of processes through which nitrogen and its compounds are converted in the environment and in living organisms. This cycle includes nitrogen fixation, nitrification, denitrification, and ammonification, which together facilitate the movement of nitrogen through the atmosphere, soil, and living organisms. Understanding this cycle is crucial because it highlights how nitrogen is transformed into various chemical forms that are vital for life and illustrates the interconnectedness of ecosystems.
Nutrient Retention: Nutrient retention refers to the ability of soils to hold and store essential nutrients, preventing them from being leached away by water or lost through other processes. This process is crucial for maintaining soil fertility, as it directly influences the bioavailability of nutrients for plant uptake and supports the cycling of nutrients within ecosystems.
Organic matter decomposition: Organic matter decomposition is the process by which dead plant and animal material is broken down by microorganisms, fungi, and other decomposers into simpler organic and inorganic substances. This essential process plays a key role in nutrient cycling, as it releases nutrients back into the soil, making them available for plant uptake and maintaining soil health.
Phosphorus cycle: The phosphorus cycle is the series of processes through which phosphorus moves through the lithosphere, hydrosphere, and biosphere, primarily in the form of phosphate ions. This cycle is crucial for the growth and development of living organisms, as phosphorus is a key nutrient for DNA, RNA, and ATP, linking it closely to both biogeochemical processes and nutrient cycling in soils.
Redox conditions: Redox conditions refer to the state of oxidation and reduction in a given environment, particularly focusing on the balance between oxidizing and reducing agents. These conditions are crucial in determining the chemical behavior of substances, influencing processes like sorption, ion exchange, and nutrient cycling within ecosystems. They play a significant role in how contaminants interact with soil and groundwater, as well as in the bioavailability of nutrients essential for plant growth.
Soil moisture: Soil moisture refers to the water content held in the soil, which is crucial for plant growth and various biochemical processes. This water is retained in the soil's pore spaces and affects the availability of nutrients to plants, as well as influencing microbial activity and overall soil health. Proper levels of soil moisture can enhance nutrient cycling, making essential nutrients more bioavailable for uptake by plants.
Soil Organic Matter: Soil organic matter is the organic component of soil, composed of decomposed plant and animal materials, microorganisms, and their byproducts. This material plays a critical role in nutrient cycling and bioavailability, influencing soil structure, water retention, and the overall fertility of the soil ecosystem.
Soil pH: Soil pH is a measure of the acidity or alkalinity of the soil, expressed on a logarithmic scale ranging from 0 to 14, where lower values indicate acidic conditions and higher values indicate alkaline conditions. Soil pH is crucial for nutrient availability, microbial activity, and overall soil health, affecting processes such as nitrogen fixation, nutrient cycling, and cation exchange capacity.
Soil testing: Soil testing is the process of analyzing soil samples to determine their nutrient content, pH level, and overall fertility. This analysis helps in understanding the soil's ability to support plant growth and its nutrient cycling capabilities, which are crucial for sustainable agriculture and environmental health.
Temperature: Temperature is a measure of the average kinetic energy of the particles in a substance, influencing various chemical and physical processes in the environment. It plays a crucial role in determining reaction rates, solubility, and the behavior of contaminants in different environmental media.
Tissue Analysis: Tissue analysis refers to the examination and evaluation of plant or animal tissues to assess their composition, nutrient content, and overall health. This process is essential for understanding how nutrients cycle through ecosystems and how their availability affects soil and plant interactions.