Soil Nutrient Cycling and Indicators
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Questions and Answers

What is the role of soil in nutrient cycling?

Soil stores and moderates the release of nutrients from one pool into another in a cyclic manner.

Name three categories of indicators used to assess soil nutrient cycling.

Fertility Indicators, Organic matter indicators and Soil reaction indicators.

What is the function of decomposers in nutrient cycling?

Decomposers break down organic materials in the soil, leading to transformation and cycling of nutrients.

Give two examples of Organic Matter Indicators.

<p>Total soil organic carbon/matter, C:N ratio, decomposition rate, microbial biomass carbon, particulate organic matter, soil enzymes activity, lignin:N ratio, or total extractable polyphenol:N ratio.</p> Signup and view all the answers

What is the role of arthropods and earthworms in decomposition?

<p>They chew the organic material and mix it with the soil, increasing the surface area available for microbial attack and distributing organic matter throughout the soil profile.</p> Signup and view all the answers

Describe the overall process of decomposition.

<p>It is a stepwise process involving soil organisms, where complex compounds are broken down into simpler components, releasing nutrients back into biological circulation.</p> Signup and view all the answers

What is the importance of the portion of plant and animal residue that is not completely broken down during decomposition?

<p>It plays a crucial role in soil structure, water retention, and long-term carbon storage.</p> Signup and view all the answers

What are the complex organic compounds formed from decomposed organic matter that can persist in soil for extended periods?

<p>Humic substances</p> Signup and view all the answers

Besides the atmosphere and above-ground biomass, where else is carbon importantly stored?

<p>Soil</p> Signup and view all the answers

Through what biological process do plants convert atmospheric carbon dioxide into organic carbon compounds?

<p>Photosynthesis</p> Signup and view all the answers

What happens if plant residue is added to the soil at a faster rate than soil organisms convert it to $CO_2$?

<p>Carbon will be sequestered.</p> Signup and view all the answers

How do human activities like tillage affect soil organic matter?

<p>It triggers increased biological activity, rapid decomposition and loss of soil organic matter.</p> Signup and view all the answers

Name the two primary carbon-based atmospheric gases discussed in the context.

<p>Carbon dioxide ($CO_2$) and methane ($CH_4$)</p> Signup and view all the answers

What organisms synthesize atmospheric $CO_2$ into organic material?

<p>Autotrophic organisms (mainly plants), as well as photo- and chemo-autotrophic microbes</p> Signup and view all the answers

How can soil aggregates contribute to long-term carbon storage in soil?

<p>They enable SOC persistence in soil for decades, centuries, or even millennia.</p> Signup and view all the answers

Besides the decomposition of SOM, what other process related to root exudates can cause carbon loss from soil?

<p>Liberating organic compounds from protective mineral associations.</p> Signup and view all the answers

What are the two primary ways carbon is transferred from soils to rivers and oceans?

<p>Dissolved organic carbon (DOC) and erosion material.</p> Signup and view all the answers

Name two climatic conditions that directly influence the decomposition of organic matter in soil.

<p>Soil temperature and water content.</p> Signup and view all the answers

What is the primary mechanism by which soil organic carbon (SOC) is lost from the soil?

<p>Respiration as a result of microbial mineralization.</p> Signup and view all the answers

Besides chemical recalcitrance, what other process affects soil organic matter (SOM) persistence in soil?

<p>SOC stabilization in the soil matrix through its interaction and association with soil minerals.</p> Signup and view all the answers

What are the three different pools soil organic matter can be divided into based on turnover time?

<p>Active pools, intermediate pools, and slow pools.</p> Signup and view all the answers

Name two reasons that explain long turnover times of organic compounds in soil.

<p>Anaerobic conditions and incorporation of SOM components into soil aggregates.</p> Signup and view all the answers

What happens to a large proportion of fresh organic carbon added to the soil within 1-2 years in the fast (labile) pool?

<p>It is lost due to decomposition.</p> Signup and view all the answers

How long is the turnover time range for the intermediate pool of soil organic carbon?

<p>10-100 years.</p> Signup and view all the answers

What is the turnover time for the slow pool of SOC?

<p>100 to &gt;1,000 years.</p> Signup and view all the answers

Flashcards

Soil Nutrient Cycling

The process where soil stores and releases nutrients cyclically.

Fertility Indicators

Measures that assess nutrient availability in soil, like nitrogen and phosphorus levels.

Organic Matter Indicators

Metrics indicating the amount and quality of organic matter in soil, e.g., C:N ratio.

Soil Reaction Indicators

Metrics like soil pH and electrical conductivity that determine soil chemical properties.

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Decomposition

The breakdown of organic materials by soil organisms, releasing nutrients.

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Role of Soil Organisms

Organisms like bacteria and worms that help in the process of decomposition.

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Nutrient Cycling Importance

Nutrient cycling ensures the availability of essential nutrients for plants and organisms.

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Dissolved Organic Carbon (DOC)

Carbon lost from soils as dissolved organic material.

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Soil Organic Carbon (SOC)

Carbon stored in soil contributed by organic matter.

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Microbial Mineralization

Process by which microbes break down organic material, releasing CO2.

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Active Pool (Fast Pool)

SOC that turns over in months to a few years.

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Passive Pool (Slow Pool)

SOC with turnover times of thousands of years.

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Turnover Time

Time needed for organic matter to fully decompose in soil.

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Greenhouse Gases (GHGs)

Gases from soils that trap heat in the atmosphere.

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Soil Carbon Stabilization

Process that enhances SOC retention in soil through interactions with minerals.

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Anthropogenic Activities

Human actions that impact soil carbon dynamics, affecting GHG emissions.

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Humus

Highly complex organic compounds that persist in soil, improving structure and nutrient storage.

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Terrestrial carbon cycle

Cycle illustrating how carbon is stored and cycled through soil and the environment.

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Soil carbon origin

Soil carbon comes from organic compounds made by plants during photosynthesis.

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Soil organic matter

Organic compounds in soil from dead plants and animals that support life.

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Carbon sequestration

Process of storing carbon in soil to reduce atmospheric CO2 levels.

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Tillage

Agricultural practice that aerates soil, increasing decomposition of organic matter.

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Autotrophic organisms

Organisms, primarily plants, that convert CO2 into organic materials.

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Heterotrophic microorganisms

Microorganisms that transform dead organic material into soil carbon.

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CO2 emission

Release of carbon dioxide back into the atmosphere during organic matter decomposition.

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Study Notes

Soil Nutrient Cycling

  • Soil nutrient cycling involves biogeochemical processes
  • These processes cycle nutrients between different pools in the soil
  • Nutrients are transformed into plant-available forms during cycling
  • Some nutrients are held in the soil, or lost to air or water
  • Assessing soil nutrient cycling involves measuring fertility indicators, organic matter indicators, and soil reaction indicators

Fertility Indicators

  • Fertility indicators include mineral nitrogen (total soil N), potentially mineralizable nitrogen, available N (ammonium and nitrate), available phosphorus, exchangeable cations (e.g., potassium, calcium, magnesium, sodium), sulfur, boron, zinc, and micronutrients (e.g., Mn, Fe, I, B, Cu, Zn)

Organic Matter Indicators

  • Organic matter indicators include total soil organic carbon or total soil organic matter, C:N ratio, decomposition rate (e.g., basal respiration or net N or C mineralization or immobilization), microbial biomass carbon, particulate organic matter, soil enzyme activity, lignin:N ratio, total extractable polyphenol:N ratio, and etc

Soil Reaction Indicators

  • Soil reaction indicators mainly include soil pH and electrical conductivity

Global Nutrient Cycles in Soil

  • Global nutrient cycles in soil include carbon, water, and nutrient cycles (e.g., N, P, S).
  • During nutrient cycling, nitrogen, phosphorus, and other nutrients are stored, transformed, and cycled through soil

Decomposition in Soil

  • Decomposition is the breakdown of organic material in the soil
  • Decomposition is driven by soil organisms, transforming and cycling nutrients through the environment (soil, water, and air)
  • Decomposition returns carbon and nutrients to biological circulation, making them available to plants and other organisms
  • Decomposition degrades potentially harmful compounds, preventing pollution of groundwater and surface water

Decomposition Process

  • Soil organisms (e.g., arthropods, earthworms, fungi, bacteria) are involved
  • Decomposition is stepwise, with each organism further breaking down the material and obtaining energy or nutrients from the process
  • The portion of plant and animal residue remaining after decomposition forms complex organic compounds (humic substances) that persist in soil
  • Humus is important for soil structure and nutrient storage

Terrestrial Carbon Cycle

  • The carbon cycle demonstrates the role of soil in cycling carbon
  • More carbon is stored in soil than in the atmosphere and above-ground biomass combined
  • Soil carbon originates from organic compounds created through photosynthesis (plants converting atmospheric CO2 into organic compounds)

Soil Organic Matter Dynamics

  • When plants/animals die and their organic material enters the soil, soil organisms consume the material, extract energy and nutrients and release water, heat, and CO2
  • If no new plant material is added, soil organic matter gradually disappears
  • If plant residue is added at a faster rate than soil organisms can decompose it, carbon is removed from the atmosphere and stored in the soil
  • Human activities (e.g., tillage) can trigger increased biological activity, rapid decomposition, and loss of soil organic matter

Carbon-based Greenhouse Gases (GHGs) and Soil

  • CO2 and methane (CH4) are the main carbon-based GHGs emitted by soil
  • Another GHG is nitrous oxide (N2O), and its emission from agricultural soils and livestock facilities is increasing
  • Anthropogenic activities can cause soil to either be a net sink or a net source of GHGs
  • The different gases have varying global warming potentials (CO2 has GWP = 1, CH4 GWP = 28, and N2O GWP= 265 according to IPCC 2014)

Soil Organic Carbon (SOC) Sequestration

  • SOC sequestration is the process where carbon is fixed from the atmosphere via plants or organic residues and stored in the soil
  • When dealing with CO2, SOC sequestration involves three stages:
    • Removal of CO2 from the atmosphere via plant photosynthesis.
    • Transfer of carbon from CO2 to plant biomass.
    • Transfer of carbon from plant biomass to the soil where it is stored as SOC in the most labile pool.

Soil Carbon Saturation

  • Soil carbon saturation means that the soil carbon stock has reached its maximum carrying capacity for storing carbon inputs
  • Soil carbon saturation depends on many factors including soil properties, their interactions, and abiotic factors
  • When maximum soil carbon saturation is reached, SOC sequestration stops; soils will no longer act as a net carbon sink, and may become a net carbon source

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Nutrient Cycling in Soil PDF

Description

This quiz explores the biogeochemical processes involved in soil nutrient cycling, highlighting the transformation of nutrients into plant-available forms. It covers fertility indicators, organic matter indicators, and the role of these elements in measuring soil health and fertility.

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