Cycles: from the Plant to the globe

Nutrient and carbon cycling is a big deal here on planet Earth! As you would have guessed - plants are a BIG part of what makes these cycles go 'round. We will be looking at global biogoechemical cycling as well as plant-scale nutrient cycling. The main constituents we are following around are Carbon (C), Nitrogen (N), and Phosphorus (P).

Nitrogen is a component of chlorophyll and therefore essential for photosynthesis. It is also the basic element of plant and animal proteins, including the genetic material DNA and RNA, and is important in periods of rapid growth. Plants use nitrogen by absorbing either nitrate or ammonium ions through the roots.

Plant Macronutrients - Hydrogen, Carbon, Oxygen (from water or CO2), Nitrogen, Potassium, Calcium, Magnesium, Phosphorus, Sulfur (from soil)

Plant Micronutrients- Chlorine, Iron, Boron, Manganese, Zinc, Copper, Nickel, Molybdenum

Global Cycles

Global N & P: This video takes a brief look at the global N and P cycles.

Global C - start at 5:15: This videos take a brief look at global C cycling.

plant scale cycles

Plant-Scale Nutrient Cycling: This is a look at nutrient cycling at the scale of a single plant. Plants engage in cooperation with the soil food web to make soil nutrients available (inorganic form) so that plants can take them up through the roots.

Plant-Scale C cycling: Understand the mechanism by which plants pump carbon into the ground

diagrams

Global Carbon Cycling

Methane Cycle - part of the Carbon Cycle

Phosphorus Cycle

N Cycle

Important terms for Biological transformations of N:

Nitrification – conversion of ammonium-N (a cation held in soil by CEC) to nitrate-N (a soluble anion easily lost in runoff or leaching)
Denitrification – conversion of plant-available nitrate-N to N-gases that are unavailable to plants and easily lost from soil
Mineralization – biological breakdown of organic-N and release as plant-available ammonium-N
Immobilization (assimilation) – uptake of inorganic-N from soil and incorporation into organic-N compounds in microbes (N becomes unavailable to plants)
N-Fixation – conversion of N-gas in the air to inorganic-N that becomes available to plants (performed by bacteria associated with roots of legumes and other plants, and some free-living soil microbes)

root nodules and N fixation

Examples of root nodules produced by many plants in the family Fabaceae that provide oxygen poor well-provisioned habitat for N-fixing bacteria (though 4 genera in Rosaceae also do this!)

Nitrogen fixation: Atmospheric N2 is reduced to NH4+ for use by organisms. Only carried out by certain bacteria -free-living and symbiotic types

Rhizobium and Bradyrhizobium - invade the roots of legumes

(Nitrogen to form protein for carbon-molecules as energy)

Within legume root nodules, nitrogen gas (N₂) from the atmosphere is converted into ammonia (NH₃), which is then assimilated into amino acids (the building blocks of proteins), nucleotides (the building blocks of DNA and RNA as well as the important energy molecule ATP), and other cellular constituents such as vitamins, flavones, and hormones. Their ability to fix gaseous nitrogen makes legumes an ideal agricultural organism as their requirement for nitrogen fertilizer is reduced. Indeed, high nitrogen content blocks nodule development as there is no benefit for the plant of forming the symbiosis. The energy for splitting the nitrogen gas in the nodule comes from sugar that is translocated from the leaf (a product of photosynthesis). Malate as a breakdown product of sucrose is the direct carbon source for the bacteroid. Nitrogen fixation in the nodule is very oxygen sensitive. Legume nodules harbor an iron containing protein called leghaemoglobin, closely related to animal myoglobin, to facilitate the diffusion of oxygen gas used in respiration.

Diagram describing the growth of root nodules

synthetic fertilizers

Ways we try to adjust how fertilizer N transitions in the soil:

1. Nitrogen Stabilizer – substance added to a fertilizer which extends the time the nitrogen component of the fertilizer remains in the soil in the ammoniacal form.

2. Nitrification inhibitor – substance that inhibits the biological oxidation of ammoniacal nitrogen to nitrate nitrogen

3. Urease inhibitor – substance which inhibits, block or delay the hydrolytic action on the urea molecule by the urease enzyme

4. Slow or Controlled Release - chemical binding with urea or ammonia in a form which delays its availability for plant uptake and use after application, or which extends its availability to the plant.

Delayed release of urea using a sulfur coating

Slow release of urea through a porous polymer coating

how nitrogen gets into plants - technical description

Nitrogen Assimilation by Plants:

Plants absorb nitrogen from the soil in the form of nitrate (NO₃⁻) and ammonium (NH₄⁺). Ammonium ions are absorbed by the plant via ammonia transporters. Nitrate is taken up by several nitrate transporters that use a proton gradient to power the transport.
Every nitrate ion reduced to ammonia produces one OH⁻ ion. To maintain a pH balance, the plant must either excrete it into the surrounding medium or neutralize it with organic acids. This results in the medium around the plants roots becoming alkaline when they take up nitrate.
To maintain ionic balance, every NO₃⁻ taken into the root must be accompanied by either the uptake of a cation or the excretion of an anion. Plants like tomatoes take up metal ions like K⁺, Na⁺, Ca²⁺ and Mg²⁺ to exactly match every nitrate taken up and store these as the salts of organic acids like malate and oxalate.^[16] Other plants like the soybean balance most of their NO₃⁻ intake with the excretion of OH⁻ or HCO₃⁻.^[17]

Almost all nutrients pass through the soil solution and up the roots in order to be brought into plants. To some degree there is foliar uptake of Nitrogen as well.

other resources....

Page updated

Report abuse