Midterm Lectures 11-12 Flashcards
Explain the 4 factors of soil dynamics?
Radiation budget (heat transfer and energy balance)
Water budget (precipitation, evaporation and transpiration, soil moisture storage and hydraulic conductivity)
Nutrient cycling (litterfall, decomposition, leaching losses from soils)
Gas exchange (soil aeration)
What is the nutrient equation?
soil solid <–(release of nutrient from solid phase)–> soil solution <–(exhange of ions at root surface, transport of nutrient to root surface–> plant root
The equation takes into account the release of nutrients from the soil solid phase to the soil solution and then the transport to the plant root (and vice-versa)
What is the role of nitrogen and what are its available forms?
macronutrient
building blocks for proteins, chlorophyll, nucleic acids, major fertilizer
NH4+ and NO3-
What is the role of phosphorus and what are its available forms?
macronutrient
Energy transfer, proteins, nucleic acids
HPO4(2-) and H2PO4-
Major fertilizer
What is the role of potassium and what are its available forms?
macronutrient
regulatory role in photosynthesis, carbohydrate translocation, nutrient uptake (exchange ions)
K+
What is the role of calcium and what are its available forms?
macronutrient
cell wall constituent
Ca2+
What is the role of magnesium and what are its available forms?
macronutrient
Chlorophyll and enzyme activator
Mg2+
What is the role of sulphur and what are its available forms?
macronutrient
building blocks for protein formation
SO4(2-)
What is the function of copper and what is its available form?
micronutrient
catalyst in respiration
Cu2+
What is the function of iron and what is its available form?
chlorophyill and enzymes
Fe3+
micronutrient
What is the function of manganese and what is its available form?
redox controls
micronutrient
Mn2+ and Mn4+
What is the function of zinc and what is its available form?
enzyme systems
micronutrient
Zn2+
What is the function of boron and what is its available form?
Sugars and carbohydrates
micronutrients
BO3(3-) borate
What is the function of molybdenum and what is its available form?
N2 fixation
micronutrient
MoO4(2-) molybdate
What is the function of chlorine and what is its available form?
O2 in photosynthesis
micronutrient
Cl- (chloride)
What is the nutrient cycling in natural ecosystems depending on?
Input output –> internal cycling
Overall budget
Residence time of nutrients in diff. ecosystems
Effect of disturbance
transformation from organic to inorganic and vice-versa
What interactions happen between the atmosphere, terrestrial environment and hydrosphere in N cycling
Nitrogen inputs to terrestrial ecosystems from atmosphere via nitrogen fixation
Internal cycling of nitrogen within terrestrial environment
Weathering of rocks –> sediments contain N
Volitization of N back to atmosphere and pathways of nitrogen losses to hydrosphere
What are the two stable isotopes of N
N14 and N15
Why is nitrogen important for biomolecules?
Building block for proteins in particular enzymes, nucleic acids, many secondary metabolites, caffeine
Limiting nutrient for many terrestrial ecosystems (such as agricultural crops)
Ammonium
NH4+
Solid or in solution
Important nutrient
Oxidation state -III
Nitrous oxide
N2O
Intermediate of denitrification and nitrification, important GHG (high global warming potential)
oxidation state -I
Nitrate
NO3-
Important nutrient, high leaching potential, at high concentrations hazard for water quality (can block hemoglobin in infants which leads to internal suffocation)
Yellowing leaves in plants is a symptom of what?
Nitrogen limiting –> cannot produce enough chlorophyll
Young leaves take up all the N, leaving older leaves to yellow
What is the significance of N in biogeochemistry?
Anthropogenic pertubations of global N cycle have resulted in:
- Increased emissions of N2O (a GHG) to atmosphere through denitrification of nitrate fertilizers
- Increased emissions of NOx to atmosphere through the burning of fossil fuels
- Increased atmospheric deposition of NH4+ and NO3-
- Evidence that absorptive capacity of forests in Europe and North America are exhausted, leading to ecological changes and loss of N to aquatic systems
- NO3- leaches base cations from soils (Ca, Mg, K)
- Nh4+ oxidation results in soil acification
- Increased export of N to estuaries/coastal oceans has led to changes in composition, functioning and fisheries
Explain the terrestrial N-cycle (natural additions)
Biological N2 fixation, atmospheric deposition and lightning –> microbial/plant sink
Microbial/plant sink –> OM, NH4+ pool, dissolved organic nitrogen pool
NH4+ pool –> ammonia volatization (loss), NO2- pool (nitrification)
NO2- pool –> NO3- pool (can be leached –> loss)
NO3- pool –> N2O, NO, N2 (denitrification) can be leached (loss)
Explain the anthropogenic influences on terrestrial N-cycle
Industrial N2 fixation brings in NO3- and NH4+
Plant/animal residues bring in organic H
Fossil fuel combusion brings in NOx (bring can be brought back to the atmosphere)
What are the losses possible in the N-cycle
Ammonia volization
Leaching
Denitrification that converts NO3- to N2, NO, and N2O and brings it back to the atmosphere
Explain N cycle using terms
Nitrogen fixation from bacteria in plant roots and soil bacteria converts nitrogen to ammonium
Through nitrification, ammonium is converted to nitrites. Nitrifying bacteria turn nitrite into nitrate
Through assimilation, plants take up the nitrate (and also ammonium) and convert it to energy that is consumable.
Animals consume the nitrogen
Decomposers take animals and plant waste/ dead matter and convert this nitrogen back to ammonium through ammonification
Denitrifying bacteria take nitrate and convert it back to N2 and release it in the atmosphere
Biological N fixation
This process converts atmospheric N2 to reactive nitrogen that becomes available to all forms of life
Carried out by a limited number of bacteria, including several species of rhizobium and cyanobacteria
The key to biological nitrogen fixation is the enzyme nitrogenase, which catalyzes the reduction of N2 gas to ammonia (important plant nutrient)
Symbiotic N2-fixation by rhizobial bacteria
Legumes provide the major biological source of fixed nitrogen in agricultural soils
They do so in a symbiotic association with rhizobial bacteria in root nodules
Anthropogenic N-fixation
Haber-Bosch N fixation
Land transformation into cropland
Reactive N from fossil fuels
Atmospheric N deposition
N is deposted mainly as ammonium and nitrate (NH4+ and NO3-)
3 main processes:
- wet deposition in precipitation
- dry deposition in dust or aerosols
- cloud water deposition in water droplets
Nitrate inputs as nitric acid and ammonium inputs, if followed by nitrification, acidify soils.
What did an increased N deposition in North America and Europe do?
Accelerated by emissions of NOx to the atmosphere by human activities
Application of urea as fertilizers leads to NH3 volatization, which is converted to NH4+ in the atmosphere
Compared to SO4(2-) deposition, less reduction in deposition, but now effective at least for NO3-
How is dissolved organic nitrogen (DON) produced and what happens to it?
OM –(extracellular decomposition by proteolytic enzymes)–> DON –(gross mineralization)–>NH4+
DON is directly taken up by plants and microorganisms to meet demand for N
When microbial growth is C limited, DON is used as C source and NH4+ is secreted as waste product
Other N limited microbes or plants may absorb this NH4+ (immbolisation)
Fine-scale heterogeneity in soil nitrogen availability
What is the fate of NH4+?
Uptake by plants and microorganisms (immobilization)
Absorption to the soil matrix (clay minerals)
Occlusion
Volitization as NH3 at pH>7 (loss from the system)
Nitrification to nitrate –> increases acidity since it releases protons into the soil environment
Nitrification
ammonium – (+2 e-) –> NH2OH –(+4 e-)–> NO2- –(2 e-)–> NO3-
review slide 22 lect. 11
Conditions:
- the concentration of NH4+ must be high enough
Aerobic (soil must be well aerated) –> oxygen availability
What is the fate of NO3- (nitrate)
Uptake by plants and microorganisms (immobilisation)
Leaching (loss from the system)
- Reduces the efficiency of fertilization
- Contamination of fresh water bodies (groundwater)
Nitrite accumulation at T<5 degrees C and pH>8
Denitrification (loss from th esystem)
-nitrate is denitrified when organisms use nitrate as electron acceptor instead of O
What are the conditions necessary for denitrification?
Anaerobic (soil not well aired, water saturated)
Oxidized nitrogen species are electron acceptors
Can occur in anaerobic microsites in aerobic soils
Where is denitrification rate higher in rapiarian wetlands?
Closer to the riverbank because there is a higher rate of inundation
Water in river is rich in O –> increases redox potential
groundwater less oxygenated
Denitrification is also increased through anthropogenic influence
What are the pathways of nitrogen loss?
Ammonium volatilization if pH>7
- NH4+ + OH- <–> NH3 + H2O
Production of NO and N2O during nitrification
Production of NO, N2O and N2 during denitrification
Production of NOx during fires
Leaching of dissolved organic nitrogen and NO3-
Erosion of organic and inorganic N
Review the processes of N cycling
N2 fixation: warm, moist, light, P, s and Mo
N2 –> OM
Mineralization: warm , moist
OM –> DON –> NH4+
Nitrification: high conc. of NH4+ and O2
NH4+ (lost through BH3)
NH4+ –> NO2- –> NO3-
Denitrification: low conc. of O2, and high conc. of DOc
NO3- –> NO2- –> NO (lost through NO)
NO –> N2O (lost through N2O)
N2O –> N2 (lost through N2)
What is phosphorus?
Phosphorus is an essential nutrient for all living organisms
P is a building block for many biomolecules
- ATP as energy reservoir
- DNA and RNA as storage of genetic info
- Phospholipids for structural support
Plants and microorganisms usually take up P from the soil solution as inorganic phosphate PO4(3-)
What is the normal oxidation state of phosphorus?
+5 as phosphate PO4(3-)
Phosphate occurs in the envr. either as inorganic or organic compounds
- usually considered to be redox insensitive (no donation or accepting of electrons under normal conditions –> there are exceptions)
What is the only stable isotope of phosphorus?
P31
What are common radioisotopes of P?
P32 and P33
Can be used as direct tracers of P in the envr.
However, due to their short half lives, their capability is limited to shorter experiments
What is the difference between phosphorus and phosphate?
Phosphorus is the element
Phosphate is the molecule that usually occurs in the envr.
Is orthophosphate organic or inorganic?
Inorganic
Has an oxidation state of +5
Is phosphate ester organic or inorganic?
Organic
Has an oxidation state of +5
What is the only gaseous phase of P
phosphine
Inorganic P
Explain the phosphorus cycle in the soil-plant system
Enters the soil from dissolution/solubilization of soil minerals –> orthophosphates PO4(3-) is bioavailable and can be taken up by microorganisms and plants
Plants and microorganisms will grow and prodcue biomass which will produce soil organic P
Soil organic P can be recycled by microorganisms that will mineralize it with enzymes –> creates PO4(3-) once again –> cycle continues
Orthophosphate can also be lost through erosion, leaching or by export of plants (crop harvest for humans or livestock) –> P is lost from the system
There can also be the sorption back to soil mineral phase (particularly clay minerals)
Where do we find high levels of P?
Arid regions
Volcanic soils rich in P as well (Japan, Hawaii)
- however, high amount fo total P does not mean that all of it is available P
What are the phosphate bearing minerals called and why are they important?
Apatite
Important in terms of solubilization of phosphate from minerals
What are the secondary minerals of P?
Iron and aluminum phosphates
Not easy to solubilize
What are some factors that control the availability of P in soils?
Solubility of primary minerals
Fixation of phosphate anions
soil pH
Mineralization of SOM
Atmospheric P inputs
Anthropogenic activity
How is the solubility of primary and secondary minerals related to pH?
Primary minerals’ solubility increases at pH<7
Solubility of seconadry minerals increases at pH>7
Therefore, in acidic soils, the iron and aluminum phosphates will not become dissolved
How does the fixation of phosphate anions impact P availability in soils?
Precipitation reactions
Anion exchange reactions (P molecule will be the dissolved ion)
Reactions with Al or Fe oxide surfaces
Formation of stable binuclear bridge (P molecule becomes immobile)
How does pH impact availability of P in soils?
As pH decreases, calcium will be solubilized which leads to an increase in the availability of phosphates in the soil solution (about pH 6-7)
However, at a certain point, the available phosphate will begin to react with silicated minerals and bond to the surface of the clay minerals (phosphate fixation)
As pH continues to decrease, phosphates also fixes with Fe, Mn, and Al –> becomes dominante form at pH<6
Chemical fixation by soluble Fe, Mn, and Al at low pH –> Phosphate is chemically fixed and is not available anymore
How does the mineralization of SOM control the availability of P in soils?
Mineralization depends on temperature, soil moisture and land use
In temperate regions, mineralization of organic phosphate is comparable to annual demand by plants
Important in highly weathered soils (tropics) –> mineralization of organic phosphate is often the most important source of P for plants
Organic phosphate in soils
Proportion of organic P varies between 5-95%
Three main groups:
1) Phosphateersters (inositol-hexakisphosphat)
- can make up 10-50% of total organic phosphate in soils
- plant seeds filled with this compounds –> storage unit for young seeldings to acquire P in their immediate vicinity
2) Nucleic acids
3) Phospholipids
- cell walls of plants and microorganisms
What is pedogenesis?
Development of soils
Explain the form of P in relation to pedogenesis
Total amount of P decreases with time –> occurs because of leaching P into groundwater and more importantly because of erosion of particles on soil surface
Beginning of soil dev. P mostly occurs in the form on primary minerals, changes with ongoing cellular dev. –> transform primary minerals into secondary minerals which will then form non-occluded (available P if you have iron exchange).
You will also have an increasing amount of P that is fixed to soil particles or that is even completely occluded.
The amount of organic P will also increase with time –> more and more plants are growing and dying (contributing to SOP)
When ecosystems becomes old, primary minerals and available P becomes completely depleted –> only SOP and occluded P –> plants rely on mineralization of organic P for their nutrition
review graph on slide 9 lect. 12
Atmospheric P inputs
Phosphorous is transported via dust for more than 6000 km from the central asian deserts to the islands of Hawaii
Ecosystems poor in P –> atmospheric deposition of dust
P eventually deposted with rainfall onto Hawaii
Winds brings dust across the Atlantic to the Amazon basin
- one continent feeds the ecosystem of another
How is food security impacted by anthropogenic changes to the P-cycle?
Too little available phosphate in the soil causes poor crop yields and can threaten food secuirty
In order to maintain current food production we need to apply fertilizer
Our consumptive habits require ever-more productive systems and fertilizers to feed humans, livestock and produce biofuels
What is a typical sign of phosphorus deficiency in plants?
Purple colouring
Mostly pronounced in older leaves –> younger leaves take up all the P
Briefly explain the history of P fertilizer
Onset of industrialization: manure
Guano becomes more common
Begin to mine rock phosphate (geological deposits rich in apatite)
After WW2: several important rock phosphate mines are discovered –> production/application explodes
Sharp drop in 1990: collapse of soviet union
Picks up again since
P mobilization in water saturated soils
In anoxic sediment, Iron 3 becomes iron 2 –> solubilizes phosphate
If plants do not require P –> transported via subsurface flow in aquatic systems
What is the consequence of excess P in waterways?
Algal blooms are caused by excessive P inputs due to inefficient fertilizer practices –> can become toxic if algal blooms are cyanobacteria
Southern shores of the Great Lakes are strongly affected by this (farming practices are less regulated in USA)
Affects freshwater access –> leads to water poverty
What are two problems associated with the solution of applying less fertilizer?
Need to convince public/farmers to produce crops in a more sustainable (and more expensive) way
P is fixed in soils for a very long time –> excess P is the result of fertilizer practices that happened years ago
What can be a solution to the P problem?
Wetland restoration: wetlands retain P in their soils and biomass and help to sequester P before it enters larger lakes and rivers
(also filters out N and pesticides)
Where is P found?
Rock derived nutrient and has therefore been mined either from biological waste (sea birds that eat fish rich in P) or geological sources
Now, majority of mining occurs in geological deposits. Mineral deposits are dug up and rock phosphates are transported through a conveyor belt to the ocean hwere is can be shipped and then further processed.
Why do we need to rethink our use of P?
Rock phosphates are a finite resource
Estimated to peak by the end of 2030 (will become depleted after)
This will have ecological as well as socioeconomic impacts
Case study of changes of soil P pools after land-use change
The hypothesis was that P would dramatically decrease since crop harvest takes P away from the system
However, surprinsingly concentrations did not change
Organic P decreases, but inorganic P increased –> creation of a new stable system after about 60 years (just different proportionate amounts of organic vs. inorganic P)
How does land use change impact organic P concentrations?
Enhanced erosion due to plowing creates dusts which destroys soil structure
Plowing also leads to stronger erasing of topsoil –> enhanced mineralization of organic P
Crop harvest –> organic P is no longer recycled through plant litter bur rather taken away (P leaves the system)
