Soil exam #3

3/10/25

pH range of soils and other materials 

Soil organisms - grouped by size 

• Macro-organisms  (>2mm) > MESO - organisms (>0.1-2mm) > MICRO organisms (<0.1mm)

• Worms, termites, mice > springtails, mites > tardigrades, nematodes, fungi, bacteria, archaea

• Soil organisms - grouped by metabolism 

  • Metabolic grouping of soil organisms based on source of energy  and carbon 

  • Source of carbon - combined organic carbon - biochemical oxidation 

    • Chemoheterotrophs , all animals, plant roots, fungi, actinomycetes and most bacteria

      • Earthworms, fungi, water bears

  • What are most of these organisms getting their combined organic carbon from?

    • Both chemoheterotrophs and Photoautotrophs 

  • Carbon dioxide or carbonate - solar radiation

    • Photoautotrophs plant shoots, algae, cyanobacteria

• CARBON can be cycled through an intermediate consumer before it is consumed by Chemoheterotroph

• Chemoautotrophs that use carbon dioxide and carbonate

  • Ammonia oxidizers and sulfur oxidizers 

  • Are doing it as an energy source transformation

  • Getting carbon from inorganic sources 

• Trophic levels and energy transfer 

  • Primary consumers in soil 

    • Herbivores : eat live plants

      • Larvae of cane beetle which feeds on living sugarcane plants in all stages of life cycle

    • Detrivores: eat remains of dead plants and microbes on them

    • Saprotrophs: microorganisms that consume detritus, corpses and feces

  • Secondary consumers in soil 

    • Carnivores : eat other animals

    • Microbivores feeder : eat microbes 

      • Protozoa, which graze on soil microbes 

• Trophic levels and energy a( and carbon transfer) of belowground communities 

• Other microbes exist in soil that arent as involved in the soil organic matter 

  • Process of transformation 

• Trophic cascade of aboveground communities 

  • 10% of energy is lost every time 

  • SMall part of period is small itty bitty animals compared to plants as largest energy source 

• In a given amount of soil there is a certain amount of biomass

3/24/25

Soil organic matter (SOM)

• Contains the elements in living biomass (CHNOPS)

• SOM is about 

  • 58% carbon by mass

  • 1-6% nitrogen by mass

• SOM is one pool ofhte global carbonand nitrogen cycles

Pools and fluxes 

• We can think of cycles of C,N water ( and other material cycles in terms of pools and fluxes 

• Pools describe the amount of material that is present in a given compartment

• Fluxes during the amount of material that is moving song the compartments during a given period of time
  

Turnover rate and residence time

• We can define turnover rate of a given pool as the proportion of that pool which leaves during a given period of time  

  • (turnover rate = output/pool)

• We can define the residence time of matter in a pool as the average length of time that a given molecules will remain in that pool 

  • (=pool/output)

• These measurements are critical for understanding soils and turnover rate of nutrients in fields 

School example 

• (turnover rate = output/pool) , (residence time =pool/output)

• Pool = 1000 students 

• Input/ output = 500 students / year 

• Turnover rate = 0.5  = 50%

• Residence time = 2 years 

The global carbon cycle 

Carbon cycles constantly between land oceans and atmosphere although its residence time in various reservoirs can vary greatly, black arrows show natural fluxes and red arrows show human fluxes 

Key points::: Soil Contain almost twice as much carbon as the atmosphere and terrestrial vegetation combined 

• Global:

  • Units in Pg (C(pools))

  • Pg C yr 1 fluxes 

• Averages per ha land area 

  • Units in Mg C *pools

  • Mg C yr *(fluzes)

Som SOM persits 

When carbon is deposited by a plant - major pathewat in which carbon enters soil 

• Plant as a photosynthetic organism is drawing carbon from atmosphere, c

  • creating organic carbon, both living and dead is depositing into soil

• Respired by microorganisms within a few hours

• Or may persist as SOM for hundreds of years 

• In other words, SOM has heterogeneous residence times

Reasons why SOM persists in the environment

• Have been radically revised in past 10-20 years

• Older concepts and terms are still around.

• Carbon dating etc 

Evolving views of SOM origin and persistence 

Favorability of microorganisms in different environments can change how quickly things are absorbed and carbon is cycled

We have redundant terminology to talk about fast vs slow cycling pools

Formation of POM (particulate organic matter)

A root or shoot enters soil (plant death or turnover) → +CO2 expelled → decomposer activity reduces size and mass of root, and alters chemical composition → CO2 expelled, remaining is >2mm> → Remaining fragments of root become POM 

Formation of MAOM (mineral associated organic matter)

A root deposits carbon such as through exudation → plant carbon is directly adsorbed to minerals or processed by microbes → organic matter associated with minerals become MAOM

Some proportion can be respired in this instance, some remains in cell of microbe and then die, which generates NECROMASS (dead material ) 

POM = ~ 20% of organic carbon in arable soils 

MAOM =~ 80% of organic carbon in arable soils 

TOTAL SOC pool is a function of…..

Carbon inputs 

Aboveground litter, animals waste, organic amendments 

Roots, root exudation (aka rhizodeposition 

| V |

Carbon outputs  

Respired C(CO2)    Harvested C (agricultural)   Erosion   Leaching of dissolved organic C 4 

Important function of crops to be able to supply soil through their roots

Protection mechanisms of SOM

• Limitations of microbial access or activity that enable SOM persistence (Ie extend time before decomposer attack, consumption and respiration) → respired C(CO2) → protection mechanisms reduce this output flux from the SOC pool

  • Some carbon outputs in soul can persist for thousands of years 

• Limitations on microbial access

  • SOM can be protected from decomposition by being occluded within aggregates  which limit microbial access 

  • Material inside an aggregate has longer residence time - something protected in an aggregate  = Occluded POM

  • Shorter residence time= free POM

  • SOM that is absorbed to mineral surfaces aka (MAOM) is less vulnerable to microbial decomposition than a counterpart molecule that is in a soil solution 

  • Oldest carbon is often that that is adsorbed into a mineral 

• Limitations on microbial activity - Low O2

  • Anaerobic conditions, lack of O2, slows down microbial activity 

  • Soils can become anaerobic if they are::

    • Permanent wetlands

    • Seasonally wet

    • Have anaerobic microsites 

      • An aggregate w POM that has lower O2 in aggregate and higher O2 on outside 

• Limitations on MICROBIAL activity - low temp

  • Cold conditions - less optimal for microbial activity 

    • Can be measured using a laboratory incubation that measures different temperatures and CO2 production/ day 

    • Happens in GELISOLS as they help protect carbon in cold climates

• Limitations on microbial activity - Low pH 

  • Acidity inhibits microbial activity - like a pickle :) 

  • The Hoosfield acid strip - single application of lime in late 18000s created pH gradient 

  • Histosols (Bogs) can become acidic if they are fed from rainwater rather than groundwater (which is rich in base cations) or develop acidic parent materials

• Peat bogs 

  • Acidity delays decomposition, this applies to organic matter inputs from non plant resources 

    • Which is why things can be preserved in the bogs 

    • Peat bogs are so dense in preserved plant organic matter they can be burned for fuel 

    • A non renewable resource as peat takes thousands of years to develop.

• Limitations on microbial activity — HIGH C:N

  • Plant litter 

    • Not protected by association with minerals or occluded in aggregates 

    • Can be characterized by its ratio of carbon atoms per one nitrogen atom (C:N ratio)

    • Red clover C:N = 15 , SPruce C:N = 50, corn leaves C:N = 100, sawdust C:N = 500

  • Microbes 

    • Litter C:N influences rate of microbial decomposition 

    • C:N ~ 20/30, is usually ideal for microbial needs → rapid decomposition 

    • HIgher C:N corresponds to more difficult to decompose material → slower decomposition 

  • WHat connection can we make between litter inputs and historical vs emerging views of SOM? 

    • Historical - above ground - now - more root and belowground inputs 

Evolving view so litter chemistry role for SOM

Emerging views of TEXTURE role for SOM

SOC distribution reflects historical balance between inputs and outputs of C

Dry areas don't have enough moisture to support plant growth to provide carbon to soil 

No output if not input

SOC los due to 12,000 years of human land use 

• Land use includes both grazing and cropping 

• Why does this matter

  • People have come in and added water and plant areas and bumping up growth and increase in soil carbon 

Atmospheric CO2 concentrations 

• CO2 is a greenhouse gas whose presence in the atmosphere causes greater heat retention 

• Together combustion of fossil fuels and C loss from soil + terrestrial biomass are major drivers of atmospheric CO2 increase and climate change 

• Less carbon in soil means

  • Carbon needs to go to other compartments of the global C cycle including the atmosphere (contribution to global climate change )

  • Potential for reduced soil health 

• Soil health = SOC 

  • Defined as the “continued capacity of soil to function as a vital living ecosystem that sustains plants, animals and humans 

  • Although many measurements can be used to describe soil health, SOC is the  most common 

  • These practices maximize soil health are the same as those that maximize SOC within agricultural systems

• Why SOC loss with land use change from native vegetation to arable agriculture 

  • Annual crops reduce root inputs compared to native perennials, reduce C inputs 

  • Tile drainage increases time under aerobic conditions, increases C outputs

    • Wetlands are great at accumulating carbon because they have a lot of water, not possible to grow crops though so you must irrigate 

  • Tillage disrupts aggregates and their protection of SOM, increases C outputs

• SOC loss under conversion to agriculture and SOC regeneration

Regenerative practices that increase SOC 

• Cover cropping vs bare fallow

• Crop rotation with perennials vs only annuals 

  • Different rotations of crops can affect soil differently 

• Reside (material left behind by crop after harvest) return vs removal 

  • Valuable organic material harvesting or leave to add carbon back into soil, 

  • Removing accelerate carbon 

• Organic amendments vs only inorganic fertilizers 

  • Organic amendments 

    • Plants, cow manure/ animal waste

  • Inorganic amendments 

• Inorganic amendments vs ZERO fertilizer

  • Growth decline in carbon inputs

• NO till, or reduced till, vs tillage (contentious

  • Tillage can contribute to soil carbon lost by speeding up its output pathway 

  • Potential for no till to lead to soil carbon stocks/storage is now considered unclear!

Three pairwise comparisons of SOC concentrations in tilled vs no tilled systems

Why might not till plots have greater difference between SOC concentration between different parts of topsoil?

Tilled system its being incorporated into the soil, no till the plant matter status on surface

No tilled system concentrates carbon in surface layer 

Two ways of measuring a SOC pool

1. Concentration - mass SOC /mass soil

   1. g SOC/kg soil or

   2. g SOC/ 100g soil

   3. Can be determined directly from sampled soil 

2. Stock = mass SOC / area and depth of soil 

   1. g SOC / M^2 or 

   2. Mg SOC/ ha

   3. Requires bulk density measurement and depth of sampling. 

3. TAKE SOIL CORES 

   1. Composite cores

   2. Sieve to 2mm

   3. Analyze soil for its % organic C 

   4. Mass SOC /mass soil 

4. All of these steps to get concentration and → 

Tillage reduces bulk density 

• Bulk density affects measurement of SOC sock 

  • Sampling to the same depth in soils that differ in bulk density will capture different masses of soil 

  • Even if SOC concentration are identical between these two soils, which soil will have greater SOC stock in the top 30 cm 

  • Denser soil will have greater stock in top 30 cm even if top concentration is the same

• Connections interlude 

  • POC → bulk density 

    • Particulate organic carbon is 

  • CO2 → MAOC

• SOM fractionation  taking soil and causing disruption→ many schemes 

  • Physical fractionation:

    • POC/MAOC disrupts soils into primary particles 

      • Size fractionation POC > 53 microns: MAOC < 53 microns

      • Density fractionation: POC < 1.86g/cm^3

    • Aggreagtion fractionatipn - less disruptive than POC/MAOC fractionation 

      • Size cutoffs range form 53 microns to >2mm

    • CHemcial fractionation

      • FOundation of historical view of SOC persistence

      • Strongly alkaline extracts produced apparent large, complex molecules

    • Biological fractionation 

      • Incubate soil with living microbes to assess biological accessibility 

  • Causing a detectable increase in SOC after changing management can take > 5 years 

    • Because potential to increase C inputs above C outputs is very small compared to the total SOC pool

    • Amount of C inputs are inexces of outputs each year 

Nitrogen soil 

Four types of biomolecules  - ALL REQUIRE CHO

• Protiens 

• Nucleic acids – require N

• Lipids 

• Carbs

Photosysnthies depends on nitrogen 

• RuBisCo

  • Most abundant enzyme (a type of protein) in the world 

  • Catalyzes the conversion of CO3 to organic carbon during photosynthesis

• Nitrogen often limits plant growth 

  • In other words, plant growth increases with more N applied 

  • N limitation more common in temperate soils

  • In tropical soils, P can be limiting

Plants get their nitrogen from soil via root uptake 

• Forms of nitrogen is usually that plants take up = plant available nitrogen 

• Plant available nitrogen is usually “mineral” nitrogen (not associated with carbon 

• Two forms of nitrogen in soil 

  • Ammonium (NH4) a reduces electron rich N 

  • Nitrate (NO3) oxidized or electron poor N species 

• Nitrogen soils exist primarily in SOM 

  • N in SOM 

    • Protiens petitdes nucleic acids 

    • Complex N forms not directly available to plants 

  • These compounds can compromise 95-99% of the N in soils

    • 725 g N m − 2 , as organic N in SOM, compared ~ 6 g N m − 2 for mineral N forms

Process by which N in SOM becomes available to plants = Mineralization 

Extra cellular enzyme degrade large molecules of SOm so they are small enough to enter microbial cells

Rates of N mineralization 

• Increase with temp

• Have an optimal moisture 

  • If water potential is too low, microbial activity cannot proceed 

  • Very high water filled pore space promotes anaerobic conditions, which slow down mineralization 

• Increase with soil pH

A SOM POOL 

• In top 30 cm 

• In - plant C input (shoots, roots, exudates) 

• Out - Respiration, harvest, leaching 

Rate of N mineralization 

Assuming SOM is 58% C and 5% N 

And what else ??? NH4 === NITRIFICATION

• NH4+ will be oxidized to NO3- for energy by microbes known as nitrifiers 

• Nitrifiers are chemoautotrophs

  • Chemo = energy from chemicals

  • Auto= carbon from inorganic sources 

• These microbes use O2 as terminal electron acceptor 

• So, nitrification is an aerobic process

  • No air

  • Optimal = ~60% water filled pore space

Immobilization: microbial uptake of mineral N 

• IMporant for controlling N availability to plants

  • wN in microbes is not available to plants

  • Plant and microbes are sometimes thought to compete for mineral N 

• Microbes immobilize mineral N when they have access to C substrates but cant meet their N needs

NH4+ fixation by soil colloids 

• Adsorption of ammonium ions by the mineral or organic portion of the soil in a manner that they are relatively unexchangable by the usual methods of cation exchange

• Colloids can “hide” NH4+ i n this way

NOTABLE FEATURES FOR N CYCLE 

• Precipitation> evapotranspiration

• Limited potential for plant uptake of N

• Lets assume soil has moderate/high CEC

• There is both NO3 and NH4

• WHich N species is more likely to leace through leaching 

  • NO3 is more  likely to leave because it is negatively charged that doesent readily bind to osil particles

4/2/25

N leaching loss 

• Transport of dissolved N out fo soil and into riverine or coastal waters 

  • Predominatnly NO3- due to its inavilty to be held inc ation exchange, but leaching can include NH4+ as well 

• N leaching (aka runoff) occurs when 

  • Mineral N pools in soil > plant uptake 

  • Precipitation>evaporation

• ENvironmental hazard due to Eutrophication

Soil nitrogen cycle 

What is a highly oxidized species of N good for

DENITRIFICATION

• Process by which nitrate (NO3-) ions are converted into gaseous forms of nitrogen 

  • Carried out by denitrifiers 

• Most identifiers are heterotrophs

  • So, process of denitrification requires organic carbon compounds

• Using NO3- as a terminal electron acceptor is less energetically efficient relative to using O2

  • When soils are well aerated, denitrification is limited

  • When soils are poorly aerated, denitrifiers can use NO3- for their growth

  • What is a terminal electron acceptor -

    • When organisms are respiring - building up a gradient of electrons

    • THe final molecule in an electron transport chain that receives electrons,

    • Last molecule in a chain of molecules that electrons go through 

• Denitrification - in  an anaerobic incubation

  • h

Reaches complete denitrification if it gets all the way to N2, otherwise is incomplete

N2O is less abundant, but more potent greenhouse gas 

Warming potential is almost 300 times greater than CO2

How will denitrification change with %WFPS (x axis)

 Denitrification will increase with x axis 

Denitrification will increase as percent of pore volume filled with water, as it is poorly aerated.

N2

• 78% of earths atmosphere

• Strongly bonded

  • Three covalent bonds →

  • Inert unreactive molecule 

Return from N2 to soil organic matter via BNF, and death turnover and decomposition

BIological nitrogen fixation (BNF)

• Important pathway for atmospheric N2 to enter terrestrial cycle

• Commonly carried out by LEGUMES

• BEAANS BEANS THE MAGICAL FRUIIIT

BNF 

• Faculatice association of N fixing bacteria(rhizobia) and a host plant(bean)

  • Legume supplies C compounds to fuel costly breakage of N2 bonds

• 13 genera of the rhizobia that vary in their effectiveness as nodulation/N2 fixation

  • Rhizobia are naturally occurring in soil

  • Attempts were made to coat seeds with optimal strains (didnt work

• Rhizobia use the nitrogenase enzyme whose activity ceases in the presence of O2

Rates of BNF

• Proportional to host biomass

• Impeded by low pH low P, Ca and K availability 

• Decline with increasing mineral N availability 

• Legumes do not need to derive all their N from BNF

Symbiotic N fixation is a more important flux than free living N fixation

Haber-Bosch process

• Developed cirva 1913

• Convers atmospheric N2 to ammonia NH3 using hidrygen gas under high heat and pressure  (usually done with fossil fuels)

• Critical source ov N to support current human population

• Increasing reactive N in biosphere carries environmental costs (leaching N2O emissions)

Denitrification - in the field  arrows indicate fertilizer applied as inorganic N

4/4/25

• If nitrate NO3- is the most oxidized form of N (loss of electrons) why does it have a negative sharge

  • Net charge (~ cation vs anion ) = overall electric charge of molecule 

  • Oxidation state = number of electrons that have been lost or gained from the atom

  • N in NO3- has given away electrons to oxygens

  • N in NH4+ has taken electrons from hydrogens 

• Arrows indicate fertilizer applied as inorganic N 

  • Key point: N2O fluzed are charachterized by “hot moments”” (shows here) when conditions in soil are right - also hot spots (not shown)

Crop response to N fertilizer 

• Why positive response 

• Widespread N limitation on plant productivity 

  • And

  • Modern crop varieties are bred in environments with large quantitites of availbale soil N to take advantage of this N 

• Why is optimal N rate lower in corn-soybean compared to corn-corn

Fate of N fertilizer

• For a cropped field in the upper mississippi river basin 

• A lot more leaching compared to other pathways 

TIMING of N fertilizer applications

• Fall (greater loss vulnerabilities 

• Spring (preplant - closer to time of plant uptake)

• Sidedress(even closer to plant uptake, bc plant is growing)

• Excess water can draw through the soil and drain and gets leached

Plant-avaibale N changes over growing season 

• Key points: plant available N spikes with fertilizer applications, is drawn down with plant uptake and loss pathways 

• PSNT - pre sidedress nitrate test

Oganic N INPUTS 

• ORGANIC N INPUTS  are lsss mobile than mineral N inputs 

  • Rate of mineralization depends on environmental controls (temp moisture)

  • And C:N ratio of organic N 

    • Manure, compost 20:1

    • Grass hay 40:1

• Exogenous organic N inputs 

  • BNF → field one —> cattle grazing → cattle waste deposit → field 2

• Recycling of N 

  • CHickens consumer alfalfa← → chickens contribute litter total soil N 

Volatilization of NH4 +

• Process of NH4+ losing a hydrogen to OH - in soil solution and becoming ammonia NH3 gas

• Soil colloids can adsorb ammonia if converted to NH4+

• Ammonia volatilization increased with 

  • Warmer temps

  • Higher soil colloids

  • NH+ availability on soil surface

    • EX surface fertilizer application not incorporated either mechanically or by water 

  • Nitrogen decomposition from the atmosphere 

    • Decomposition of ammonia and 

    • Acid rain →

      • Combustion and fossil fuels lead to production of CO2 and fossil fuel combustion

      •  

• N balances: N in vs N out 

  • N balance: 

    • Multiple types of N balance, possible system boundaries 

    • Commonly: sum of anthropogenic N inputs - N exports from crop harvest

  • This N balance provides a metric of whether

    • Excess N enters a system (--> environmental consequence of reactive N)

    • Insufficient N enters a system (--> ‘mining’ SOM: reduced yield 

    • N balance = inputs - outputs from harvest 

• Global excess of N inputs 

  • Heavily fertilized systems - crops may still take up the majority of N from SOM

DOes mineral N fertilizer ‘ kill soil”

• NO - many studies show microbial biomass increase with N fertilizer in ag systems 

• If we want to understand microbial abundance and activity between different systems, we need to 

  • Clarify which systems are being compared

  • Identify multi faceted influences on microbial communities that vary between systems (NOT just N)

4/7/25

Does mineral N fertilizer ‘kill” soil 

• No — many studies show microbial biomass increases with N fertilzer in agricultural systems 

• If we want t understand microbial abundance and activity between different systems we neeed to 

  • Clarify which systems are being compared 

  • Identify multifaceted influences on microbial communities that vary between systems (not just N)

• Wheat corn zero N fertilzer  vs wehat corn 150kg N ha-1yr-1

•   WHat can we infer about these two systems in terms

  • Crop productivity  and C inputs to soil

  • N losses (denitrification and leaching

  • Likelihood of steady state SOM vs not steady state SOM

  • Implications for soil as microbial habitat

  • When we make generalized statements about influence of mineral soils, we need to directly account for other influences and where Nirtogen is coming from

Four types of biomolecules 

• Protiens—------------------

• Nucleic acid —- >        -

• Lipics—- > requires P    - all requires CHO

• Carbs—--------------------

  • Also requires P: ATP

P limitations can limit BNF 

• CHallenge of phosphorus acquisition 

  • P is critical for plants despite its low leves (

    • 0.2-0.4% of dry matter in plant leaves (1/10th the level of N)

  • Total soil P is relatively low 

    • 500-10,000kG P in upper 50cm of soil

  • Most of total soil P is unavailable, because it is insoluble 

    • Only 10-15% of fertilizer P might be taken up bya plant that year 

• Soil phosphorous cycle 

  • Plant←– uptake Soil solution P

• P in soil solution

  • Present at low concentrations relative to other nutrients 

    • 0.001-1mg/L

  • Mix of inorganic phosphate ions (HPO42- and H2PO4-) and small organic P molecules

  • Ionic form determined by soil PH 

  • Phosphate ions relatively immobile in soil 

    • Roots quickly deplete P in nearby soil solutions as they grow

  • Plant uptake therefore aided by mycorrhizal associations

• Inorganic P (not in soil soilution)

  • P added to soil solution rapidly ‘fixed’ in soil via chemcia rxns 

  • Nature of reactions depends on soil pH 

    • In pacific soils H2PO4 precipitates with Al, Fe, or Mn ions and forms 

    • In alkaline soils , HPO4 2- reacts with Ca to form insoluble complexes

    • Reaction with silicate clays across broad pH range

    • 
      
      

Organic P

• Can be mineralized and immobilized by microbes 

  • Similar to soil N in this way 

• Rates of P mineralization correspond to rates of plant uptake in temperate systems

Gains and losses from P in soil 

• Losses 

  • Plant p removal ((5-50 kg ha-1 yr-1)) agricultural 

    • Harvest

  • Erosion (P-containing particles, 0.1-10 kg ha-1 

  • P dissolved in surface runoff (0.01 – 3.0 kg ha-1 yr-1)-1)

  • P leaching (0.0001 – 0.5 kg ha-1 yr-1) (not shows in P cycle diagram)

• Gains

  • Dust (0.05-0.5kh ha-1 yr-1)

  • Fertilizer inputs (variable agricultural) 

P fixation capacity of a soil 

• Describes tendency of soil to adsorb P into soluble forms 

• Is finite due to a finite number of P fixation sites in soil 

• Varies between soils 

  • Fe and Al ozxides contribute a LOT to P fixation

  • Organic matter contribute little to P fixation

04/09

Soil P, K

Phosphorus dynamics across climates 

Temperate soils 

• More total P

  • Lesser extent of weathering

• Added P is more available 

  • Fewer Fe, Al, oxides to fix P

• Higher efficiency of P fertilizers 

Tropical soils

• Less total P 

  • Greater extent of weathering

• Added P is more fixed 

  • Presence of Fe, Al oxides

• Lower efficiency of P fertilizers 

Historical accumulation of P

• Added P that accumulates in soil beyond plant needs is termed ‘legacy P’

• Legacy P accumulated in excess of soil P fixation capacity may provide slow-release P to waterways - even after P fertilization has ceased 

  • One contributor of eutrophication and associated hypoxia in chesapeake bay 

• Is eutrophication caused by excess nitrogen or phosphorus ?

  • Phytoplankton response to nutrient additions in whole-lake experiments 

Key point: there is no atmospheric pool of P

• P moves from terrestrial to marine systems and is returned through uplift over geologic time scales 

• Via parent materials, plants, movement 

Peak phosphorous 

• An open phosphate mine, there is no growth in phosphate observed 

• There is no substitute for phosphorous, its in our dna

• Historical view shows we will reach peak mining around 2040

What soil order has most total P 

• Aridisol> mollisol> utisol 

  • Aridisol has less weathering to wash away P

  • Mollisol has lots of organic matter extremely highly weathered 

  • Ultisol is also highly weathered

Potassium (K)

Potassium is none  of these biomolecules 

• It is in the solution of cells and activates cellular enzymes 

• Leaf K concentration in plants range from 1-4% dry matter

Potassium in plant nutrition and the environment 

• Potassium uptake by plants is high relative to P

• Soil K is high relative to P but similar to P most of it is unavailable 

• Plant- available K exists in soil as cation K+

• Soil K cycling mostly controlled by CEC and weathering, not microbiological transformations (unlike N to some extent P)

• Runoff does not cause eutrophication 

Potassium supports plant defence 

• Pine beetles killing forest stands

• Different from tree growth 

• Addition of nitrogen increasing tree mortality 

  • Could have to do with chemistry of leaves 

  • The addition of K and N allows plants to withstand these bugs and baddies more 

Potassium forms in soil 

• Total amount of soil K is greater than any other nutrient element 

  • Solution (as K+ 0.1-0.2%)

  • Exchangeable (1-2%)

  • Fixed in 2:1 clays,, NON exchangeable (1-10%

    • Soil colloids , 2:1 clays, in between sheets of 2:1 structures in clays 

    • Sometimes water

    • Sometimes K 

  • Micas primarily minerals, feldspars (unweathered parent materials) (90-98%)

    • Fine grained mica - non expanding 2:1 colloid with K in between mineral layers 

• Gains and losses of potassium from soil 

  • Gains 

    • Fertilizer 

    • Exogenous K from other systems (animal waste, residues)

  • Losses

    • Leaching

    • Plant uptake and removal 

  • N+P+K are big nutrients needed 

    • K included because rate of mineral weathering may not be sufficient to meet the needs of a plant 

• Luxury consumption of K 

  • What is the key point of this figure 

    • As potassium availability increases, so does the relative content of plants and also growth 

  • 1 what's the X axis

    • K available in soil 

  • 2 what are both the Y axis

    • K content of plants

    • Relative plant growth of yield 

  • What do each of the lines mean, how do they change, over coordinate space 

    • Lighter line is relative plant growth

    • Darker line is potassium content of plants

    • Shaded area  is luxury potassium 

    • Dotted line is potassium required for optimal growth 

4/11/25

Changes to global nutrient cycles 

Inputs and outputs are changing throughout the years

• Denitrification produces nitrous oxide which leads to moire climate change

• AS more N that isnt used is denitrified it can lead to more 

• N surplus 

• WIth P surplus 

Differing loss pathways for N and P 

• N carried by runoff 

• P carried by runoff 

  • Primary pathway for phosphorous 

• N leached in drainage water - much more easily leached because it is soluble and moves through soil profile 

  • Tile drains can lead to the ability for water to leave the system 

• P leached in drainage water

• Transport of N and P from land to water

• N is deposited in atmosphere and transferred via precipitation

Some general practices of nutrient management 

• Control total nutrient inputs 

  • INFORMATION from nutrient balance soil testing, crop testing

• For N: substitute leguminous N for mineral N 

  • Leguminous N is less mobile - carbon and nitrogen are already together

  • Sub Leguminous N for a mineral form when you need more movement 

• Prevent overland flow/erosion (esp for P)- NO TILL 

• Tie up nutrients in living biomass (esp for N)

• Modify landscape to prevent nutrient delivery to waterways (N+P)

  • Because water is such a prevalent way of movement for so many nutrients

Difference in nutrient retention capacity 

• Perennial grasses vs annual 

• Nitrogen surplus - supplied - removed 

• At same level of nitrogen surplus the annual crops are making more leaching per surplus vs perennial 

• Why would an annual crop produce more lost  nitrogen than a perennial 

  • Less soil mixing in perennial system prevents release via leaching 

  • Volatilization 

  • Thinking that microbial activity 

Incorporating perennial grasses  into arable landscapes reduces nutrient export because more nutrients available makes it so that 

Riparian buffers - next to streams

Multiple pathways for preventing nutrient flow into waterways

Inorganic forms of nutient in soil

Nitrogen

• Prevalence

  • More prevalent thatn phosphorus , greater pools (1-5%)

  • More present in organic form than in inorganic form

• Plant available?

  • inorganic forms are plant available

Phosphorous

• Prevalence

  • Inorganic P is most of soil P (30-70%), organic compounds make the rest

• Availability to plants

  • Most inorganic P is not Plant available as it is fixed in Al, Ca or Fe

Mineralization and immonlization

1 meaning and 2 impliactiosn for plant nutient

• Nitrogen

  • Immoblization

    • MICROBES CONSUME SOM

    • Mineralization nitrogen complex turns into NH4 (inorganic)

  • Implications for plant nutrient avaiblity

    • Immobilzation : microbail

• P