RENR 445 soil fertility midterm 2

What type of nitrogen makes up 78% of the atmosphere?

N2 gas (inert nitrogen)

What are the major sources of reactive N (N available to biota)?

• inorganic fert produced via Haber-Bosch synthesis

• biologically fixed N

• atmospheric deposition of N that previously volatized elsewehere

• release of N from soil organic matter mineralization

Total nitrogen (TN): total organic nitrogen (TON) + inorganic nitrogen. Where is organic nitrogen and inorganic nitrogen found? Where is Organic N in the soil profile?

ON is found in SOM - 80% of N is ON

inorganic (mineral) N (NH4+ and NO3-) accounts for 1-2% of soil TN and is altered by fertilization

Organic N in soil profile is found nearer to the surface, down to bout 40 cm

Total N in AB soils

• more N in fine soil (vs coarse)
  more N in luvisolic vs chernozemic soil

What is soil organic nitrogen (SON) comprised of?

amide, amine, and hertocyclic N bonds . 90% SON is sorbed to soil minerals or protected within aggregates

SON is also present in:

• particular organic matter (POM; 1-4%), which is a transitory pool of decomposing organic material

• dissolved organic nitrogen (DON; 0.1-2%): the soluble N in the soil solution

How is dissolved organic nitrogen (DON) divided?

high molecular weight (HMW) —> proteins

Low molecular weight (LMW) —> amino acids, peptides

What is depolymerization in the context of HMW DON?

HMW DON require extracellular enzume mitigated degradation to LWM DON

LWM DON can be taken up by microbes and plant roots

What is the main source of N for most crops?

inroganic N
- ammonium (NH4+)

nitrate (NO3-)

What is the preferred form of N for plant uptake?

nitrate

analyze and explain the transformations of N in the soil and how those transformations are controlled

industrial fixation

industrial conversion of molecular N to ammonia NH3

atmospheric fixation

lightning and photochemical conversion of molecular N to nitrate N2 —> NO3-

biological fixation

prokaryotic conversion of molecular N to ammonia
N2 —> NH3

plant acquisition

plant absorption and assimilation of ammonium or nitrate

immobilization

microbial absorption and assimilation of ammonium or nitrate

ammonification

bacterial and fungal catabolism of SOM to ammonium

nitrification

bacterial oxidation of ammonium to nitrite and bacterial oxidation of nitrite to nitrate

denitrification

bacterial conversion of nitrate to nitrous oxide and molecular nitrogen

mineralization

bacterial and fungal catabolism of SOM to mineral N through ammonifcation

volatilization

physical loss of gaseous ammonia to the atmosphere

nitrate leaching

physical flow of nitrate dissolved in gw out of the topsoil and eventually into the oceans

N2 fixation

• biological nitrogen fixation

• lighting

• haber-bosch process

N2 fixation by lighting

N2 + O2 —NOx

NOx + H2O —> HNO2 or HNO3

falls to the surface in rain

source of NO3-

haber-nosch process

developed in 1900s by haber, modified in 1912 by bosch

uses high pressure, N2 and H2 to form NH3

biological N2 fixation

• carried about by a group of prokaryotes (Diazotrophs) using the enzyme nitrogenase

• requires a large energy investment: 16 moles of ATP for each mole of N

• symbiotic and nonsymbiotic N2 fixation

  

  c N2 fixation

symbiotic biological N2 fixation

• species-species association in root nodules between Rhizobium species and legumes, also Frankia and some non-legume trees

  • legumes (alfalfa, beans, chickpeans, soybeans, peas, lentils)

• plants exchange energy (sugars and CHO) for fixed N from bacteria

• meets 25-90% of plant N requirements

• can meet some of the N requirements of subsequent plants through minerlization oh host plant residue

  • depends on the quantity of N2 fixed and the amount of reside returned

what is symbiotic biologcal N2 fixation affected by:

• pH: low pH impairs rhizobial activity

• nitrate: preferred alternative to biological fixation as a source of N for the host plant

• other limiting factors (water, other nutrients)

• species: forages > pulse crops

nonsymbiotic biological N2 fixation

• free-living or associative

• non-specific association between micro-organisms and plants

• ex: cyanobacteria and rice in paddy soils

• fixation rates are much lower than those of symbiotic bacteria

atmospheric N desposition

• source of reactive N

• natural and anthropogenic sources of atmospheric N deposition:

  • lighting, vocanoes

  • combustion of fossil fuels, industrial processes

  • agricultural practices

  • biomass burning

• NOx, HNO3, NH3, NH4+, NO3-

There are two ways of atmospheric deposition which are:

wet deposition

• rain, snow

dry deposition

• dust

• gaseous adsorption

What is cloud scavenging?

the condensation of water vapour on aerosol particles during the formation of cloud droplets

Below cloud scavenging: raindrops dissolve particles during their fall

spatial patterns of N depositition?

• need to look into more

• depends on emission sources, atmospheric transport, prec. patterns
  
  more in industrial areas and agricultural hotspots

environmental impacts of Atm N deposition

• reduces biodiversity, contributes to eutrophication, acidification, declining air and water quality

components of SOM

~50% SOC, 5% SON

What is SOM? SOM Sources?

sources:

• aboveground biomass

• belowground biomass

• soil organisms

microbes us OM decomposition as a source of energy and nutrients

SOM impacts:

• nutrient supply

• soil structure

• available water holding capacity

• reduces erosion

• biological activity

• nutrient retention and buffering capacity

• C sequestration

• pollutant buffering

SOM decomposition

• performed by hetertrophic bacteria to derive energy

• hetertrophs cannot produce their own food, they extract energy from organic carbon and nitrogen (plant litter, SOM)

• hetertrophs oxidize C and N-rich compounds to release chemical energy required for metabolic functioning

• decomposition of SOM depends on extracellular enzymes produced by microorganisms

rate of SOM decomposition is dependent on:

• temp

• soi moisture

• microbial community composiiton (oligotrophs vs copiotrophs)

• pH

In order to be transported to microorganisms and transferred across microbial membranes, organic substrates must be in what form?

dissolved form

mineralization

organic N —> mineral N via biological oxidation

performed by hetertrophic soil microorganisms

2 types of mineralization

aminization, ammonification

aminization

macromolecules of organic C —> amino acids, amines, urea

ammonification

amino acids, amines, urea —> NH4+

rates of mineralization dependent on:

• quantity of substrate

• quality of substrate

• soil management practices

• temp

• soil moisture

aminization and ammonifcation chemical process

immobilization

mineral nitrogen —> organic nitrogen via microbial assimilation

reverse of mineralization

Mineralization and immobilization

• depends on the C:N ratio of the substrate vs the microbe

• hetertrophic microbes seek to maintain a C:N ratio of 8:1

• during decomposition, microbes oxidize ~50% of substrate to CO2

  • carbon use efficiency (CUE)

• therefore, if initial C:N ratio is >16:1, then microbes take up mineral N from the soil (immobilization)

• but products of substrate decomposition will have a smaller C:N ratio because 50% of C has been oxidized

• the lower the initial C:N ratio of the residues, the more rapid the initial decomposition process

net mineralization calculation

how to build-up and maintain SOM

• reduced or no till

• cover crops

  • clover, fall rye, radish, ryegrasses, alfalfa, hairy vetch, winter wheat,

• fertilizer, manure

soil health

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

characteristics of a healthy soil affected by SOM

• sufficient depth

• good water storage and drainage

• sufficient (but not excess) nutrients

• large pop. of beneficial organisms

• resistance to degradation

• resilience in unfavorable conditions

common soil constraints affected by SOM

• soil compaction

• poor aggregation

• low water and nutrient retention

emerging view vs previous view of SOM persistence

emerging view: biological, physical and chemical transformation processes convert SOM into products that form associations with minerals

previous: ‘humification’ creates large, complex, recalcitrant ‘humic substances’ to make soil ‘humus’

the existence of humic substances has not been verified by direct measurements

soil humification model - humification

further transformation of initial decomposition products into large, dark-colored compounds

• results in macromolecules that are rich in C and N (‘humic substances’

  • resistant decomposition and are stable

Soil Continuum Model

• SOM as a continuum

• plants and fauna are broken down via decomp

• a continuum exists from large, energy-rich compounds, to small, energy-poor compounds

• excludes any secondary s

  

  ynthesis of ‘humic substances’

SOM protected by:

• physical protection

• chemical protection

  • microbial processing increases polar groups

  • polar groups= sorption onto soil mineral surfaces and inclusion in aggregates

• spatial heterogeneity

• molecular diversity

• temporal mismatch

fates of ammonium:

• immobilized

• adsorbed to surfaces and soil particles

• fixed in expanded latices of clay minerals

• nitrification

• volatilization (N loss)

soil colloids (reactive fraction)

finer size fractions of the soil (clay and OM)

• most chemically active portion of soil

<2.0 micrometers

high SA

colloidal properties of soil (imparted by clays and OM)

• nutrient retention

• shrink/swell

• water holding capacity

• buffering capacity

cation exchange capacity

the number of centimoles of positive charge that can be adsorbed per unit mass

• determined by the amounts of different colloids

cations commonly held:

• NH4+, Ca2+, Mg2+, K+, H+, Al3+

cation exchange

cations in soil solution exchange with cations attached to colloids

NH4+ adsorption

• results from the charge on clay minerals

• Al3+>H+>Mg2+>K+ = NH4+>Na+

NH4+ adsorption is affected by:

• CEC

• concentration in soil solutions

• presence of other cations

• pH

NH4+ fixation

• ammonium replaces interlayer cations in expanded lattices of clay minerals

• slow equilibrium with echangeable NH4+

affected by:

• wetting-drying, freezing-thawing expands lattices, promotes fixation

• K+: closes lattices, reduces fixation

nitrification

2 steps

• performed by chem0autotrophic bacteria

• generation of H+ causes soil acidification over time

  • sustained use of NH4+ fertilizers requires periodic liming (CaCO3)

• the repeated addition of ammonium-based fertilizers causes nitrification rates to be higher than under natural conditions. Nitrification —> soil acidification

factors affecting nitrification

• supply of NH4+

• soil pH

• O2 supply

• soil moisture

• soil temp

denitrification

• carried out by facultative anaerobic bacteria and fungi

• performed because microbes require a TEA

• removes Nr from soil and returns N2 to the atmosphere

• NO3- used as TEA

factors affecting dentrification

• OM

• water comtent

• pH

• temp

• No3-

• plant growth

N2O emission is promoted by:

• fert application

• soil wetting

• soil warming

What produces nitrate, and what removes nitrate?

produces:

• nitrification

• fertilizer

removes:

• immobilization

• dentrification

• plant uptake

What causes nitrate leaching? Negative effects of leaching?

• precipitation and irrigation exceeds soil water holding capacity and evapotranspiration, causing downwards water movement

negative effects of leaching:

• pollution, eutrophication

start at Feb 14

nitrogen volatization

the loss of N through the conversion of ammonium to ammonia gas, which is release to the atmosphere

At pH > 9.3 [NH3] > [NH4+]

At pH <9.3 [NH3] < [NH4+]

factors affecting volatization

• soil pH

• soil buffering capacity

• water content

• temperature

anhydrous NH3

• 82% N

• basc buildinf block of most chemically derived N ferilizer

• haber-bosch using N2 from air and H2 from natural gas

• most is used to manufacture other N fert

• easily volatilized

urea

• 46% N

• most widely used source N

• when applied to soil, urea is hydrolyzed by the enzyme urease to NH4+

• subject to volatilization in high pH soils

aqua NH3

• 20-24% N

• bulky, limited use

ammonium nitrate

33-24% N

highly explosive

Beirut explosion

ammonium sulfate

21% N

decreases soil pH more than other sources

ammonium phosphates

more often used as a P source

ammonium chloride

25% N

acid forming

important for rice

only for Cl- tolerant crops

NO30 salts

immediately available

easily leached

which of these ferts raises and which lowers pH?

urea hydrolysis and ammonifcation raise pH

nitrification decreases pH

pH changes due to N fertilizer can impact:

• availability of other nutriens

• NH3 volatization

• solubility of SOM

slow release N fert

ex. ESN, polymer-coated ureafert

• reduce N2O emissions

coated in a semi-permeable polymer or sulfur OR altered chemical formulation to decrease solubility

synchonization with plant N uptake

• nutrient requirements lower in early plant stages

• sufficient N required 2-3 weeks after emergence

nitrificatio and urease inhibitors

Prevent or delay hydrolysis of urea by inhibiting urease

• Inhibit nitrifying bacteria (ex: Nitrosomonas), delaying NH4

+  NO3

-

manure, compost, biosolids, plant resides

• slow release over time

• provides N and other nutrients (50% N, 20%, nearly all required K)

• improve soil health

• hetergenous

• can be high salt

• losses from NH3 volatilization and NO3- leaching

start at february 24

Alberta’s agricultural operation practices Act (AOPA)

to ensure the salts in manure do not affect plant growth:

• manure may not be applied at rates that would result in a one ds/m increase in EC in the top 15 cm of soil

• manure application is prohibited if the EC of the soil in the top 15 cm is greater than 4 dS/m

AOPA setbacks from common water for manure application on laand

4% slope or less: 30 m

4-6% slope: 60 m

6-12% slope 90 m

min setback distances for manure application

• 150m away from residence

• 30 m away from water well

• 10.m away from common body of water if subsurface injection is used

• 30 m away from a common body of water if surface applied

• incorporated within 48 hours of application

under AOPA, soil NO3- N limits have been set for the top 60cm of soil. The max allowable level depends on:

• productive potential

• soil texture

• depth to water table

• soil type

fertilizer nitrogen use efficiency (NUE)

amount of N fert taken up by the crop compared to how much N fert is applied

• there is a lack of spatial and temporal synchonization between N supply and crop N demand

• waterlogged soils promote N loss

goals of nutrient management

1. maintenance of SOM

2. ensure plant N requirements are met

3. minimize environmentally damaging N losses from the soil plant system

4R nutrient stewardship

• right source

  • select the correct source of nuttrient

  • balance supply of essential plant nutrients

• right rate

  • consider the availability of nutrients from all sources

  • perform annual soil testing

  • apply nutrients to meet requirements, accounting for nutrients already in the soil

• right time

  • avoid application when losses could be high

  • plan on an annual basis

• right place

  • setback distances for applications near waterways

  • place nutrients where they can be taken up by growing roots

Right source

• fert type

  • conventional, EEF, organic amedments

• recognize synergism or antagonism among elements

  • N and P = synergistic growth

  • P fert can reduce Zn availability

right rate

• law of diminishing returns

• soil test

  • soils are highly variable

  • timing is critical

  • send to lab for: available N, P, K, Mg

calculating N fert requirements

Nirtrogen fertilizer rate = plant N requirement- availalble soil N beginning of the season - N mineralized during the season

spring vs fall N application

• spring application supplies N closer to when the crop needs it

• winter and early spring leaching in colder climates

• less N2O emissions in spring

Right Place

• address root-soil dynamics

• nutrient movement

• spatial variability within the field

• kinds of application

  • broadcast

  • foliar

  • localized

    • banded

    • seed placed