NITROGEN CYCLING & NITROGEN FIXATION



Lecture Plan (5 lectures)
1. Ecosystem N cycling - The N cycle (revision)
2. Biology of Nitrogen Fixation - Symbiotic 1
3. Biology of Nitrogen Fixation - Symbiotic 2
4. Biology of Nitrogen Fixation - Free living (Non-symbiotic)
5. Nitrogen Fixation in Tropical Agriculture

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Lecture 1. Ecosystem Nitrogen Cycling

Lecture Plan
The N Cycle
Losses and Gains
Important Terms
N in global terms
How microbes do it
N balance sheets (inputs = outputs)


The N cycle (revision)


Important points about nitrogen cycling

All soil N originates from the atmosphere (not from the rocks like other nutrients)

C, P and S cycling are closely associated with the N cycle

Sustainability is defined as : N inputs = N outputs (i.e. no net loss of N from the system)
N Gains and Losses
N Gains
N2 fixation
Rainwater (dissolved NH4+ and NO3-)
Animal manure and plant residues (indirect)
N Losses
Crop removal
Leaching
Gaseous losses (volatilisation)
Erosion and runoff

Do these inputs provide enough N for sustainable agriculture in the UK?

We know cereal crops need 100-200 kg N Ha-1

Using current practices we are not sustainable - Why ?

Too much N loss from leaching (zero crop cover in the winter)
Loss of N from animal manure as NH3
Not enough crop rotation and N2 fixation
Rainfall inputs are small (10 kg/Ha/y: only 10 % of what's needed)
Therefore we need fertilzers - but at the moment we use them inefficiently

With future cropping practices it is also unlikely - Why ?

We remove too much N with the crop (we are too greedy)
We don't give the soil time to recover (this loses the farmers money)
Not everybody wants to eat N2 fixing plants (i.e. beans)
N2 fixing plants need to be supercharged to increase N inputs (we haven't done this yet)
We are pushed more by economics more than sustainability (esp. by the EEC)

We need to optimize gains and minimize losses

e.g. enhance N2 fixation
e.g. reduce leaching

The problem is that we know what to do but it is not economic.
Virgil in 40 BC told us what to do: To prevent depletion of fertility, rotate crops, leave fallow or plant legumes

Important N cycle terms - learn these !
Biological N2 Fixation conversion of N2 gas to organic N forms
Immobilisation uptake of inorganic N and conversion to organic N
Mineralization conversion of organic N to inorganic N by microbes
Ammonification conversion of organic N to NH4+
Nitrification conversion of NH4+ to NO3-
Denitrification conversion of NO3- to N2
Fire conversion of Organic N to N2 gas

Organic Matter N (what is it and what is it composed of ?)

20-50 % amino acids/proteins
5-10 % as amino sugars
1 % as nucleic acids
50 % is unknown (probably mostly microbial breakdown products e.g. bits of lignin)

Most N transformations in soil are carried out by microorganisms.
Microbes control the N cycle.
 Exceptions to this (chemical/physical reactions) includes
Fixation of NH4 (+ve charge) by clay particles (-ve charge)
Leaching of NO3-
Fire (caused by lightning strikes)
Text Version
Now lets look at where all the N is in the world to fill in some of the pool sizes in the N cycle


Where is all the N in the world ?
Global N statistics N content

(x 1016 kg)

Atmosphere 386
Ocean 2.3
Soils 0.02
Plants and Animals 0.05
i.e most is in the atmosphere


Where is all the N in the soil ?
Soil-Plant-Animal Component % in each pool
Plant Biomass 3.5
Animal Biomass 0.06
Litter 0.6
Soil organic matter 90
Soil Biomass (microbial & mesofauna) 0.15
Fixed NH4+ 4.8
Soluble NH4+ and NO3- 0.3
i.e. most is in the soil organic matter





Effect of soil type on organic N content of soil

Soil Type % of soil N as organic N
Upland Peat (Histosol) 99
Spruce forest (Podzol) 90
Deciduous forest (Cambisol/brown earth) 90
Tropical forest (Oxisol) 90

The average residence time for N in soil has been estimated at 175 years


Nitrogen as a Plant Nutrient (sse previous lectures too !)

Nitrate is the predominant form of N taken up by crop plants (exception is rice; NH4+)
The enzymes involved are nitrate reductase and nitrite reductase

NO3- NO2- NH3 Amino acids

Ammonium is the predominant form of N taken up by trees and microbes

N uptake is closely regulated: supply=demand

Most plants do not accumulate N

75 % of the N taken up by a plant is removed with the crop (see below)



Here's a typical N use efficiency curve for cereal crops. I'm sure you've seen this before but to recap, as you increase fertilizer N inputs, crop yields increase significantly until crop growth becomes limited by some other factor unrelated to N (e.g. P uptake , photosynthesis)


Nitrogen Losses

N is the most mobile nutrient in the soil

At least 30 % of all fertilizer is lost

Bacterial Ammonification and Nitrification

Ammonification

When microbes have too much N for their own requirements they excrete the excess as NH4+ into the soil. This happens mainly when the microbes are degrading crop residues with low C:N ratios e.g. dead nodules high in N

In high pH soils NH4+ (ion in solution) is unstable and changes to NH3 (gas) which can be lost via volatalization

Nitrification

This is an aerobic reaction carried out by autotrophic bacteria which converts ammonium (NH4+) to nitrate (NO3-).

Maximal nitrification rates occur at neutral pH and high temperatures (factors that favour the bacteria involved in this process - Nitrosomonas and Nitrobacter)

The nitrification process is shown below




Bacterial Denitrification

Microbial reduction of NO3- to NO2- or N2
e.g. Pseudomonas

Use NO3- instead of O2 as a terminal electron acceptor

Denitrification is accelerated under anaerobic (flooding) conditions and high fertilisers and also to a lesser extent the following:

high temperatures
neutral pH
high organic matter


Results in environmental pollution (destroys ozone)

It also contributes to global warming (nitrous oxides; minor effect)

Ammonia volatilisation

Chemical process converting ammonium (NH4) to ammonia (NH3) which is accelerated by wind, high pH (>7) and temperature


It occurs largely in arid calcareous soils

Also occurs in soils with low exchange capacity (e.g. sandy soils)as NH4 is not firmly held

It's prevalent in soils to which farmyard manures or urea have been added (you can smell the NH3 coming off chicken manure for example)

Leaching, runoff and erosion

This occurs mainly as NO3 (as it's sorbed weakly to the soil)

Accelerated under high NO3 fertilzer applications (supply> plant demand)

NH4 is only leached from sandy soils (otherwise it's firmly held on the exchange phase)

Leaching requires high rainfall to move NO3 away from the rooting zone

Runoff - occurs when there's high rainfall, soil compaction and no groundcover

Erosion - wind and water carrying the soil away

Methodology for N research

1. Identify N-fixing micro-organisms and plants via growth in N-poor media (you also culture predators, however)

2. Measure total N in plant, soil or microbe by Kjeldahl-N digests (effective but insensitive)

3. Labelled 15N (N2, fertilizers)

4. Acetylene reduction (N2 fixation) (see later lectures)

NN 2NH3

HCHC H2C=CH2

(acetylene) (ethylene)
Adapted from a site on Nitrogen Cycling and Nitrogen Fixation.