Soil Nitrogen Cycling

Introduction

Two lectures on soil nitrogen cycling, with a third lecture focusing on nitrous oxide emissions.
Nitrogen fixation will be integrated into the first two lectures.

Forms of Soil Nitrogen

Most soil nitrogen is in organic form, which is not directly usable by most plants.
Exceptions exist among native plants, but agricultural plants cannot directly use organic nitrogen.
A small proportion exists as ammonium (NH4+), which can be absorbed by negatively charged clay minerals.
A very small proportion is in soil solution in mineral form, readily available to plants.

Nitrogen Inputs

Nitrogen Fixation

Significant in Australia.
The air is 79% molecular nitrogen (N2), which plants cannot use directly.
Microorganisms, such as rhizobia in symbiotic relationships with legumes, can fix nitrogen.

Industrial Nitrogen Fixation

Production of nitrogen fertilizers.
Molecular nitrogen in the air is the primary source, fixed either biologically or industrially.
Processed nitrogen cycles through feed, animal returns, and crop returns.
Microbial returns or decay of microbial biomass also contribute.

Atmospheric Deposition

Minor input in Australia but substantial in polluted areas like Europe.
Nitrogen oxides in polluted air dissolve in rainwater, forming nitrates and nitrites.
Molecular nitrogen (N2) is inert and does not dissolve in water.
In the UK, precipitation can contribute significantly (e.g., 80 kg N/hectare/year).
Nitrous oxides produced from lightning: a relatively minor input.

Nitrogen Cycling Processes

Crop and animal residues add nitrogen to the soil.
Inputs include industrial fertilizers and both symbiotic and non-symbiotic nitrogen fixation.
Non-symbiotic nitrogen fixation: various microbial genera fix nitrogen independently of plants.
Atmospheric deposition is a relatively minor input.

Ammonium (NH4+)

First form of nitrogen in soil cycling processes, resulting from mineralization of residues and wastes.
Plants can take up ammonium, although some, like canola, may suffer from ammonia toxicity.
Wheat can tolerate 100% ammonium-supplied nitrogen.
Ammonium can be nitrified to nitrate (NO3-).

Nitrate (NO3-)

Produced through nitrification, also from atmospheric deposition and fertilizers.
All plants can take up nitrate without adverse effects.

Nitrogen Losses

Ammonium can be lost through volatilization.
Nitrate can be lost through leaching into deeper soil layers and groundwater.
Denitrification is another significant loss pathway for nitrate.

Microorganisms and Nitrogen

Microorganisms take up both ammonium and nitrate, leading to immobilization.
Nitrogen becomes unavailable to crops during microbial expansion.
After the microbial expansion phase, microbial biomass decays, releasing nitrogen.
The cycle completes when crops can take up the released nitrogen.

Detailed Processes

Ammonification

First step in mineralization of nitrogen.
Ammonium is converted to nitrate.
Immobilization and mobilization depend on the microbial growth cycle stage.
Denitrification and volatilization are also crucial.
Ammonification: organic nitrogen forms (e.g., proteins) are converted to ammonium (NH4+).
Microorganisms perform this process to obtain energy and food.
The balance between mobilization and immobilization depends on the nitrogen content in organic matter relative to microbial needs.

Nitrification

Second step: ammonium is converted to nitrate in a two-step process by specific microorganisms.
Nitrification has very little redundancy; few microbial taxa can perform it.
Requires plenty of oxygen; inhibited in waterlogged soils.
Ammonium is converted to nitrite (NO2-) and then to nitrate (NO3-).
Energy is a byproduct, driving the process.
The first step (ammonium to nitrite) is slower and rate-limiting.

Nitrogen Immobilization

Reverse of decomposition: inorganic nitrogen is converted to organic as microorganisms assimilate it.
Dependent on microbial growth cycle stages.
Initially, rapid microbial growth occurs with the addition of carbon-based food.
If nitrogen is insufficient in the organic matter, microorganisms take it from the soil, causing immobilization.
Microorganisms are more competitive for nitrogen than plants.
The balance between mineralization and immobilization determines whether net mineralization or immobilization occurs.

Denitrification

Occurs when oxygen is limited, such as in waterlogged soils (e.g., paddy rice) or after heavy rain.
Requires carbon and oxygen in organic material.
Products include nitrous oxide (N2O) and nitrogen gas (N2).
Complete denitrification results in nitrogen gas, which replenishes atmospheric nitrogen.

Soil Stratification Example (Beverly and Avondale, Australia)

Demonstrates stratification of mineralization and nitrification rates down the soil profile.
Significant drop in rates even within the top 2.5 cm (1 inch).
Low microbial life and strong stratification of organic matter content in Australian soils.
Most activity occurs in the top 2.5 cm.

Calculations

Rough calculations to estimate nitrogen from decomposition.
$1 \mu g/cm^3 = 1 g/m^3$
$100 \text{ days of activity}$
Approximately 100 kg N/hectare/year is produced.
Not all nitrogen is retained due to losses from leaching, volatilization, etc.

Organic Matter Decomposition

16 tons of organic matter per hectare results from ~3 tons of wheat grain harvested.
Approximately 5% nitrogen in organic matter.
About 10% of total organic matter is decomposed in one year (range: 5-50% depending on conditions).
Decomposition can be quite fast and a substantial nutrient source.

Carbon to Nitrogen (C:N) Ratio

Different organic matter types have different C:N ratios, indicating quality.
Low C:N ratio indicates high-quality organic matter, which decomposes faster.
Microorganisms release nitrogen, encouraging growth and faster decomposition.
High C:N ratio indicates poor-quality organic matter, leading to slower decomposition and nitrogen immobilization.

Reference Table

Examples:
- Lucerne, compost, and rotted manure: high quality, low C:N ratio.
- Maize, wheat: higher C:N ratio.
- Sawdust: very high C:N ratio.

Relationship between C:N Ratio and Mineralization

Between C:N ratios of 20-40, mineralization rates are relatively constant.
Below a C:N ratio of 22, mineralization increases with decreasing C:N ratio (increased quality).

Schematic Representation of Decomposition

Microbial biomass expands initially, causing a drop in nitrogen levels due to immobilization.
Microbial biomass peaks when carbon becomes limiting, then declines.
As microorganisms decay, nitrogen is released, eventually resulting in a net gain compared to initial levels.

Nitrogen Depression Period

The period when microorganisms immobilize nitrogen, making it unavailable to crops.
In some systems, farmers apply nitrogen fertilizer to feed microorganisms and speed up decomposition.

Practical Examples

Vineyards: grape mark (skins, seeds, stocks) is composted and returned to the soil.
Composted grape mark initially causes a nitrogen decline, followed by a release of nitrogen as it decomposes.

Compost Quality

Compost quality varies based on feedstocks.
Higher nitrogen content generally indicates greater quality but requires careful management to prevent nitrate loss.
Increased organic matter and nitrogen lead to shorter times for nitrate release.