Page 28 - Nutrient Management Planning Guide

24

Soil Nutrient Cycling

Adapted from Jones and Jacobsen 2001a

Figure 2.1.3 The Agricultural Nitrogen Cycle

Seven forms of N are involved in the N cycle: atmospheric N gas (N

2

), ammonium (NH

4

+ ), ammonia (NH

3

), nitrate (NO

3

- ), nitrite (NO

2

- ), nitrogen oxide gases (NO, N

2

O) and organic N. Each form of N exists in a pool. For example, organic N is part of the organic pool, NO

3

- exists in the soil solution pool and NH

4

+ can be present in the soil solution or exchangeable pool. Plants can only directly use inorganic N (NH

4

+ and NO

3

- ) to meet their N requirements.

Commercial N fertilizers are produced through industrial N fixation. Atmospheric N

2

is reacted with hydrogen gas (produced from the steam treatment of natural gas) under high pressure and heated to form NH

3

, which can be used directly as a fertilizer (i.e., anhydrous ammonia) or can undergo additional processing to produce other forms of N fertilizer.

s i d e b a r

Atmospheric N

2

makes up 78% of the gases in the atmosphere. While N frequently limits crop production, thousands of tonnes are present in the air. For crops to access this pool of N it must be converted to a plant-available form. Legumes (e.g., alfalfa, clover, peas, beans) are able to access atmospheric N

2

through a symbiotic relationship with Rhizobium bacteria (Figure 2.1.4). In exchange for energy, Rhizobium convert atmospheric N

2

to plant-available forms in a process called fixation.

Crop removal

Plant uptake

Immobilization

Mineralization

Organic N

Clay

Volatilization

NH gas

3

NH

4

+

Denitrification

Leaching

Exchange

Plant uptake

Nitrification

NO

3

-

NO

2

-

N

2

Fixation

Plant residue

Erosion