Page 29 - Nutrient Management Planning Guide

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Chapter 2.1

Photo courtesy Surya Acharya, AAFC

Figure 2.1.4 Root Nodules on a Legume Inhabited by Rhizobium Bacteria

Organic N compounds are an important source of N for crops. These compounds, which are part of soil organic matter, must undergo decomposition before the N they contain is plant available. Soil organisms (e.g., insects, small animals, and microorganisms) gradually break down complex N compounds into simpler forms in a process called mineralization. In the process of N mineralization, organic N compounds are converted to NH

4

+ , which can be taken up by plants from the soil. However, NH

4

+ is usually converted quickly to NO

3

- by bacteria in the soil through a process called nitrification.

Ammonium has a positive charge and can be temporarily held by negative charges on soil particles. This type of reaction is called cation exchange and will be discussed further in Chapter 2.2. It is important to know that exchange reactions are critical to the nutrient holding capacity of the soil and the ability of the soil to replenish nutrient concentrations in the soil solution. Adsorbed NH

4

+ makes up a portion of the exchangeable pool of N, but as adsorbed NH

4

+ it is unavailable for plant uptake until released into soil solution.

Nitrogen in the soil can also be temporarily tied up by the microbial biomass, in a process referred to as immobilization. Soil microbes require N to decompose crop residues and can get this either from the residue or soil solution. Residues with higher carbon to nitrogen ratios and more lignin, like cereal straw, decompose more slowly, immobilizing N for longer periods. Eventually decomposition will slow and microbial biomass will release the N, increasing plant available N.

Nitrogen can be lost from the soil in four alternative ways depending on the chemical form. NH

4

+ can be converted to NH

3

gas and lost to the atmosphere through volatilization. Situations that favour NH

3

volatilization include alkaline soil pH, low buffering capacity (directly related to cation exchange capacity) and warm moist (but drying) soil conditions. NO

3

- can be lost from soil through denitrification – the conversion to N

2

O or N

2

gas through microbial activity when soil oxygen levels are low. Soils that experience anaerobic (low oxygen) conditions (e.g., water logging) are more subject to denitrification.

Nitrogen is also lost from soil systems through leaching and runoff. Nitrate is one of the most mobile nutrients in the soil system and readily moves with soil water. Leaching losses of NO

3

- in Alberta are limited because of the semi-arid prairie climate, although exceptions do exist. Leaching can occur in years of abnormally high rainfall, under irrigation and on fallowed fields, especially when these conditions occur on coarse-textured soils.

Another mode of N loss from soil is through surface runoff carrying dissolved nutrients or sediments. This can then enter surface water ecosystems, contributing to eutrophication.