Soil Nitrates

Nitrogen (N) is an important nutrient for plant growth; it is a major part of chlorophyll (crucial for photosynthesis) and a key building block of proteins. Nitrogen can often be the main limiting nutrient in soils and as a result is applied as fertiliser.

Nitrogen is only available to plants in inorganic form as either ammonium (NH₄) or nitrate (NO₃).

The following information is adapted from RB209.

The Soil Nitrogen Supply (SNS) is the amount of nitrogen (in kilograms per hectare) available for uptake from the soil by the crop over its entire life.

The SNS includes three separate components:

1. Soil Mineral Nitrogen (SMN) – ammonium plus nitrate content of the soil within the crop’s rooting depth
2. Estimate of nitrogen already in the crop – total content of nitrogen in the crop when soil is sampled for SMN
3. Mineralisable soil nitrogen – estimated amount of N which becomes plant-available from mineralisation (decomposition) of organic matter and crop debris during the growing season after sampling for SMN.

The SNS depends on a range of factors which can vary between fields and seasons. It is therefore important to assess SNS each year. The key factors are:

• Nitrogen residues leftover from previous fertiliser applications
• Nitrogen residues from previous organic manure applications
• Soil type and organic matter content
• Losses of nitrogen by leaching and other processes
• Nitrogen made available for crop uptake from mineralisation of organic material during the growing season

There are two methods of determining the SNS Index of a field:

1. Field Assessment Method – based on field specific information (e.g. soil type, previous cropping, fertiliser), the SNS Index is read off a table
2. Measurement Method – involves soil analysis, useful when you are uncertain of SNS

The Field Assessment Method involves predicting the SNS Index from a table in RB209 based on soil type, previous crop and rainfall. Adjustments can be made based on fertiliser residues – regular applications of manure increases SNS by 1-2 levels; lower the index level if there is any reason to believe fertiliser residues are smaller than usual (e.g. through excessive leaching).

Learn more about nutrient management planning.

The three key inputs of nitrogen (N) for plant growth are:

• Atmospheric N – made available through legumes
• Artificial fertiliser
• Organic sources – includes soil organic matter, crop residues and manures. These organic sources must be converted to inorganic forms of N through decomposition by microorganisms.

Nitrogen can be lost from the soil through:

• Leaching – water soluble nitrates move out of soil with excess water
• Denitrification – primarily occurs in anaerobic (i.e. waterlogged) soils, nitrates are converted to nitrous oxide gas and lost to the atmosphere
• Volatilisation – N from surface-applied fertilisers is lost to the atmosphere as ammonia gas
• Crop removal – N in plants is lost as the crop is harvested
• Soil erosion and runoff – N can be lost with soil through erosion and runoff.

Soil type will influence how nitrogen is lost. Sandy soils may lose N through leaching while heavy, wet soils lose N through denitrification.

There are management practices that will reduce N losses:

• Drainage – reduces waterlogging and anaerobic soils so prevents denitrification
• Incorporating/injecting manure and fertiliser – reduces volatilisation
• Returning crop residue to the field – returns N to the soil
• Soil conservation practices reduce soil erosion

Source: Understanding Nitrogen in Soils

Agricultural soils contain a lot of nitrates which represent the main source of greenhouse gas (GHG) emissions in the UK. Nitrates (NO₃-) in the soil are converted into the potent greenhouse gas nitrous oxide (N₂O), during a process called denitrification. Denitrification happens under oxygen limiting conditions.

Nitrate is water soluble so can leach out of soils and pollute watercourses. The Nitrates Directive and Water Framework Directive aim to protect water quality by promoting good farming practices and preventing nitrates polluting water. As part of these directives, 58% of England is in a Nitrate Vulnerable Zone.

A guide to management practices to reduce water pollution from agriculture is available here.

Nitrates in the soil can originate from three different sources:

1. Nitrates added from fertiliser application
2. Newly formed nitrates from ammonia, and
3. Existing or ‘native’ soil nitrates.

Scientists at North Wyke are carrying out research to determine the factors controlling the utilisation of those different pools of nitrates.

To achieve this they are using a denitrification incubation system (DENIS, pictured below left) which allows them to measure directly the nitrogen-containing gases (NO, N₂O and N₂) by keeping soil cores (pictured below right) under a nitrogen-free atmosphere, to investigate how microorganisms utilise different soil nitrate pools as a source of available N.

The differences between nitrification (when ammonia is converted to nitrates), and denitrification are examined through looking at different soil moisture contents and varying carbon to nitrogen ratios (C:N) to determine the potential switch between the utilisation of fertiliser nitrates and native nitrates.

Go to Rothamsted’s DENIS Laboratory to find out more.

What does this mean for farmers?

Ultimately this project will lead to a better understanding of nitrous oxide emissions, improved nutrient management and possibly less reliance on fertiliser nitrates. The answer could simply be related to adding a carbon source to the soil for the ‘native’ N to be utilised by the crop.