Denitrification in cereal crops

What is denitrification? 

Cereal crops need a lot of nitrogen to grow well and produce high yields with good grain protein. Most nitrogen found in the soil is nitrate (NO3) and, to a lesser extent, ammonium (NH4). These forms come from fertiliser and the breakdown of organic matter in the soil. 

When soil becomes waterlogged, oxygen is quickly used up by microbes. To survive, denitrifying bacteria switch to using nitrate instead of oxygen. This sets off a chemical process that converts nitrate into nitrogen gases: 

• nitric oxide (NO)
• nitrous oxide (N2O)
• dinitrogen (N2) 

These gases then escape into the air. This means the soil loses nitrogen, and the crop may not have enough left to meet yield and protein targets. 

Measuring nitrogen loss 

It is hard to measure nitrogen gas (N2) losses in the field, because the air already contains 70% nitrogen. Measuring nitrous oxide (N2O) is easier, so it is often used to indicate that denitrification has occurred. There is no standard ratio of N2O to N2, and actual nitrogen losses can vary widely, even within the same paddock. 

Several factors affect how quickly denitrification happens, including: 

• Soil temperature Warmer soil speeds up the process.
• Organic matter. Bacteria need this as a food source. 
• Soil type. for example, vertosols in northern cropping areas drain poorly, which increases denitrification risk 

Denitrification can happen even in cool conditions. While low soil temperatures may slow the process, they will not stop it. Research shows that long periods of waterlogging or flooding can quickly reduce nitrogen in the topsoil. Less denitrification occurs deeper in the soil due to lower organic matter and fewer microbes. 

Other ways nitrogen moves in wet soils 

Nitrate is very mobile in the soil. After rain, water can move nitrate deeper into the soil instead of escaping as gas. This deep nitrogen may not help yield in the short term but could improve grain protein later in the season. 

If conditions stay wet, crop roots may not be able to reach deeper nitrogen. For this reason, it is important to include subsoil reserves when estimating total nitrogen available. 

Checking nitrogen after waterlogging 

To find out how much nitrogen is in the soil after waterlogging: 

• Wait until the soil dries out.
• Test the soil to the full depth of the crops root zone. 

This will help you decide if you need to add more nitrogen fertiliser. 

For cereal crops still in early stages of growth, this information can support a more accurate and cost-effective fertiliser decision. 

Things to consider before applying extra nitrogen: 

• Crop stage affects results. Later applications are less likely to increase yield but may still improve grain protein.
• Fertiliser cost can reduce profit. High prices may make extra nitrogen less cost-effective.
• Grain prices and protein premiums influence value. Higher returns may make extra nitrogen more worthwhile. 

If you plan to plant a summer crop, it is also useful to reassess soil nitrogen after waterlogging. This will help you understand how much nitrogen remains and where it is in the soil profile.