Denitrification in cropping (cereals)

Following a wet weather event, the complex process of denitrification due to waterlogging can lead to nutritional deficiencies in cereal grains.

The denitrification process

Cereal grain crops require large amounts of Nitrogen (N) to produce high yields and protein. Much of the readily available N in soils is held in the Nitrate (NO3-) and to a lesser extent Ammonium (NH4+) forms. Both forms are produced from fertiliser N and the decomposition of soil organic matter (organic N). Waterlogged soils are rapidly depleted of oxygen (O2) by soil microbes, after which denitrifying bacteria use NO3- as an alternative source of oxygen for cell metabolism. The series of denitrification reactions converts the NO3- into N gases – first nitric oxide (NO), then nitrous oxide (N2O), then dinitrogen (N2). Once produced, these gases escape to the atmosphere and represent a net loss of N from the soil-crop system and crops may be left with insufficient N for the original yield and protein target.

Emissions of these N gases during denitrification are difficult to measure in the field - especially emissions of N2 against an existing background of 70% N2 in the air. Measurements of N2O emissions are more straightforward and have often been used as a “marker” for denitrification events, although there is no set ratio of N2O: N2 gases in the total of N losses by denitrification. Denitrification is also highly variable even within a paddock making it difficult to predict the extent of losses in a particular situation. What is known is that the rate at which denitrification occurs like many soil processes is dependent upon the temperature with the rate increasing with higher soil temperatures in summer. A food source (organic matter) is also required for the denitrifying bacteria. The vertosols of the northern cropping region have limited drainage, which favours the denitrification process. Lower soil temperatures at present will slow the rate of denitrification but will not stop the process. Previous research-trial observations have found that prolonged waterlogging/flooding can rapidly deplete soil N from the upper parts of the soil profile-less denitrification occurs lower in the profile due to less available organic matter and lower microbial populations at depth.

Remember that N (as NO3-) is highly mobile and recent rains may have moved plant-available N down the profile as well as or instead of causing denitrification. This N may not be accessed by the crop until later in the season resulting in a protein benefit more than a yield benefit. N budgeting needs to take account of deeper N reserves. However, if wet conditions continue, then crop roots may have limited access to deep N in the profile.

The impact of waterlogging on soil nitrogen

The only way to determine the impact of waterlogging on soil N is to soil sample in crop to the full depth of root exploration once waterlogging ceases and determine if there is sufficient N to fulfil yield and protein targets. For cereal crops that are still in early growth stages growers can then make an informed decision if additional N fertiliser is needed or justified. This decision will need to take account of several factors:

• Crop growth stage – as crops progress the yield benefit will decline as any additional N fertiliser is used by the plant to boost grain protein
• The current high cost of N fertiliser- this will reduce the economic gains of additional N
• Grain price and protein premiums – will also influence the profitability of additional N.

More information on denitrification

Download the Local Land Services Denitrification in Soils factsheet PDF, 225.11 KB.

This GRDC factsheet PDF, 225.11 KB also provides further information on denitrification. Growers are advised to seek assistance from an agronomist where required.

Download the factsheet on nitrogen strategies for kikuyu ryegrass pastures PDF, 2106.86 KB.

Learn more about soil nitrogen testing and fertiliser decisions.

Or contact:

Bill Manning, Senior Land Services Officer, North West Local Land Services on William.manning@lls.nsw.gov.au or 0428 607 731.

Tim Bartimote, Senior Land Services Officer, Central West Local Land Services on tim.bartimote@lls.nsw.gov.au or 0409 833 914.

Dr Graeme Schwenke, Senior Research Scientist (Soils) DPI on graeme.schwenke@dpi.nsw.gov.au or 0418 636 421.