Nitrogen loss in flooded soils

29 March 2023

As flood waters recede, it’s natural to want to get straight back to business as usual. While it can be easy to see the damage done above ground, losses that lie beneath the soil can sometimes be overlooked.

Nitrogen is an essential nutrient for plant growth. To be utilised by plants, soil microorganisms must convert nitrogen into forms that are able to be taken up by the roots.

However, in some forms, nitrogen can be very mobile in the soil and is prone to being lost, particularly in very wet or waterlogged conditions. The way in which nitrogen is lost in a flood depends on soil type and how long the soil remains saturated.

Sandy textured soils generally have limited nutrient holding capacity. Nitrogen is prone to leaching in sandy soils as water flows through the more porous structure. This soil type can commonly display nitrogen deficiencies and is made worse in wet conditions following flooding or heavy rains, with nitrogen flushed below the root zone and unavailable to plants.

Clay and loam soils have higher nutrient holding capacities in comparison to sand. However, these soils can limit deep drainage and become waterlogged in wet conditions. Sand over clay subsoil can also become waterlogged in floods.

Nitrogen in the form of nitrate (NO3) is most prone to leaching. Nitrate nitrogen losses from leaching can be significant where inorganic fertilisers, particularly those containing nitrate, have been applied.

Losses can also be high in spring as the soil warms up, increasing soil microbial activity and decomposition of plant residues and soil organic matter which releases nitrate (mineralisation).

With extended periods of waterlogging, oxygen availability to the soil is restricted and nitrogen may be depleted through the process of denitrification.

Aerobic microorganisms are responsible for nitrogen conversion but cannot survive without oxygen. Consequently, in waterlogged conditions soil nitrogen is converted by anaerobic bacteria (microbes that don’t need oxygen) into its gas forms, which are released from the soil and no longer available for plant uptake.

The longer the soil is waterlogged, the greater the potential loss of nitrogen from the soil profile. Losses can begin to occur if soil is waterlogged for more than two days and become significant after more than a week of waterlogging.

Nitrogen losses through denitrification will also be higher in soils that are high in soil organic matter, nitrogen and reduced oxygen. Denitrification is driven by soil microorganisms, with warm temperature resulting in more rapid nitrogen losses to the atmosphere.

Soil testing close to when you plan to sow crops and pastures will help determine nitrogen fertility levels. Separate soils samples should be taken in previously flooded and non-flood affected areas for comparison.

Deep soil testing (to 60 cm) can indicate if nitrogen has been leached deeper in the soil profile, but still within the root zone and available for plant uptake. Knowing the soil type or types you have on your property may assist you in identifying the possible impacts on nutrient availability caused by flooding and help with target zones to conduct soil tests.

A soil test will be able to provide you with an overview of your soil fertility and indicate whether additional fertiliser applications are required.

For further information on your soil type, please visit the soil page on the Victorian Resources Online website or the soil maps page on the Irrigating Agriculture website.

Media contact: Sarah Hetherington

Phone: 0409 405 639