The soil nitrogen supply often provides the bulk of a plant’s needs, but to maintain a healthy soil, the supply needs to be replenished regularly.

Key points

• Nitrogen is being gradually mined from Australian soils by conservative application of nitrogen fertiliser
• Soil stocks of nitrogen need to be nurtured and replenished on an ongoing basis

To maximise profit, nitrogen fertiliser is typically applied at rates that are lower than those required to maximise yield. This sufficiency approach means that nitrogen mined from soil organic matter is not replenished, causing a long-term decline in the soil nitrogen bank.

A critical component of soil health, soil organic matter contains an intrinsically balanced ratio of carbon and nitrogen. It supports the soil microbial community, which is essential for nutrient cycling – particularly nitrogen – and promotes water infiltration and retention.

Once soil organic matter is lost, it takes a long time to rebuild. To use a financial analogy – it is easy to run up a lot of debt, but trying to reverse that situation is a very difficult task.

A more-sustainable approach is to maintain and nurture soil organic matter levels. To achieve that, inputs must exceed exports and losses.

Building soil organic matter has the added benefit of ensuring that sufficient credit is available to meet a sudden need – such as the high nitrogen requirements of a bumper canola crop.

Conversely, if the system has been run down, a lack of soil organic matter makes it more prone to nitrogen losses.

Conservation farming and stubble retention is gradually rebuilding organic matter in Australian soils, but this requires a supply of both carbon and nitrogen. Growing productive crops generates biomass to return carbon to the soil as it decomposes, provided there is also sufficient nitrogen and a healthy microbial population.

Rebuilding soil nitrogen

There are three ways that growers can potentially improve soil organic matter and fertility:

• nitrogen banking;
• legume crops and pastures; and
• cover or break crops.

Nitrogen banking is achieved by increasing fertiliser rates to ensure nitrogen is not limiting. The strategy has already proven successful in the Victorian Mallee, where losses are minimal and any fertiliser not utilised by the crop in the year of application is mostly carried over to the following season.

Losses are generally low in most dryland environments, but the approach will need to be tested in wetter environments where losses are more likely.

While legume crops export the bulk of fixed nitrogen as grain, crop residues that remain have a much-higher proportion of nitrogen than cereal residues. Nitrogen fixation by pastures is another source of soil nitrogen, but research suggests that more could be done to optimise nitrogen fixation by pasture rhizobium.

It is important to remember that rhizobia are at their most efficient when soil supplies of nitrogen are deficient. Increasing the soil nitrogen supply has the potential to reduce the value of nitrogen fixation by legumes.

Unlike harvested crops, break crops or cover crops are returned to the soil and provide a slow release of carbon and nitrogen as they break down, boosting microbial populations.

Recent Landcare and GRDC-supported research has shown that cover crops reduce ‘lossy’ nitrate concentrations, which might reduce the risk of greenhouse gas emissions and leaching, while soil organic nitrogen and microbial nitrogen pools increased. In turn, these pools then become plant-available during the subsequent growing season.

Substantial research into microbial nitrogen processes is being undertaken by CSIRO Agriculture and Food as part of the national GRDC investment led by the University of Queensland. This work seeks to greatly improve our ability to predict nitrogen availability and vulnerability to loss, and thus improve fertiliser management decisions and increase profitability while limiting environmental impacts.

More information: Dr Mark Farrell, 0451 596 148, mark.farrell@csiro.au, GRDC Update paper: Addressing the rundown of nitrogen and soil organic carbon