Parts of Australia’s northern grain-growing region could be at a heightened risk of losing nitrogen via denitrification after excessive rainfall has caused waterlogging across many paddocks.

NSW Department of Primary Industries (DPI) senior research scientist, Graeme Schwenke, says a lot of northern growers would have had nitrogen-depleted soils after last year’s record harvest.

“High grain yields remove large amounts of nitrogen, and this can leave soils depleted of nitrogen reserves unless N fertilizer is applied to address the nitrogen needs of the next crop,” he says.

However, Dr Schwenke says that some of the ‘top up’ nitrogen fertiliser applied before the current season was at increased risk of loss, due to ongoing wet conditions in parts of the northern region.

“In really wet years we often see growers produce low protein cereal crops, or crops that didn’t yield as well as expected. A common reason for this is that the crop didn’t have a sufficient supply of nitrogen from the soil and fertiliser,” he says.

“If growers applied nitrogen fertiliser ahead of the season and experienced waterlogging after application, they could have lost a significant portion of the product through a process called denitrification. How much will really depend on individual circumstances though.”

A recent review of Australian grains cropping research found that on average, 28 percent of applied nitrogen fertiliser was lost from denitrification, although this varied enormously (range: 0–54%), with the higher losses occurring in wetter years with waterlogged paddocks.

Dr Schwenke says the denitrification process, which releases dinitrogen gas (N2) as well as some nitrous oxide (N2O), can happen at any time of the year, but the rate of loss will be faster in warmer months.

Losses are triggered by microbial activity when soil oxygen levels are low, and so commonly occur in saturated soils.

“Following really wet, waterlogging conditions, growers should look at soil testing once it’s dry enough to get on paddocks so they can gain a better understanding of how much nitrogen is left within the soil and how much might need topping up,” Dr Schwenke says.

“Where in-crop nitrogen applications are practical, this top up can occur by top dressing during the crop season. In drier regions and on the heavy clay soils, this top up is more effective prior to sowing.”

To better understand the complete picture of nitrogen cycling and the conditions under which growers might lose nitrogen from their soil, GRDC has launched a four-year, $11.9 million investment titled ‘Predicting nitrogen cycling and losses in Australian cropping systems’ led by the University of Queensland and including the Queensland University of Technology, CSIRO and state agencies in NSW, Qld, SA, Vic and WA.

GRDC Senior Regional Manager - South, Stephen Loss says the outcomes from the project will help growers understand when losses occur, and what might be done to minimise these and maximise fertiliser use efficiency.

“It will also help them understand how much nitrogen they’ve potentially lost if they’d already applied fertiliser,” he says.

With the rising cost of inputs like nitrogen becoming a major concern for growers, Dr Loss says it was essential that GRDC looked at understanding all the ways in which nitrogen can leak from soils and how growers can manage this risk.

Data from the project will also be used to improve the current Agricultural Production Systems sIMulator (APSIM) model, with the project focusing on upgrading the soil nitrogen cycling and loss routines.

“Strengthening the APSIM model with a new national dataset will help us forecast how nitrogen is utilised throughout the season and make better predictions on how much nitrogen growers need to apply in certain circumstances to grow a profitable crop,” he says.

“Better fertiliser decisions arising from an improved model can help to achieve better nitrogen use efficiency and reduce the direct effects of emission of the potent greenhouse gas nitrous oxide.”

Dr Loss says, in addition to the nitrous oxide losses, the manufacturing of nitrogen fertiliser is very energy-intensive and losses of any kind can increase the environmental footprint of our crop industries.

“Improvements in nitrogen use efficiency will reduce the impact of Australian grain production on the environment, which will enhance the ‘clean and green’ reputation of the industry,” he says.

More details on this project, including how trials will be conducted to understand nitrogen loss, will be promoted as it progresses.