- Nitrogen is a mobile nutrient and can be lost downwards (leaching), sidewards (erosion) and upwards (gas emissions)
- To reduce losses avoid unnecessarily high nitrogen rates
- Delay or split nitrogen fertiliser application so that peak nitrogen availability coincides with peak crop demand
- Using legumes in crop rotations will also reduce nitrogen losses
Nitrogen fertiliser is one of the most significant input costs for northern grain growers, so understandably growers want to minimise applied nitrogen losses through better management to maximise return on investment.
Professor Mike Bell says legumes are becoming an important part of lifting nitrogen rates in soils.
This quest is being assisted by GRDC-supported research investigating how nitrogen is lost from cropping soils and effective management options.
The research is being led by Queensland Alliance for Agriculture and Food Innovation principal research fellow Professor Mike Bell and Dr Graeme Schwenke, senior research scientist from the New South Wales Department of Primary Industries (DPI).
Nitrogen for crop production can come from soil organic matter, crop residues (especially legumes), manure and fertiliser. The amount of plant-available nitrogen (mineral nitrogen) released from microbial breakdown of soil organic matter, crop residues and manure depends on the amount of organic matter in the soil, the amount of crop residue remaining and its nitrogen concentration, and the amount and type of manure applied and how it is applied. However, as the nitrogen release process is undertaken by microbes, temperature and moisture availability can also influence the rate of release.
In the north, growers either measure this ‘mineralised’ nitrogen in the soil profile towards the end of a fallow period using soil testing or estimate it from soil organic matter levels, fallow rainfall and rotation histories.
They then derive a fertiliser nitrogen requirement based on the difference between this soil mineral nitrogen and the likely crop demand based on expected yields. Nitrogen fertilisers such as urea are then applied either directly into the soil (banding), or broadcast onto the soil surface and then incorporated.
In contrast to the soil organic nitrogen reserves, fertiliser nitrogen is either immediately available for plant use (in ammonium or nitrate forms) or soon available after conversion in soil (for example, from urea to ammonium and nitrate).
Any loss of nitrogen will reduce the pool of nitrogen that a crop can use to produce biomass and grain yield.
How nitrogen is lost
“Essentially, cropping systems are ‘leaky’ and nitrogen (especially when in the nitrate form) can be lost via downward, sideward or upward movement,” Dr Schwenke says.
Researchers Professor Mike Bell and Dr Graeme Schwenke are working with the grains industry to improve grower understanding of how applied nitrogen is lost.
PHOTO: Nicole Baxter
“Downward movement via leaching (the drainage of water through the soil profile) is a greater problem in lighter-textured soils than the medium/heavy clay soils dominating the northern grains zone, but previous research has demonstrated some losses can occur this way.
“Sideways movement can occur rapidly through erosion of organic-matter-rich topsoil during intense rain events, or more slowly through lateral movement of nitrate in soil water.
“The main upwards nitrogen loss pathways consist of gaseous losses through ammonia volatilisation or denitrification of nitrate (a biological process occurring within the soil profile wherever there is sufficient available nitrate, labile carbon substrate and low oxygen conditions, such as in slowly draining soils).”
Dr Schwenke says understanding nitrogen-loss pathways and how they are influenced by seasonal conditions and management strategies is a critical first step in optimising the efficiency and profitability of applied nitrogen use.
“Losses to ammonia volatilisation can occur soon after fertiliser is surface-applied to soil,” Dr Schwenke says.
“In previous research on northern NSW clay soils, we found losses from broadcast urea averaged 11 per cent when applied to fallow paddocks, five per cent when applied in a wheat crop (mostly dry soils) and 27 per cent when applied to pasture.”
He cautions that these figures were specific to the conditions present in the measured paddocks at the time and were influenced by factors including clay content, soil pH, fertiliser rate, rainfall in the week after application, presence of crop canopy, fertiliser placement and wind speed after application.
Dr Schwenke says nitrate denitrification losses can be large, but require a combination of low soil oxygen (an extreme example is when soils are waterlogged for an extended period), high soil nitrate concentration (soon after soils have been fertilised) and readily available (labile) carbon to support an active microbial community.
“Clearly this set of circumstances don’t coincide every year, but when they do, denitrification losses can be high, with nitrogen losses typically higher when soils are warmer in spring and summer.”
Reducing nitrogen losses
Over the past three years, the research team has conducted six experiments with isotope-labelled (15N) urea fertiliser in northern NSW and a further 11 in southern Queensland, all focused on measuring the fate of applied nitrogen fertiliser in summer sorghum.
According to Professor Bell, the results showed that between 0 and 44 per cent of the applied nitrogen was not able to be recovered in the soil or plant at harvest, with in-season rainfall, both timing and amount, and soil carbon and nitrogen status having a major impact on these seasonal losses.
“The research also found that avoiding unnecessarily high nitrogen rates, delaying or splitting nitrogen fertiliser so that peak nitrogen availability coincides with peak crop nitrogen demand, and relying on residual nitrogen from legume rotations all significantly reduced gaseous nitrogen losses from dryland sorghum,” Professor Bell says.
However, he warns that the effectiveness of these management strategies varies with seasonal conditions.“Depending on the season, delaying/splitting nitrogen applications gave either no yield benefit (dry season) or a significantly greater yield (good in-crop rainfall).
“Much of the unused nitrogen after a dry season remained in the soil and, provided loss events were not experienced during the fallow, significantly benefited the following crop.”
The trial work also investigated the impact of using nitrification-inhibitor-coated urea, which significantly reduced nitrous oxide emissions in all studies, but did not improve grain yields enough to justify the additional cost on an agronomic basis.
One management strategy to reduce nitrogen losses
A trial at Kingaroy in southern Queensland explored the impact of crop rotation (grain or grain-legume pre-histories) on fertiliser nitrogen requirements and nitrogen use efficiency during a subsequent sorghum crop in 2014-15. The pre-histories were sorghum/peanuts/soybeans in the 2013-14 summer, all harvested for grain.
In the second summer crop year when sorghum was planted, the fertiliser nitrogen rate required to achieve a sorghum grain yield of 6.3 tonnes per hectare was reduced by 50 per cent after the peanut rotation and the need removed entirely after soybeans. Specifically, sorghum following sorghum needed 120 kilograms of nitrogen (N)/ha, sorghum following peanuts needed 60kg of N/ha and sorghum following soybeans required no nitrogen fertiliser at all.
“Fertiliser nitrogen losses were negligible at the optimum nitrogen rate in the peanut or sorghum histories on this friable soil, with each history recovering 65 to 70 per cent of the applied nitrogen in crop biomass in these high-yielding crops,” Professor Bell says.
“Fertiliser is a major input cost for northern growers and will continue to be so as the region’s soil organic matter and associated mineralisable nitrogen reserves continue to decline. This will continue to be the case unless legume frequency in crop rotations increases substantially.”
Professor Mike Bell
07 5460 1140
‘Understanding and managing N loss pathways
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‘Nitrogen volatilisation losses
Denitrification fact sheet
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