Nitrogen use efficiency

Take home message

Nitrogen losses from dryland cropping soils are largely determined by soil type, rainfall intensity and the timing of fertiliser application.

In cracking clay soils of the northern grains region, saturated soil conditions between fertiliser application and crop growth can lead to significant N losses from the soil through denitrification. The gases lost in this case are nitric oxide, nitrous oxide and di-nitrogen. Isotope studies in the northern region have found these losses can be > 30% of the N applied. Direct measurements of nitrous oxide highlight the rapidity of loss in this process.

Insufficient rainfall after surface application of nitrogen fertilisers can result in N losses from the soil through volatilisation. The gas lost in this case is ammonia.  Direct measurements of ammonia loss have found these losses were generally < 15% of the N applied, even less in in-crop situations. An exception was the application of ammonium sulphate to soils with free lime at the surface, where losses were > 25% of the N applied. Recovery of N applied in-crop requires sufficient in-crop rainfall for plant uptake from otherwise dry surface soil.

Some nitrogen terms

Nitrogen use efficiency (NUE): the efficiency with which soil nitrate-N is converted into grain N. The nitrate-N comes from fertiliser, crop residues, manures, and soil organic matter, but it is the efficiency of conversion of fertiliser into grain that is generally of greatest concern to growers. Efficiency is reduced by seasonal conditions, crop diseases, losses of N from the soil as gases, N leaching or immobilisation of N into organic forms.

Mineralisation: the microbial production of mineral nitrogen in the form of ammonium (NH4+) from soil organic matter. Ammonium-N is subsequently converted to nitrate (NO3-) through the microbial process of nitrification.

Immobilisation: the conversion of mineral forms of soil N, nitrate (NO3-) and ammonium (NH4+), into organic forms (microbial biomass). This occurs when plant residues of low N content (e.g. wheat or sorghum straw) are decomposing in the soil. Immobilisation represents a temporary unavailability of mineral N in the soil for growing plants to access. Applying fertiliser N in a band separated from crop residues is one means of reducing immobilisation.

Denitrification: in waterlogged soils, nitrate (NO3-) is converted by soil microorganisms into the gases nitric oxide (NO), nitrous oxide (N2O) and di-nitrogen (N2). The total loss of N from the soil varies according to conditions and whether there is also a ready source of carbon available. Nitrogen (N2) is the main form of the gas that is lost, but the proportion of the different gases produced is dependent on soil pH and water content. In acidic soils, more is lost as N2O whereas in alkaline soils most is lost as N2. Once in gas form the N is no longer available to plants in the soil.

Nitrous oxide emission: loss of N from the soil to the atmosphere, chiefly as a result of denitrification during waterlogging events. Total losses of nitrous oxide (N2O) as a proportion of fertiliser N applied are typically less than 1% in dryland cropping systems but may be higher in situations of prolonged heavy rainfall and waterlogging. Nitrous oxide is a long-lived gas in the atmosphere whose concentration is steadily increasing. It is harmful to the ozone layer and is a greenhouse gas which is 300 times more potent than carbon dioxide in terms its global warming effect. Small emissions of nitrous oxide are normally indicative of much larger N2 emissions.

Ammonia volatilisation: loss of N as ammonia (NH3) from the soil to the atmosphere as a result of chemical reactions at the surface and environmental conditions. Unless it is being applied as anhydrous ammonia fertiliser, ammonia gas is generally only produced in the soil at very high pH, which occurs as urea is converted to ammonium N, or when ammonium sulphate comes in contact with calcium carbonate in the soil. Soils with a high clay content can buffer changes in pH and also have an affinity for ammonium adsorption making them less at risk of converting to ammonia gas and being lost.

Leaching: downward movement of nitrate with water in the soil. Loss of soil N from the plant root zone is more likely in coarse textured (sandy) soils than clay soils, but can still occur. If the nitrate is still within the root zone then it is not lost from the system, but past research in clay soils has demonstrated that nitrate can move below the root zone in years of high rainfall, especially during long fallows.

Timing: Nitrogen fertilisers are applied either at sowing, pre-sowing, in-crop or at some combination of these, e.g. some at sowing and some more in-crop as determined by the growing season. Early application is often more convenient and allows N to move lower into the root zone where it will be accessed later in the season and thus be used more for grain filling than for plant growth, but it is also then at greater risk of denitrification loss should waterlogging occur. Even applying at planting can carry this risk if heavy rains occur before plants begin to use the N from the fertiliser. In-crop applications can allow N fertiliser use to better match the season being experienced, but still require follow-up rainfall to get the applied N taken up by the plant.

Application method: Solid and liquid N products are applied either directly to the soil surface, placed or injected into the soil (including anhydrous ammonia), or surface applied then covered in a separate operation. In-crop applications are either surface-spread as solids or sprayed as liquids. Physical separation of added fertiliser from crop residues via banding can minimise immobilisation of N. Surface applications by broadcasting may be at risk of ammonia volatilisation, particularly in coarse textured soils and those with thick stubble layers that prevent the fertiliser from soil contact as it dissolves.

New research on nitrous oxide emissions and denitrification: (UNE000012, DAF00004-15)

Our research into nitrous oxide (N2O) emissions is focussed on reducing agriculture’s contribution to global warming since a major source of N2O emissions is associated with the application of nitrogen (N) fertilisers to agricultural soils. Currently, the national estimate for the proportion of fertiliser N emitted as nitrous oxide from dryland cropping is 0.3% of that applied. This is an average figure from previous research in Victoria and WA, and our own measurements range from 0.08-1.31% of the fertiliser N applied. Obviously, losses this small will not affect NUE greatly, but since most of this comes from denitrification, these amounts indicate potentially greater N losses as di-nitrogen (N2). The ratio of N2 to N2O loss in alkaline clay soils can be as much as 40:1, so for every kg of N2O-N lost, another 40 kg may have been lost as N2 (Rochester 2003) or even as high as 80:1.

At Kingsthorpe (Qld), in a wheat-cotton rotation on a heavy black clay soil, losses of N2O increase (exponentially) in cotton as more N fertiliser is applied but there is no major yield benefit (Figure 1). Similar plateaus in yield are apparent in sorghum at Kupunn (Figure 2) providing further evidence that any N applications above 80-120 kg N/ha were unnecessary and potentially costly in terms of wasted N when you consider that 80 kg mineral N was found in the top 120 cm of soil prior to sowing.

GRDC Project code: UNE000012 and DAF00004-15, DAN00144, UQ00066 and DAF00004-05