GRDC research queries N losses on northern soils
Author: Sarah Jeffrey | Date: 18 May 2015
- Research on northern-grains region cracking clay soils found that fertiliser nitrogen (N) losses via ammonia volatilisation may not be as high as previously thought.
- Surface application of nitrogen fertilisers, applied either pre-sowing or in-crop, risks gaseous loss of the applied N via ammonia volatilisation but the magnitude of N loss in northern grains region had not previously been measured.
- The amount of N lost is affected by a number of factors including fertiliser compound, fertiliser form, type of application, timing of application, soil properties, rainfall amount and intensity, and the temperature and windiness after application.
- A rethink on fertiliser application practices could save time and operational costs.
Grain growers in Australia’s northern cropping belt could save valuable time and operational costs by rethinking their fertiliser application practices, with research showing that fertiliser nitrogen (N) losses via ammonia volatilisation may not be as high as previously thought.
Figure 1. A summary of the cumulative N loss via volatilisation in this study, grouped by paddock type and fertiliser type in 19 separate paddock experiments (219 site-months data). Bars are means (+ standard error) of results from 2–8 paddock experiments (number of paddocks in each group is given under bar). U = urea, UAN = urea ammonium nitrate solution, GU = Green urea®, AS = ammonium sulfate, LU = urea solution, LAN = ammonium nitrate solution.
Northern growers typically apply nitrogen fertilisers such as urea directly into the soil (banding) or broadcast it on the surface then incorporate.
This is done to reduce the potential for ammonium-containing (eg. sulfate of ammonia) or ammonium-producing (eg. urea) fertilisers volatilising into the atmosphere as the gas ammonia.
Ammonia volatilisation is a chemical process that occurs at the soil surface when ammonium ions are converted to ammonia gas at highly alkaline pH - as occurs when urea is converted to ammonium.
The quantity of N lost as ammonia depends on a range of factors including fertiliser compound, fertiliser form, type of application, timing of application, soil properties, rainfall amount and intensity, and the temperature and wind after application.
Global estimates for ammonia volatilisation losses from fertiliser average 10-19% of the N applied across all soil types, with an average of 7% loss in industrialised countries like Australia.
However, losses at the paddock scale can be much greater with losses of >40% recorded under some extreme conditions.
Previous research in southern Australian cropping soils has shown N losses ranging from 1.5–25% with highest losses recorded on soil fertilised soon after rain or from paddocks with a substantial ashbed from burning of rice stubble.
In the northern grains region it is commonly accepted that surface application risks gaseous loss of the applied N via ammonia volatilisation, but the magnitude of N loss from soils had not previously been studied.
Dr Graeme Schwenke from the New South Wales Department of Primary Industries (NSW DPI) conducted 19 separate field experiments in north west NSW farmers’ paddocks during 2011-2013 as part of a Grains Research and Development Corporation (GRDC) supported research project.
“Since both soil properties and environmental conditions are known to influence ammonia volatilisation, our measurements needed to be field-based and non-intrusive,” Dr Schwenke said.
“We used a micrometeorological technique to measure cumulative ammonia loss over a month-long period after application of 2–6 fertiliser products in 10 fallow paddocks, 7 mid-tillering wheat crops, and 2 perennial grass-based pastures. (Cumulative ammonia volatilisation results for all fertiliser treatments in all field experiments are summarised in Figure 1.)
According to Dr Schwenke the N lost from urea as volatilised ammonia averaged 11% when applied to fallow soils, and 5% when applied to tillering wheat crops.
A soil’s affinity for ammonia/ammonium is one of the most influential factors governing the potential for ammonia volatilisation through both cation exchange of ammonium and physical adsorption of ammonia in dry soils.
“The overall lowness of N losses in the current study is likely to be related to the medium to high cation exchange capacity (CEC) of most Vertosols used in the study,” Dr Schwenke said.
“Critical CEC values of 25–32 cmol kg-1 have been proposed elsewhere, below which ammonium adsorption is minimal, and above which ammonia loss is substantially reduced. The CEC of most soils in the study were above these critical thresholds.”
In the study, losses from ammonium sulfate were typically less than those from urea, except when the soil contained >2% calcium carbonate (lime).
At five of the eight low calcium carbonate fallow paddocks, ammonia loss from ammonium sulfate averaged 52% less than from urea.
However, where soils contained >10% calcium carbonate, N losses averaged >20% from ammonium sulfate applied to fallow soils. Calcium carbonate content did not affect losses from urea or other nitrogen fertilisers trialled.
“A combined statistical analysis of low calcium carbonate plots showed that ammonia volatilisation loss in the study was principally affected by the presence of a crop, fertiliser type, and the average wind speed at ground level,” Dr Schwenke said.
“Losses were greater in fallow paddocks than in-crop and greater under windy conditions.
“As a consequence of the time of year of measurement, the in-crop paddocks (winter) tended to be drier soils, had less rainfall after application and lower temperatures during the measurement period than fallowed paddocks measured during autumn or spring.
“The in-crop paddocks also had crop canopies that provide protection from wind speed at ground level, the next most influential factor affecting volatilisation loss in our study.”
After the month of volatilisation measurements, Dr Schwenke said most of the non-volatilised applied nitrogen was recovered in the topsoil or plant tissue.
“The exception was where paddocks had had intense rainfall which likely caused nitrate leaching and denitrification,” he said.
Nitrate leaching is a physical process that occurs with the drainage of water through the profile. While nitrate movement within the profile is common in cracking clay soils, large-scale loss of nitrate below the root zone is minimal in most conditions.
Nitrate denitrification is a biological process that occurs within the soil profile wherever there is sufficient available nitrate, labile carbon substrate, and low oxygen conditions such as in slowly draining soils. Losses are minimal in most dryland cropping soils, but may be high in waterlogged conditions.
“In contrast to the cropping paddock results, the two pasture paddocks trialled averaged 27% N loss from urea because the spread fertiliser granules were caught in the foliage and thatch of the pasture and did not contact the soil. There was little rain after spreading on the pastures,” Dr Schwenke said.
Additional detail on the N volatilisation research is available in Dr Schwenke’s GRDC Update paper.
Graeme Schwenke NSW DPI research officer
02 6763 1137
Sarah Jeffrey, Senior Consultant Cox Inall Communications
GRDC Project Code DAN00144