Researchers probe impact of nitrogen mineralisation
Recent rainfall is likely to have encouraged soil nitrogen (N) mineralisation in most Western Australian cropping soils, freeing up N from organic matter for coming crops.
But this does not necessarily mean applied N fertiliser levels should be reduced at seeding time, according to WA researchers involved in a national Grains Research and Development Corporation (GRDC) funded project under the More Profit from Crop Nutrition II initiative (MPCNII).
Soil scientists say good soil moisture levels and extra N made available through mineralisation could increase the early vigour of crops, stimulating demand for N and setting up high yield potential, so planned N fertiliser regimes at seeding may be best left unchanged.
Post-sowing N applications should be reviewed as the season progresses.
The University of WA (UWA) soil scientist Louise Barton presented a paper at the 2016 GRDC Grains Research Update, Perth, which has helped to quantify the supply and loss of N from soils in WA dryland cropping systems.
Co-authored by Frances Hoyle, of UWA, and Craig Scanlan, of the Department of Agriculture and Food (DAFWA), it reviewed scientific literature and collated WA soil N mineralisation rates, as well as rates at which N is lost from cropping soils through NH3 volatilisation, N2O and N2 emission, and N leaching.
“Between-season net N mineralisation rates (the net balance between N supply from soil organic matter and N losses) have ranged from 0.1 to 0.2 kg N/ha per day,” Dr Barton said.
“Significant accumulation of mineral N before sowing has often been reported in response to summer rainfall (and when plant N uptake is negligible).
“The length of the fallow season and the extent of summer rainfall events strongly influence the contribution of between-season mineralisation to annual net N mineralised.
“By contrast, in-season net mineralisation rates have ranged from -0.16 to 0.5 kg N/ha per day.
“Overall, annual net N mineralisation is estimated to range from 43 to 122 kg N/ha in the surface of WA soils.”
Dr Barton said current soil tests would provide growers with information about the amount of plant available N (ammonium and nitrate) present in their soil and the capacity of the soil to supply more N from mineralisation, which is estimated from soil organic carbon.
This could help them manage fertiliser applications rates to ensure adequate N was then supplied to the crop.
GRDC Western Regional Panel deputy chairman and scientist Mike Ewing said that while N fertiliser was the biggest and most expensive input for Australian grain crops, it generally represented one of the highest returns on investment when well managed.
“Accurate predictions of N fertiliser requirements are therefore very important to growers and advisers,” he said.
“However, there can be uncertainty over N management decisions, and past experience is not always totally applicable as seasonal scenarios change and systems evolve.
“Factors such as changing rainfall patterns (warm, moist autumns) and improved summer/autumn weed management practices are positive for available N accumulation.
“Declining soil organic matter, a lower legume frequency in farming systems and increasing adoption of zero tillage/stubble retention systems are generally associated with less available N.
“There are also issues of residual N from previous fertiliser applications, depending on rate and type.
“The project ‘Organic matter and nutrient availability’, within the MCPNII initiative, aims to assist grain growers and their advisers to reduce the uncertainty and financial risk associated with N management.”
The GRDC Grains Research Update paper - Where does the nitrogen go? Soil sources and sinks in Western Australia cropping soils – is available here.
Louise Barton, UWA
08 6488 2543
Mike Ewing, GRDC
0409 116 750
ContactNatalie Lee, Cox Inall Communications
08 9864 2034, 0427 189 827
GRDC Project Code UQ00079