Nitrogen 'win-win' with precision irrigation
Scientists in the National Agricultural Nitrous Oxide Research Program (NANORP), which is administered by the GRDC, coordinated by Queensland University of Technology and funded by the Australian Department of Agriculture, have been exploring how irrigation scheduling can reduce nitrogen (N) losses in irrigated broadacre cropping systems, and be used as a nitrous oxide (N2O) mitigation strategy.
Irrigated agriculture is predicted to make an increasingly large contribution to food production, nationally and globally, in response to demands from a growing human population in coming decades.
This raises two particular challenges. Intensive cropping, with its high water and N fertiliser inputs, has the potential to emit large amounts of N2O (the most potent greenhouse gas) and contribute further to global warming.
Conversely, because water and N inputs can be accurately applied to irrigated crops, plant demand can be met more accurately, resulting in high N and water use efficiency.
Consequently, irrigated farming systems offer the potential to be able to reduce fluxes of N2O to the atmosphere and other soil N losses, while at the same time ensuring high yields.
Research measuring this has been carried out at a unique facility at the CSIRO Agriculture Flagship, Griffith, New South Wales. The facility enables replicated manipulation of soil–water–plants–fertiliser configurations at and beyond field conditions.
It has been developed and validated to investigate relationships between soil type, irrigation schedule, soil moisture, N2O emissions and N losses in drainage below the root zone.The overall aims of the research have been:
- to improve understanding of N2O emissions under a range of soil types;
- prove N2O emissions can be reduced through optimised irrigation scheduling and fertiliser application at close to field conditions;
- develop data sets representing soil–crop conditions at and beyond those possible in field trials; and
- provide comprehensive data sets for the improved calibration of computer models that predict agricultural greenhouse gas emissions from various cropping scenarios.
The facility has 12 weighing lysimeter soil monoliths (0.7 metres in diameter and 1.2m deep) installed in a set of below-ground concrete silos with walk-in underground access.
Each monolith is fitted with an automatic chamber connected to a gas chromatograph, for measuring N2O and methane, and to a carbon dioxide analyser. Eight measurements of each gas are made daily.
There is continuous measurement of water loss through evapotranspiration from each soil column, and the soil profile is monitored for volumetric water content and temperature at 10-centimetre intervals. Also the soil monoliths freely drain, so leachate is collected for analysing mineral N and dissolved N2O.
The study has examined how the frequency and volume of irrigation water can be manipulated to reduce N2O emissions from a Willbriggie clay loam in a broadacre summer irrigated crop – grain sorghum.
The approach taken was to keep the total seasonal volume of irrigation water typical of grower practice (3.6 megalitres per hectare) constant, but to apply it in smaller, more frequent events compared with the usual practice of two, or possibly three, larger events.NANORP researchers hypothesised that smaller, more frequent irrigation applications may:
- reduce N2O emissions because soil oxygen levels will stay above the potential threshold required for denitrification;
- reduce leaching losses of N and water below the root zone by better matching irrigations to crop growth requirements; and
- increase crop N use efficiency by matching water availability to crop requirement and reducing N losses.
Four irrigation treatments were selected for the experiment (Table 1).
Data obtained from the study includes that:n cumulative N2O emissions were reduced by about 50 per cent and 25 per cent when irrigation was applied in 30 and 60-millimetre events, respectively, compared with the usual grower practice of approximately120mm irrigation events;
- smaller N2O emissions in 30 and 60mm irrigation water applications resulted from avoiding large N2O emission pulses immediately following irrigation;
- losses of water below the root zone decreased from 48 per cent in the 120mm treatment to 14 per cent in the 30mm irrigation while mineral N losses through leaching decreased from 20 per cent in the 120mm to three per cent in the 30mm irrigation; and
- plant N uptake in the 30 and 60mm irrigations was significantly higher than at 120mm irrigation. The increase in plant N uptake and decrease in leaching losses of N in frequent irrigations of less than or equal to 90mm compared with 120mm irrigations suggest potential for improved crop N use efficiency using this irrigation approach.
However, it remains technically demanding at the field scale. That said, recent development of high-performance irrigated broadacre layouts (for example, terraced layouts, see Ground Cover issue 106, September–October 2013) and automated water control devices for flood and furrow irrigation mean lower volumes and more accurate scheduling of water to crops are now possible to a higher degree than previously.
The lysimeter system has provided proof of concept that requires rigorous field testing at the paddock scale to examine practical feasibility involving water and land availability, weed and disease control, different seasonal weather conditions and gross-margin analysis.
||Number of irrigations
*ML = megalitre = 106 litres
Grain sorghum (Sorghum bicolor; variety MR-Taurus) was sown on 23 December 2013.
More information:Dr Wendy Quayle,
GRDC Project Code MCC00011
Region North, South