- Nitrogen fertiliser is a significant input cost in cropping.
- Loss of nitrogen has agronomic and economic costs for growers and increases greenhouse gas emissions through nitrous oxide.
- Controlled-traffic farming (CTF) significantly reduces the risk of nitrogen being lost as nitrous oxide.
Controlling traffic can substantially reduce the amount of nitrogen lost from the cropping system.
As one of the major input costs in grain production, there is a big agronomic and economic incentive to make sure nitrogen fertiliser is not wasted.
But there is also increasing global pressure on agriculture to reduce greenhouse gas emissions from carbon dioxide, nitrous oxide and methane.
Nitrous oxide is produced in soils by two microbial processes - nitrification and denitrification - and is an important indicator of total denitrification losses in the farming system. But it is also a greenhouse gas with a global warming potential 296 times greater than carbon dioxide.
Intensively managed agricultural soils emit more nitrous oxide to the atmosphere than any other anthropogenic source.
Production of nitrous oxide relies on three things:
- nitrogen (usually from fertiliser);
- carbon (usually from crop residues); and
- a soil environment with low oxygen (such as compacted soil) that favours denitrifying bacteria.
Denitrification tends to increase as the soil approaches saturation, which is more common after irrigation or rainfall.
The most effective way to reduce emissions is to control nitrogen inputs and better manage the soil physical environment by reducing compaction and improving aeration.
Converting a paddock from a random wheeled area of 50 per cent to permanent traffic lanes of 12 per cent could reduce denitrification losses by more than 10 kilograms of nitrogen per hectare.
Recent research by the Australian Controlled Traffic Farming Association, with support from the Australian Government's Action on the Ground program, investigated the potential for CTF to reduce soil emissions of nitrous oxide.
The study measured emissions in 15 CTF growers' paddocks in Victoria, Western Australia and Queensland between 2013 and 2017.
It compared non-wheeled crop beds, permanent traffic lanes and random wheeling.
The random-wheeling treatment used a single pass on the crop bed by the seeding tractor immediately prior to seeding as the equivalent of random non-CTF traffic.
This was highly detailed work, with an average of 14 assessments of emissions per site during the growing season.
The mean daily emissions over a 200-day growing season at Inverleigh, Victoria, were 2.1, 4.9 and 6.3 grams of nitrogen (as nitrous oxide) per hectare from crop beds, permanent traffic lanes and random-wheel tracks, respectively (see Figure 1).
These were typical of the results across all sites, but slightly higher than the northern region.
In other words, random wheeling can emit nitrous oxide at more than twice the rate of non-wheeled CTF beds.
On the trafficked soil, the nitrous oxide emissions from the CTF lanes were at least 30 per cent lower than emissions from random wheeling.
Emissions from the CTF system could be even lower by ensuring no nitrogen fertiliser is applied on permanent traffic lanes, although this may be challenging when using twin-disc spreaders.
Random traffic increases the amount of nitrogen lost when compared to the non-wheeled crop bed.
The daily loss can be converted to seasonal losses (200-day crop) of about 0.42, 0.97 and 1.25 kilograms per hectare of nitrogen (as nitrous oxide) per season, from crop beds, permanent traffic lanes and random wheeling, respectively.
Random wheeling emits nitrous oxide at more than twice the rate of non-wheeled CTF beds.
Research has shown that nitrogen lost through complete denitrification can be between 10 and 70 times the amount of nitrous oxide measured.
If 30 was a reasonable average for dryland cropping, the random-wheeled area would lose about 37kg/ha of nitrogen as a conservative estimate through complete denitrification.
Converting a paddock from a random-wheeled area of 50 per cent to permanent traffic lanes of 12 per cent could reduce total nitrogen loss through denitrification by about 11kg/ha.
With urea at $400 per tonne, this loss represents about $9.50 per hectare.
However, this is only an estimate - the real figure might be anything from $3 to $22/ha and that economic benefit could be realised through fertiliser cost, crop yield or grain quality.
Regardless of how the benefits are realised, CTF improves soil structure and therefore water and nitrogen uptake, which are better for the nitrogen budget and the environment.
More information: Dr Diogenes Antille, 02 6218 3835, firstname.lastname@example.org