Queensland Department of Agriculture and Fisheries senior soil microbiologist at the Leslie Research Facility in Toowoomba, Dr Nikki Seymour, said the amount of N fixed depends on the species of legume grown, the site and seasonal conditions, as well as agronomic management of the crop or pasture.
Grains Research and Development Corporation (GRDC) and Queensland Department of Agriculture and Fisheries (DAF) co-funded research has shown that growers who optimise the growth of their pulse crops can dramatically improve soil nitrogen fixation.
With parts of the northern growing region experiencing a wet start to spring, it’s a timely reminder for growers who are contemplating planting pulse crops in 2016-2017.
In Australia, nitrogen fixation from legume-rhizobia associations is estimated at approximately 2.7 million tonnes of nitrogen (N) each year, worth about $4 billion, according to the GRDC.
On average, this equates to about 110 kilograms of N per hectare per year. However, the range of values can be from zero to 400kg of N per hectare.
DAF senior soil microbiologist at the Leslie Research Facility in Toowoomba, Dr Nikki Seymour, said the amount of N fixed depends on the species of legume grown, the site and seasonal conditions, as well as agronomic management of the crop or pasture.
“It is largely influenced by how well that crop grows. More crop biomass will result in more nitrogen fixed by that crop provided it is well nodulated,” Dr Seymour said.
“However, the amount of N fixed by a legume does not equal the amount available for the next crop. Nitrogen is removed in the harvested grain and the N remaining in the crop residue then needs to be mineralised by microbial activity before it’s available to the next crop.
“Maximising N fixation in grain legumes is dependent on firstly inoculating the legume seed with the appropriate rhizobia strain, then optimising basic agronomy to maximise legume productivity.
“Nitrogen fixation is directly related to biomass production for a particular species if that crop is well nodulated.”
Dr Seymour said other factors such as soil nitrate levels will influence the amount of N fixed by that crop.
Recent trial work funded by GRDC and undertaken by the GRDC Pulse Agronomy Initiative in the northern region has demonstrated that row spacing and time of sowing can significantly impact N fixation.
Two trials conducted in southern Queensland near Goondiwindi and Dalby were assessed for the amount of nitrogen fixation at different row spacings.
They showed row spacing in chickpeas of 25 centimetres led to greater biomass, yield and nitrogen fixation than at one metre.
Up to 59kg of nitrogen per hectare remained at the Dalby site when the chickpeas were grown on 25cm rows, but only 23kg of N/ha as the row spacing increased from 25cm to one metre.
As a crop’s demand for N increased, so did the N fixation by that crop.
Trials conducted at Grafton in northern NSW showed sowing at the optimum time to take full advantage of soil water and growing conditions to achieve maximum crop biomass led to greater amounts of N fixed.
The trials used two different soybean varieties (NF246-64 and PR443), which were planted in the middle of the appropriate planting window and then late in that window.
Results showed that as much as 150kg/ha less N was fixed due to the shortened growing season for the soybeans planted late in the window.
However, increasing the plant population becomes very important for improving the proportion of N that is fixed if time of sowing is delayed.
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