What have mill wheels and trout fishing got to do with fertiliser, asks Chris Hall?

It turns out that a favourite form of nitrogen in farm fertiliser may be harvestable by our riversides. Not only that, but United Nations data shows that gigatonnes of nitrogen await our collection.

And all thanks to a tiny microbe.

Researchers at RMIT Melbourne, UNSW microscopy labs and the University of Sydney have investigated a phenomenon which turns our thinking about nitrogen fertilisers on its head. What is more, the results might enable us to serve up nitrogen burgers to wheat, barley, oats and other crops.

The microbes in question, freshwater diatoms, convert nitrate to ammonia – a favourite form of fertiliser. The Hall family – farmers in the wheat and wool belt of NSW – only noticed the connection because of a bit of unexpected luck, during Covid.

Oxygen depletion

Because of Covid border closures, researcher Al Hall had some trouble getting 150 litres of pond and groundwater samples from NSW to Victoria, to the microscopy labs there. Hall had hoped to image microscopic life in the sampled waters, but delays had caused oxygen depletion in them. A lab technician told him to throw them out, because everything in them would have died after the first two weeks in storage. The samples had been stored for months.

But when Hall noticed total nitrogen counts in the sealed samples fluctuating, it seemed that something in them was not only alive, but kicking nitrate.

Because the samples were stored for a long time, something very strange had occurred. Microbes with strangely transparent, very geometric shapes (rectangles, circles and triangles) that looked as if they had been made in a factory – had bred up in the low oxygen conditions.

The freshwater lab habit of filtering the water before testing for nitrogen levels usually filtered out the microbes – along with their nitrate. The glitch had probably held freshwater river research back for some 40 years. And during that time, other sciences made huge discoveries in nitrogen sequestration – and much of it had been about microbes.

The measurement glitch hadn’t affected studies in saltwater environments. Marine science measured 200 and even 300mg/l nitrate in the microbes as early as the 1980s – enough nitrogen to feed corn twice. In 2022, marine scientist Peter Stief showed freshwater researchers where to look: in freshwater, nitrate was being captured by these tiny glass beads, and mainly on river bottoms.

Freshwater researchers were familiar with diatoms, but nobody suspected they would take up much nitrate. People had been filtering them out and throwing them away for decades. But it turns out the microbes contain 4,000 times as much nitrate as other microbes in river bottoms – where most of the nitrates are found.

Stief, writing in Nature, described the microbes as clustering most densely under rocks, and Hall realised they were capturing nitrate from ground seepage.

A total of 100 times more nitrate is in groundwater seepage than in river water, and a good deep channel through the soil and clays at the side of agricultural fields and rivers yields 20 to 50 mg/ll nitrate in many parts of Europe.

Risks associated with shipping ammonium nitrate

Figures vary across the world, but corn crops enjoy about 80 to 100mg/l nitrogen, while wheat crops vary – about 40-50 is common. Nitrogen fertilisers usually come in the form of ammonia, which is sold commercially in urea. Plants prefer this form, and it reduces the risks associated with shipping ammonium nitrate.

Ammonium nitrate is explosive – and part of the cost of commercial fertilisers is created by the need to ship them carefully. The appeal of a locally made, water-based product, that can be sold side by side next to dwindling supplies of quarried fertiliser, might be strong among companies and farmers – especially with nitrogen at €230 per hectare.

A total of 1,000ml of groundwater seepage yields about half a tonne of fertiliser – about 18,000 tonnes a year is processed by natural means in a 50gl river section. Not only that, but the microbes do the conversion for us, taking in nitrate, and releasing ammonia.

If the researchers could map the flow of nitrogen across fields and riverbanks, and the flows of nitrate rich waters underground, methods of nitrates harvesting could be trialled where supplies were richest.

'Hamburger' layers of substrates

As a starting point, Hall visited the super at the thermal pool lab at Bundoora Australia, where Eliecer Bonilla showed Hall how to set up 'hamburger' layers of substrates.

Abhijit Date showed Hall how to monitor the ionic pumping that Ivan Kennedy described to Hall as possibly driving nitrates into the microbes from clays.

Josh Green described to Hall how construction of a substrate had worked in an artificial pond. And while Hall had been focused on marine samples and their nitrate, Michael Carnell, head of microscopy at UNSW, zoomed in on diatoms at tree roots in Hall’s samples from Watsons Bay park, imaging the connection between low oxygen storage and diatom abundance.

Jega Jegatheesan told Hall he had seen swans feeding on duckweed in mid-winter – sign of a nitrate spike from microbial dieback. Perhaps refrigeration could be a method of extracting the nitrogen. Italy’s river Po had experienced nitrates spikes in mid-winter for a decade.

Several designs for the nitrogen burger were considered, before Hall remembered something a friend had said about water mills and free energy.

The friend was a bridge builder and general construction engineer – and he’d told Hall that trout liked rocky streams partly because they were high in oxygen. 

Diatoms predominate in deoxygenated waters, seen here at the junction between marine groundswell and freshwater rainfall runoff at a tree base in Robertson’s Park, Watsons Bay. Images: Dr Michael Carnell, University of New South Wales (UNSW).

Free energy 'pump'

He then proceeded to insult Hall, by describing how much better the trout fishing was in Scottish burns, and the tributaries of the Shannon. Despite the insult, the two noticed that at the foot of one of Munro’s bridges, some fish were feeding under an outflow that was oxygenating the water – using the free energy 'pump' provided by gravity, and pounding air into the water as it fell.

As a civil engineering solution, the free energy from gravity could provide a large scale water turnover, and riffle building with stream boulders could oxygenate the water just as Munro’s outfall had done.

Stief’s microbes are prompted to take up their nitrate in oxygenated waters, and the undersides of stones are a favourite with them. A substrate that resembled a rocky trout stream would be ideal for the microbes.

And as low oxygen levels cause the microbes to convert nitrate to ammonia, Hall had a design for the second substrate, and had trialled ammonia absorption in the lab. In the middle of the substrates lies the microbial layer – the 'meat in the burger' – where nutrients are formed.

After harvesting the fertiliser, water could be released downstream in the turbulence following a mill wheel or sluice gate.

The microbes are, in fact, sold as commercial fish food to aquariums, in the form of a white powder. Spent microbes travelling in the water mix downstream could provide fish food for trout.

The exact setup of the system could be adapted to trout farms, fresh and saltwater lakes and rivers, drainage ditches along ploughed fields, as well as existing groundwater outflows like Munro’s.

Corresponding Author: Al Hall; Jega Jegatheesan, WETT centre director, Abhijit Date, Elicier Bonilla, RMIT thermal pool laboratory. Lab Advice and Credits: Paul Woodward, Groundswell laboratories; Kate Smith, Watertest; Ivan Kennedy, the University Of Sydney; Image of a diatom in Sydney Harbour groundswell: Michael Carnell, UNSW microscopy.

Research work mentioned in the article (in order of citation)