Monitoring equipment has shown bioreactors work in the Wet Tropics, where rainfall can exceed 4,000 millimetres annually.

(ABC Rural: Tom Major) Woodchip-filled trenches hardly seem like a high-tech answer to the global problems of fertiliser run-off, but could hold the key to improving environmental outcomes for the Great Barrier Reef.

Key points:

• Nitrogen run-off, mainly from sugarcane paddocks, has been linked to reduction in coral cover in the Great Barrier Reef
• New bioreactors work by using woodchips to break down nitrates in water that flows through before it reaches waterways
• Farmers and researchers are working in cooperation to trial the new technology

The International Bioreactor Forum held in Cairns this week heard that data from bioreactors is producing valuable insights into exactly how much fertiliser from coastal farms is flowing to the reef.

A newly developed technology, denitrification bioreactors work by using naturally occurring bacteria in woodchips to break down nitrates in the water flowing through them, converting it into nitrogen gas.

Nitrogen run-off, mainly from sugarcane paddocks, has been identified as a cause of algal blooms and outbreaks of Crown of Thorns starfish that reduce coral cover.

Crown of Thorns Starfish kill corals by eating the coral tissues. The white area is where they have eaten away the tissues.

(Supplied: Lisa Boström-Einarsson)

Research agronomist with the Queensland Department of Agriculture, Rhianna Robinson, said the relatively cheap bioreactors could be a tool to intercept nitrates before they entered waterways.

"Sometimes we're finding best management practice alone isn't going to achieve water quality targets," she said.

"We're doing specific monitoring at the start and end of our bioreactors so we have a good idea of what's going in and what's coming out.

"In terms of how that influences catchments, we're not quite there yet: we've got a long way to go."

Do bioreactors work?

Initial signs are that bioreactors, already proven overseas, can form a useful tool to manage nitrogen run-off in the wet, humid north Queensland summers.

The warm weather stimulates the microbial activity in the woodchips, but high flows in a region where 300 millimetres or more can fall in a 12-hour period is challenging for bioreactor design.

Trenches are filled with woodchips to build a bioreactor, where microbes operating in an anaerobic environment remove nitrates that would otherwise flow into local rivers.

(Supplied: Terrain NRM)

Soil scientist with James Cook University, Alex Cheesman, said high nitrogen loads at the beginning of the season caused the greatest levels of nutrient run-off.

"We're dealing with a high nitrogen load coming off the paddocks at the beginning of the season, just after harvest," he said.

"We've been actively designing systems that don't interfere with the natural drainage, what we're trying to do is capture that first flush [of water] and treat it to the best of our abilities."

Barramundi farmer from Mourilyan, Marty Phillips, said the possibility of using intensified water treatment through bioreactors could help him increase production and cut costs.

"That is the end game, to re-use your water and keep it all on-farm … it closes a biosecurity loop for us," he said.

"If it's coming from a pond where we know it's coming through an anaerobic process and coming back we know it's not going to have anything nasty in it."

Currently Mr Phillips' farm uses wetlands to clean nitrogen from water before it re-enters the local creek, an area that could be repurposed to grow more fish if a closed system is developed.

"This more intensified water treatment process will hopefully let us free up some of that and potentially lead to more production," he said.

JCU researcher, Alex Cheesman, and barramundi farmer, Marty Phillips, are working together on a bioreactor that is helping to remove nitrogen from water used to grow fish.

(ABC Rural: Tom Major)

How much pollution do farms produce?

Previous data, used by the Queensland Government to draw up laws restricting fertiliser inputs on farms, modelled 'whole of catchment' nitrate levels, rather than specific levels leaving farms.

Initial data from bioreactors shows the loss of nitrate fertiliser in the Wet Tropics, where more than 4,000 millimetres can fall in a year, is about 1 milligram per litre.

Ms Robinson said information on exactly how much nitrate was entering and then being removed from sugarcane farm run-off would soon be published.

"This year, I think with all of our research across all of the organisations, I think that we'll be able to answer that towards the end of this year," she said.

Farmer Barry Stubbs from Mirriwinni, south of Cairns, volunteered sites for trial bioreactors and said he welcomed hard facts on the exact nitrate run-off from his farm.

"The results are coming up as not as bad as everyone thinks it is for the farmers — sure there's issues that need dealing with, but we can deal with them," he said.

"The scientists are learning a lot in the last two years about the release of nitrogen."

SOURCE ABC AUSTRALIA

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Taxonomy

• Fertilizers
• Fertigation management
• Fertilizer
• Nitrogen