Woodchips at the centre of bioreactor project

Nitrogen gas. It makes up 79 percent of the atmosphere and on the Sunshine Coast, pineapple growers are relying on a natural nitrogen cycling process to transform nitrate in ground water into nitrogen gas.

Amongst the peaks of the Glass House Mountains, a sea of pineapple heads point toward the sky. Beneath the crop, the sandy/loamy soil holds the nutrients, including nitrogen that the pineapples need to grow.

However, heavy rainfall can lead to leaching that flushes nitrogen from the soil in the form of nitrate into the local waterways of the Pumicestone Passage and Moreton Bay, a Ramsar listed and internationally protected wetland.

Excessive nitrate in waterways leads to algal bloom, which can be toxic. Such algal blooms can block sunlight impacting other flora and fauna by removing oxygen from the water. Too much nitrate can affect the balance of the whole ecosystem and severely degrade the health and diversity of the natural waterways.

Over the years, growers have implemented and trialled new practices to help keep nutrients in the soil where the crops need them. These include improved targeted practices for pre- and post-plant nutrient application, precision boom spraying, and use of polymers for erosion control.

However, large duration, high intensity, or unseasonal rainfall events cause soil water drainage which rapidly removes nitrogen from the crop root zone by transfers to shallow ground water tables.

To address the issue of nitrate transfer from farms into the surrounding aquatic environment, denitrification bioreactors are being constructed. This is so that farmers like Sam Pike, of Sandy Creek Pineapples, can continue to grow premium pineapples, while improving the sustainability of practices used on their farm.

On Mr Pike’s farm, wall bioreactors are being installed at the perimeter of the farm. They are strategically placed where there is a lot of subsurface water movement and high nitrate concentrations. This water will now run through the bioreactors before reaching the local waterways.

Inside the bioreactors, denitrification microbes are doing the hard work and conditions need to favour them. The three main requirements for the microbes are a carbon source, anaerobic (without air) conditions in saturated soil and nitrate.

Queensland Department of Agriculture and Fisheries’ (DAF) Dr Stuart Irvine-Brown explained the scientific goal behind the bioreactors is to achieve de-nitrification.

“The bioreactor creates a home for naturally occurring bacteria, which use the woodchips as a food source and due to the anerobic – underwater – situation, they use the nitrate in the water to respire or breathe like we do,” Dr Irvine-Brown said.

“The bacteria convert the nitrate in the water into normal nitrogen gas.”

Growcom’s Hort360 South East Queensland Project Manager Tim Wolens said the implementation of bioreactors is a pragmatic and effective choice to reduce nitrate runoff into some of South East Queensland’s most used and relied-on waterways.

“Australian pineapple industry operators are showing commitment to making improvements in the way in which they farm, so that they can demonstrate to their consumers that they are striving towards more sustainable farming practices and improve water quality in the Pumicestone Passage and Moreton Bay, which have important ecological significance,” Mr Wolens said.

The Department of Environment and Sciences’ (DES) Stephanie Cooper said the bioreactor is a win for the pineapple industry, Glass House communities, and the Pumicestone Passage and Moreton Bay catchments.

“Pumicestone Passage is an internationally important wetland and Moreton Bay is an internationally recognised Ramsar wetland,” Ms Cooper said.

“This bioreactor implementation is important for maintaining water assets for social, cultural and economic uses that South-East Queenslanders enjoy.”

Ms Cooper went on to say that the efforts of Sam Pike and the Australian pineapple industry, Growcom, as well as DES and DAF will go a long way to help set a standard for businesses to reduce nitrate in runoff into the Pumicestone Passage and Moreton Bay catchments.

“The 2021 South-East Queensland report card showed there were increasing pollutant loads in the Pumicestone Passage region,” Ms Cooper said.

For pineapple grower, Mr Pike, who is also the chair of the industry group, Australian Pineapples, maintaining the land his family farms on, is at the heart of what they do every day.

Mr Pike’s farm, now in its fourth generation, looks after and works the land for the next generation and said he sees the establishment of a bioreactor as a way to do something good for the land, the waterways and the community that his family helped pioneer.

“Our farm has gone on 100 years, I’ve got kids coming on – they might do farming, they might not,” Mr Pike said.

“This is a step toward helping our environment and helping reduce our footprint.”

The practical nature of bioreactors also means that not much maintenance is involved once construction finishes.

There are examples of bioreactors in New Zealand going on 25 years. The warmer temperatures and heavy rainfalls in South-East Queensland might shorten the lifespan of a bioreactor to a degree.

Regular monitoring and measuring of nitrate in runoff water will ensure the bioreactors are functioning properly and further guide their placement and use to intercept agricultural N leaving the farm in groundwater.

While the bioreactors are not solving the problem of keeping nitrogen in the soil under the pineapples where it is needed, it is stopping the dissolved nitrogen from exiting the farm, entering the local waterways, and impacting the environment.

Mr Pike said all stakeholders involved with the bioreactor trial – Growcom, DES, DAF & Healthy Land and Water – play important roles and bring in special expertise to develop the bioreactors.

The Pineapple Environmental Team (PET) is continuing to work with growers on solutions that will improve efficiency around fertiliser use and improvement of sustainable practices.

The PET was established to work on improving practices in pineapple production and to consolidate the work and resources in different areas and organisations. First and foremost, the group is headed by Australian Pineapples’ Chair Sam Pike, representing the growers. Other growers are also being consulted and participate in the meetings to provide input. Several growers across Queensland are hosting field trials and are actively involved in the work.

Growcom, as the peak industry body for pineapples, is providing extension support along with technical inputs and support for on-ground trials.

Healthy Land and Water is the local Natural Resource Management group in the region and thus provides NRM advice along with on-farm trial work on geopolymers for erosion control.

DAF is contributing with trial work and technical support. DES provides scientific advice and is the funding body for Growcom’s SEQ water quality project and the bioreactor implementation.