Agricultural practices for improving water quality

For many agricultural farms, applying fertiliser is needed to enhance soil fertility, meet crop nutrient demands and promote healthy crops or pasture. But heavy rainfall and intense irrigation can quickly wash nitrogen and phosphorus fertilisers off the paddock and into drainage systems and waterways, and leach into groundwater. This can harm ecosystems and put stress on habitats and aquatic species.

Decades of research is helping growers improve their practices to enhance water quality while keeping their farms profitable. To maintain soil health, growers are adjusting their practices by using rotational cropping, reducing tillage, reducing fertiliser use, or replacing synthetic fertiliser with organic nutrients to boost soil fertility.

Scientists and landholders are working together to understand the benefits of improved agricultural practices, from nutrients and microbes in the soil all the way to the plants and animals they support.

A metabarcoding snapshot

Our team is using advanced technique called metabarcoding to investigate how nutrients move through the environment, and microbes and invertebrates that play a key role in this process. This helps us better understand how nutrients are recycled and lost to waterways in agricultural systems.

The process works by targeting a specific gene that is shared by many different species. Our researchers then look for all the DNA in a soil or water sample that matches this common gene. Once the relevant DNA is found, it is sequenced and compared to a database to identify the different species present in the sample – this is then matched with the processes these species support. This allows us to gain a detailed picture of the biodiversity in an area without needing to physically collect or observe the organisms.

By understanding the microbial and invertebrate communities in soil and water, scientists can identify key farming practices that influence nutrient cycling, helping to improve farming practices and reduce nutrient loss to waterways.

Our team are applying this method across a range of projects investigating agricultural practices that improve water quality.

Nitrate leaching in Melaleuca and sugarcane

This project investigated rates of nutrient loss under different land uses on a farm near Ingham, Queensland.

The project consisted of examining differences in microbe diversity and nitrate leaching in sugarcane, mature Melaleuca, restored Melaleuca, and grass.

Our team investigated:

• Microbe diversity including bacteria, fungi, and nematodes, using soil metabarcoding.

• Soil properties such as pH, electrical conductivity, total carbon, total nitrogen, and organic matter.

• Nitrate leaching, using the ion-exchange resin method.

The team found substantially higher levels of nitrate leaching from sugarcane than all other sites. Even when the same high levels of nitrate were applied, sugarcane leached significant amounts of nitrate while mature Melaleuca did not.

These results suggest that the restoration of Melaleuca and removal of sugarcane in low-lying, flood-prone sites could eliminate nitrate leaching from these low-productivity areas when wet season floods deliver and deposit nitrogen. As this would reduce nitrogen runoff to the Great Barrier Reef, Melaleuca restoration could serve as a method to gain credits through the Queensland Government’s Reef Credit Scheme.

A report on this study is available here and a news story: The benefits of converting flood-prone cane paddocks into melaleuca plantations. This project was funded by Greening Australia.

Effects of mulch on soil nutrient retention and microbes

This project investigated the impacts of different mulch compost on nutrient retention and microbial diversity in soils on three farms around Rockhampton, across macadamia, lychee, and grazing systems.

Mulch is used to help stimulate soil microbes, which are important for nutrient cycling, breaking down organic matter, and controlling disease. Our team compared the effects of no mulch, woody mulch, and mulch made of water hyacinth (an aquatic weed), with and without the application of fertiliser on leaching and microbial diversity.

By studying soil microbes (bacteria, fungi, and nematodes) and soil properties like pH and carbon levels, our team could measure microbial diversity and nitrate leaching in each mulch type at each farm.

Results found that water hyacinth mulch increased microbial diversity associated with nutrient cycling and decomposition, but this varied depending on vegetation type and soil conditions. This means that microbes facilitate the mobilisation of nutrients so they are available for plants but may also be at risk of leaching.

While woody mulch was more closely associated with stabilisation of organic matter and suppression of nematodes, which could be useful to control plant parasitic nematodes that reduce crop health. Soil microbe assemblages were found to differ considerably between the three different agricultural systems, making any findings and recommendations context-dependent.

Outcomes of this project will address ways to practically reduce nutrient leaching and use of fertilisers and pesticides. This will have significant flow-on effects for water quality by reducing runoff at the source.

This project was funded by Fitzroy Basin Authority, the Cooperative Research Centre for Developing Northern Australia, Rockhampton Regional Council, and the Queensland Government.