A healthy soil is productive, sustainable and resilient to the impacts of farm management practices and changing climatic conditions. It also stores and cycles carbon.
Soil organic carbon is strongly linked to soil quality and productivity. How much carbon your soil can store and for how long varies depending on factors such as soil moisture, soil texture (the clay and fine silt fraction), temperature and annual rainfall (amount and distribution over the year), and importantly, farm management practices.
Soil organic carbon is a key component of soil organic matter which includes all of the organic components of the soil such as plant and animal tissue in various states of decomposition.
Soil organic carbon provides a source of nutrients through mineralisation, helps to aggregate soil particles (structure) to provide resilience to physical degradation, increases microbial activity, increases water storage and availability to plants, and protects soil from erosion.
Ultimately, increasing soil organic carbon levels can lead to better plant establishment and growth. While increasing soil carbon is highly desirable, it is also easily lost, so maintaining what you have is important. Climate is a strong driver, affecting accumulations and decomposition of soil organic matter in soils.
One of the cotton industry’s long-term senior soil scientists, Dr Guna Nachimuthu of NSW DPIRD, says healthy soil is a “superpower for your plants”.
“Healthy soil with good structure is a result of soil storing good organic matter and soil organic carbon, then cycling them, which is good for crops,” he says.
Soil organic carbon is the lifeblood of soil, and research has been underway for many years to establish how to preserve and increase it.
What determines the soil carbon content?
Many factors impact the soil organic carbon storage in broadacre cropping fields.
Guna says it’s like a living puzzle with ever-evolving pieces, and soil scientists are constantly learning more about complex relationships in soil.
“Combine this with the impact of local climate and you get a situation where a practice that boosts soil carbon storage in one area might have little effect or even hurt it elsewhere,” he said.
“To make things even trickier, some conservation techniques such as minimum till might increase carbon storage near the soil surface and decrease soil carbon deeper down, leading to an overall loss.
“This explains why crop managers might feel scientific studies on soil carbon seem contradictory, however, most scientists agree that practices like cover crops and no-till are winners for building carbon in the topsoil.
“Compared to conventional tillage and leaving bare fields, these methods keep more carbon near the surface.
“However, the jury’s still out on how deeply they impact carbon storage. In fact, some practices might even accelerate carbon loss in lower soil layers and there is less consensus for deeper soils.
“The key take away from the science so far is when it comes to soil carbon storage, we need to consider the entire soil profile and tailor our approach to specific location taking into account soil type, management practices and climatic conditions.”
While there can be local variability depending on location, some general trends occur which include:
Soils with the most potential for improvement in carbon storage tend to be the coarse texture or sandy soils with low productivity, low carbon content initially and that have historically received lower crop residues and manure.
Adoption of soil health practices such as cover cropping, minimum or no-till practices enhance soil organic carbon in top soil (top 10cm).
Irrigated systems hold more potential for carbon storage than dryland systems.
Soil carbon fluctuates depending on soil moisture, but remains at fairly stable levels on a long-term basis.
Long-term cotton systems experiments
Guna says before diving into long-term Australian Cotton Research Institute (ACRI) experiments, it’s important to understand how land-use changes impact soil organic carbon, as it depends on the balance between organic matter added to the soil (inputs) and what decomposes and releases carbon (outputs).
When land use changes, so does the carbon cycle in the soil. Converting forests or pastures to cropland often leads to a decrease in soil organic carbon as organic inputs decline. Over time, the soil organic carbon reaches a new, lower equilibrium level. Conversely, converting cropland back to pastures or forests increases organic inputs, leading to a rise in soil organic carbon and a new, higher equilibrium.
Two long-term experiments conducted by NSW DPIRD revealed a consistent decline in soil organic carbon regardless of the crop rotation or soil management techniques employed. However, the experiments also showed that implementing practices like minimum tillage and wheat rotation helped slow down the rate of decline.
While maize rotations showed some potential for increasing topsoil soil organic carbon with fluctuations both upwards and downwards over the years, these gains appeared short-lived. When rotations transitioned away from maize, the positive impact faded, and soil organic carbon levels began to decline again.
A separate 10-year study from 1998-2008 by the late Dr Ian Rochester of CSIRO found an increase in soil carbon storage. The differing soil organic carbon levels across ACRI fields, even though they’re all within the same farm, raises the question of why these fields show variations in storage.
“The field with increasing soil organic carbon is near the river and floods more frequently, potentially depositing carbon-rich sediments,” Guna says.
“Conversely, the declining soil organic carbon field has high subsoil sodicity, hindering biomass production. Additionally, differences in fallow-phase biomass turnover contribute to the variation in soil organic carbon trends.
“The declining soil organic carbon field, converted from pasture, might still be reaching its ‘cropping equilibrium’ – a stable soil organic carbon level for cotton production.
“Conversely, the high soil organic carbon field might be trending at the top end of the cropping equilibrium due to flood-deposited sediments and good fallow-phase biomass management.”
The role of water
Generating biomass for soil carbon requires water, a scarce resource with variable annual rainfall and allocated quotas for irrigation water across cotton growing regions. This limited water availability can restrict the potential for biomass production and subsequent soil carbon storage in cotton cropping systems.
A preliminary study suggests lower soil organic carbon levels in commercial cotton fields compared to research farms like ACRI. This difference might be due to water availability. ACRI, with access to purchased water during droughts, can maintain crop rotations and minimise soil organic carbon decline. Conversely, grower’s fields often rely on extended fallows, leading to carbon loss and there are limits on maintaining the soil organic carbon levels within the cropping systems.
Irrigated systems hold more potential for carbon storage than dryland systems. However, maintaining soil carbon through irrigation during droughts presents a water cost dilemma. Finding a solution to this water-carbon trade-off will be a long-term challenge for the industry.
Soil carbon, nutrient cycling, and cotton productivity
Guna says while building soil organic carbon is difficult in Australia’s inland cotton regions due to extreme weather, efforts to improve soil organic carbon should persist.
“That’s because higher soil organic carbon translates to a more fertile soil, boosting its ability to deliver nutrients and enhancing its resilience.
“Research across the globe shows that cotton plants can access a significant portion (60 to 70 per cent) of their nitrogen needs directly from the soil, highlighting the critical role of nutrient supply.
“Therefore, maintaining good soil organic carbon levels is crucial for long-term soil health and sustainable cotton production.
“This is backed up by a team of researchers from NSW DPRDI, the University of New England and CSIRO who investigated contrasting cotton fields (high vs low yield) and found a positive correlation between soil organic carbon and normalised lint yield.
“This finding underscores the significance of soil organic carbon for improving cotton production.”
This article appears courtesy of the Cotton Research and Development Corporation (CRDC). It was published in the (Autumn 2025) edition of CRDC’s Spotlight magazine: www.crdc.com.au/spotlight
What is the carbon cycle and why do cotton growers need to understand how it works?
Soil carbon cycle
Plants power the soil’s carbon cycle. Through photosynthesis, plants capture carbon dioxide (CO2) from the air and turn it into sugars for growth, releasing oxygen in the process. These sugars fuel the plants, but some are released as CO2 again during respiration.
The cycle keeps turning thanks to soil microbes. Plant leftovers like dead leaves and root secretions become a feast for these tiny decomposers. As they break down the plant matter, they release some of the carbon back to the atmosphere as CO2. The microbes themselves also contribute to the soil’s carbon stores when they die.
Carbon created by these natural processes and stored in the soil is known as soil organic carbon and is a vital part of healthy soil. It helps plants access nutrients, improves soil structure and holds water.
