Does your soil have a pH of between 5.5 and 6.5? Are you in an area of high annual rainfall like the Tropics? If so, your cane farm could potentially sequester carbon dioxide in the soil profile. The challenge is now for researchers to prove it.
In a natural process called silicate weathering, rocks rich in calcium and magnesium gradually break down when exposed to water and air and carbon dioxide becomes bicarbonate and/or carbonate to be transported harmlessly through the soil profile and groundwater. The problem is that this important process in the global carbon cycle happens on a huge time scale over thousands to millions of years.
Scientists are researching ‘enhanced’ weathering, a technique designed to speed up the natural process. Among them is Hannah Green who is working on enhanced weathering for her PhD thesis at James Cook University, supported by a scholarship funded by SRA and with assistance from Wilmar Sugar. One of Hannah’s supervisors is Peter Larsen, Agricultural Productivity Manager at Wilmar Sugar, who has been involved in all parts of the project, including facilitating Hannah’s field trial on a Wilmar farm near Ingham.
There are two approaches to enhanced weathering. The first is where suitable types of rock are crushed to a fine powder to increase the reaction surface area and are broadcast onto agricultural land. The second is where industrial by-products, such as sugarcane mill ash, are used in the same manner.
“Agricultural fields are ideal to facilitate this sequestering reaction since soil has a naturally high carbon dioxide concentration and the infrastructure is already in place to distribute large amounts of material,” Hannah said. “In addition, the potential release of nutrients from the right materials could offer growers beneficial improvements in soil health at no additional cost. In our research work we have been looking at a common hypothesis – that tropical soils have the highest potential for carbon dioxide removal via enhanced weathering. Water availability is known to drive the process forward, so we look to the Tropics as the best site for carbon sequestration using this process.
“However, while there is a high annual rainfall in the Tropics, tropical soils also tend to be strongly acidic. In terms of weathering, acidic conditions increase the reaction speed, which is good, however, carbon dioxide cannot be removed from the air if the acid that does the weathering is not carbonic acid from dissolved carbon dioxide. And we know that in soils that are really acidic with a pH of, say, less than 5.5, other stronger acids such as nitric and sulfuric acid tend to dominate over carbonic acid in these reactions. That means soils with low pH can be useless for carbon sequestration, unless there is a way to raise pH throughout the entire profile.”
The aims of Hannah’s project which finishes this year are to determine the optimum soil pH for carbon dioxide capture in the soil profile, and to investigate how other soil conditions influence the relationships in the elements for carbon dioxide capture.
“We have run experiments using a reactive transport model in a geochemical modelling software called PHREEQC. We have used data from published field studies to set up virtual soil profiles that we can then manipulate for our experiments. In two experiments we simulated the application of a local crushed basalt, a local sugarcane mill ash and a limestone, to a soil profile of between zero and 50 cm deep. In both experiments, we simulated five different soil pHs ranging from 4.5 to 8.5. In the first experiment, we compared the outcome when rainfall and other climate variables are consistent, regardless of soil pH. We then looked at what happens if we paired soil pH and climate variables like we see in the field.
“In the second experiment, we compared high and low pH buffering capacity – that is the ability of the soil to resist changes in pH, and soil carbon dioxide concentrations. The model alternates between transporting the soil solution down the profile, one cell at a time, and equilibrating the soil solution with the environment. Equilibration involves multiple reactions – ranging from elements transformed between gas, aqueous, and solid forms, through to weathering the minerals in the added material and in the soil, precipitating new minerals and enabling plants to uptake the released nutrients and water.
“In the first experiment we compared varied climate where we had rainfall and evapotranspiration which matched the soil pH scenarios like we see in the field – high rainfall in acidic conditions and lower rainfall in more alkaline conditions. We also adjusted some other soil parameters too, in the same way that we would see occurring in the field. We compared that with an experiment using a constant climate, regardless of what the initial soil pH was. We measured the cumulative carbon dioxide removed after 15 years in the different climate and soil pH scenarios and found that the effect of a varying or constant climate is a small effect. The overwhelming effect is the soil pH.
“We found there is clearly an optimal soil pH at around 6.5. Soil pH above 6.5 is too high and the weathering rate slows right down. Even after 15 years, you would not get significant weathering. When the soil pH is lower, stronger acids dominate in the process which means no carbon dioxide removal. We also tested the effects of soil pH buffering capacity and carbon dioxide concentration. Both of these varied with depth, as we would see in the field.
“We adjusted soil pH buffering capacity based on the amount of organic carbon exchange sites in the soil. Where the pH buffering capacity is low i.e., the soil doesn’t resist changes in the pH, there is more carbon dioxide removal. That’s especially prominent where the initial soil pH is 5.5. What’s happening is that the added material in the soil is alkalising the soil over time. The pH is increasing to the optimum point around 6.5.
“However, when the pH gets higher, above 6.5, some of the carbon starts to come out of solution and precipitates into a solid form. The system is in fact re-releasing carbon dioxide. So, we’re potentially seeing more carbon dioxide removal in this slightly more acidic system with the optimum being between pH 5.5 and 6.5. The potential for carbon dioxide removal is also greater when soil carbon dioxide concentrations are high. Unlike pH buffering capacity, soil carbon dioxide levels could be improved by adding organic amendments like sugarcane mill mud, alongside weatherable material.
“We calculated cumulative carbon dioxide removed directly from the change in carbon in the soil. For comparison, we also calculated it using a common method that assumes that cations (calcium and magnesium) released from the weathering material equate to carbon dioxide removed. According to this cation method, you would assume that a soil pH of 4.5 would be optimum, but in reality, there would be no carbon dioxide removed in this situation. This means that if you deployed enhanced weathering in these highly acidic soils, you would end up with net emissions of carbon dioxide.
“Importantly, this cation method cannot be used in acidic soils and is only suitable in near neutral conditions. These results are based on modelling data, however, there are currently many trials being set up to test these hypotheses in the field (including as part of my PhD). There are also many analyses still to do to ensure that deploying enhanced weathering sequesters carbon and is not a source of carbon emissions. Also, it is important that we measure carbon directly, despite the challenges of doing that, so we can properly understand the soil conditions that impact on these processes and accurately verify carbon sequestration.”
Research into enhanced weathering is working to build a scientific proof of its viability in tropical soils. Over time, this may enable the development of an approved methodology under the Australian Carbon Credit Unit (ACCU) scheme, regulated by the Clean Energy Regulator, with the potential to provide an additional income stream for growers at the farm gate.
“On the flip side, the challenge we currently face is to alert people that if the soil conditions are not right the process will not work and may even exacerbate the loss of carbon. Growers who are currently thinking about investing in expensive basalt fines to spread on their caneland should bear this point in mind before buying,” Hannah said.
