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Shedding new light on soil organic carbon in Australian grain systems

The Pampas Farming System Trial led by Drs David Lester and Lindsay Bell.
Photo: Han Weng

Soil organic carbon management has been recognised by the Australian Government as one of six priority low-emissions technologies for lowering greenhouse gas emissions with the highest abatement and economic potential. Not only can building and protecting soil organic carbon improve carbon sequestration, but it also builds soil health, fertility and resilience, thereby contributing to enduring profitability for Australian grain-producing systems.

Nevertheless, the ability to increase soil organic carbon, including in grain-producing soils, is hindered by a lack of understanding as to how organic carbon behaves in soil, including the factors that actually dictate whether carbon has long-term stability in the soil or not.

Without knowledge of the factors altering the storage and cycling of soil organic carbon, it is difficult to optimise management practices for increasing soil organic carbon. Indeed, a better understanding of these underlying mechanisms can enable optimisation of predictive carbon modelling and facilitate a paradigm shift in the way grain-producing systems are managed to store soil organic carbon and improve soil health.

A research team at the University of Queensland worked across a range of existing GRDC field experiments to better understand how organic carbon behaves in Australia’s grain producing soils, including the Free Air Carbon Dioxide Enrichment (FACE) experiment led by Professor Roger Armstrong (Agriculture Victoria) at Horsham, Victoria, the Northern Farming Systems experiment (DAQ2007-004RMX) led by Drs David Lester (Queensland Department Agriculture and Fisheries) and Lindsay Bell (CSIRO) at Pampas, Queensland, and Amelioration of Subsoil Constraints experiments (DAV1606-001RMX) led by Dr Ehsan Tavakkoli (NSW Department of Primary Industries) at Rand and Grogan, New South Wales.

Using these diverse experiments, the University of Queensland team harnessed the Australian Nuclear Science and Technology Organisation (ANSTO) synchrotron facility in Victoria to examine (1) changes in carbon forms and their microscale distribution in bulk soils; and (2) their carbon fractions, as related to climate conditions, soil types, land use and management practices.

Overall, they have found unexpected and surprising results. Traditionally, it has been assumed that increasing the stability of soil organic carbon relied upon the formation of large and chemically complex carbon structures that were difficult for soil microbes to break down.

Dr Han Dr Han Weng examined soil organic carbon at the soft X-ray spectroscopy beamline at the Australian Synchrotron with Professor Peter Kopittke from the University of Queensland. Photo: Han Weng

However, across a diverse range of grain-producing soils and management practices, the University of Queensland team and their collaborators found there was generally a marked lack of change in the carbon forms within the soil. In other words, whether carbon was stored in soil or whether it was lost from the soil did not appear to be related to the form (type) of carbon.

For instance, land use change from native vegetation to cropping caused substantial loss of soil organic carbon in two subtropical Queensland vertosols but, despite this carbon loss, there was no noticeable change in the forms of carbon in the soil. Nevertheless, such carbon loss provides opportunities to restock soil organic carbon.

Furthermore, the University of Queensland team examined the impact of farming systems on soil organic carbon over four years in another Queensland vertosol at the Pampas site. A high crop intensity (for example, six crops over four years) with high carbon inputs was found to potentially increase soil organic carbon, with this increase as occluded particulate matter.

These findings suggested that one approach for increasing soil organic carbon is the need to maximise inputs of organic matter, coupled with reducing disturbance of the soil, with this needing to be balanced among other production constraints. These surprising findings highlighted that the ability to accumulate and store carbon in soil is not related to the formation of complex carbon structures as traditionally assumed, but rather the organic carbon that is retained in soil appears to be similar in functional composition.

This was regardless of changes in total organic carbon content in bulk soils and their carbon fractions. Instead, it is likely that the retention and stability of soil organic carbon appears to be related to the ability of soil to physically protect that carbon from being broken down as opposed to chemical complexity of the soil organic carbon preventing its breakdown by soil organisms.

However, such mineral protection of soil organic carbon can be reversed in light-texture (sandy) soils under elevated carbon dioxide or under land use change from native vegetation to cropping.

For action now, these findings suggest that we should not emphasise identifying management practices that increase the complexity of carbon, but rather practices that increase the physical protection of soil organic carbon.

A change in thinking

This is a marked shift in thinking from what we have traditionally assumed and we now need to be thinking about land management practices and carbon modelling in a different manner.

Specifically, farming systems with maximised carbon input and minimised disturbance have the potential to increase soil organic carbon as particulate organic matter in the short term and improve physical protection of soil organic carbon over the long term as required for carbon sequestration and for building soil health and resilience for enduring profitability.

By working closely with GRDC managers and project leaders (including Roger Armstrong, David Lester, Lindsay Bell and Ehsan Tavakkoli), the University of Queensland team has value-added to existing GRDC projects by revealing the mechanistic complexity of soil organic carbon from a diverse range of grain producing soils.

The next users of this research will be researchers and proactive growers and their consultants who seek to evaluate and further develop management strategies to maintain and restock soil organic carbon.

Such information is beneficial to modellers in developing more-accurate carbon prediction models for maintaining and building soil organic carbon for improved soil fertility and carbon sequestration. Here, the University of Queensland team provides evidence that focus should be given to approaches that physically protect carbon within the soil and also to approaches that increase carbon inputs to soil.

More information: Professor Peter Kopittke, [email protected]; Australia's Nuclear Science and Technology Organisation.

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