Terra Preta Australis: Reassessing the carbon storage capacity of temperate soils
Research highlights
▶ Cumulic Anthroposols identified in temperate Australia exhibit characteristics of Terra Preta soils from the Amazon, and hence have been referred to as Terra Preta Australis. ▶ Terra Preta Australis sites have large quantities of stable aromatic carbon and enhanced soil chemical and physical properties for agricultural production. ▶ Findings suggest the significant potential for biochar to be used in Australian soils to sequester C and improve soil fertility.
Introduction
The upper 100 cm of the worlds soils represent an estimated 1200–1600 Gt C pool globally (Batjes, 1996, Eswaran et al., 2000, Post et al., 1982), which offers large sequestration potential (Cole et al., 1996, Lal et al., 2007) when considered in relation to the estimated 270 ± 30 Gt CO2–C emissions from fossil fuel combustion between 1850 and 2000 (IPCC, 2001). Increasing soil carbon (C) levels has benefits beyond climate change mitigation as it improves agricultural productivity and sustainability (Lal et al., 2007, Paustian et al., 1997). As these benefits relate directly to profitability, they have the potential to motivate land managers to incorporate the practices of increasing soil C without the need for high C emissions offset prices.
The potential limit for C sequestration by soils is often assumed to be the C holding capacity of native, pre-cultivation soils (Lal et al., 2007, Paustian et al., 1997), which results in an estimated global potential of 40–60 Gt (Cole et al., 1996). It has been demonstrated, however, that incorporation of high levels of organic matter, and in particular chemically recalcitrant forms of organic matter, can result in greatly enhanced soil C levels (Sombroek, 1966, Sombroek et al., 1993). Notable examples are the Terra Preta de Indio (dark earths of the Amazon), the Plaggen soils of North-West Europe (Davidson et al., 2006, Pape, 1970, Sombroek et al., 1993) and the ancient agricultural soils of the Andes (Sandor and Eash, 1995). The achievement of high soil C levels that are stable over time in both these examples has been attributed to anthropogenic amendment with charred organic matter, which resists microbial breakdown (Glaser et al., 1998, Lehmann et al., 2003b, Sombroek, 1966).
The Terra Preta de Indio soils have a clear anthropogenic origin involving the addition of charred organics, the remnants from earthen ovens used for cooking and firing pottery, to the surrounding soils (Glaser et al., 2001, Lehmann et al., 2003b, Sombroek et al., 1993). Terra Preta soils have garnered interest because of their anthropology, increased fertility over extended periods and demonstrated long-term soil C sequestration (Lehmann et al., 2003a). The enhanced fertility of Terra Preta in the Amazon has been explained by higher levels of soil organic matter (SOM), improved holding capacity of nutrients such as nitrogen, phosphorus, calcium and potassium, higher pH and higher moisture-holding capacity compared to the surrounding soils (Glaser et al., 2001, Lehmann et al., 2003a, Lehmann et al., 2003b, Smith, 1980, Sombroek, 1966, Zech et al., 1990).
These soils demonstrate the potential benefits of adding charred organics to soils, both in terms of C sequestration and improving soil fertility, and have been directly linked to the idea of “biochar” amendment to soils. In this context, biochar is defined as charred organic matter produced specifically for this purpose. Although the Terra Preta soils provide the opportunity to investigate the long-term implications of biochar addition, the outcomes will almost certainly be influenced by soil type, climatic conditions and the anthropological process by which they are created. Therefore the clear benefits of biochar amendment in the Terra Preta soils of the Amazon will not necessarily occur in other regions. Hence there is great interest in finding examples where anthropological biochar addition has occurred in different soils and climates. The European Plaggen soil has been identified as an example of a Cumulic Anthroposol demonstrating Terra Preta-like characteristics (Pape, 1970). The highly fertile nutrient status of phosphorus and calcium in this soil has been attributed to carbonised (biochar) particles present due to anthropogenic activity (Davidson et al., 2006). Ancient agricultural terraces in the Colca Valley of Southern Peru have also been found to have more organic C and N, lower pH and enriched P compared to nearby uncultivated Mollisols (Sandor and Eash, 1995).
At the same time as the pre-Columbian Indians were using ovens in the Amazon, in Australia, the pre-European Aboriginals resident in nomadic camps above the flood zone of the Murray River were also using earthen ovens to cook food. The resulting charred organics and refuse were discarded, building up into mounds over generations (Beveridge, 1869, Coutts, 1976, Coutts et al., 1979, Spencer, 1918). To date, the relevance of these anthropogenic oven mounds, or kitchen middens, to long-term C sequestration and soil fertility has not been investigated.
In this paper, these Cumulic Anthroposols are recognised as not simply oven mounds but as examples of Australian dark earth, for which we suggest the title Terra Preta Australis (TPA). We investigate these soils in terms of the impact that this anthropological activity has had on soil fertility and what C sequestration was achieved over the long-term. The findings of this investigation are considered in light of today's pressing issues of climate change, food security and agricultural sustainability.
Section snippets
Classification
The soils investigated can be classified as Cumulic Anthroposols under the Australian Soil Classification (Isbell, 2002) or under the international framework for international soil correlation and communication as Anthrosols (WRB, 2006). Anthroposols are soils resulting from human activities which have led to a profound modification of the original soil horizons (Isbell, 2002). The suborder Cumulic refers to soils that have been formed by application of human-deposited material to a minimum
Terra Preta Australis site descriptions
The Terra Preta Australis sites were identifiable as raised mounds with distinctly darker soil colouration. Sites were identified in native forests that are utilised for forestry and in cleared areas used for grazing. Glaser et al. (2000) describe the identification of Terra Preta in the Amazon by deep (40–80 cm) black A horizons which include charcoal particles, which is consistent with the identification of these Cumulic Anthroposols. The occurrence of ceramics also used as an identifier in
Conclusions
Cumulic Anthroposols exist in Australia and exhibit all the features typical of Terra Preta soils in the Amazon, with the exception of geographical location. Therefore, informal classification of these soils as Terra Preta Australis is justified. These Terra Preta Australis soils have been created via anthropological addition of charred organics (biochar) to soil hundreds of years ago. The Terra Preta Australis exist in a temperate climate with low rainfall.
The formation of Terra Preta Australis
Acknowledgements
We acknowledge the Yorta Yorta people as the Aboriginal Traditional Owners of the areas studied. We would like to acknowledge the Australian Research Council, BEST Energies Australia Pty Ltd and I&I NSW for project funding. We would like to thank Derek Barnstable for his time, local knowledge and assistance with field work. Scott Petty, Josh Rust and Glen Rangott are thanked for their assistance with laboratory analysis.
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