Terra Preta Australis: Reassessing the carbon storage capacity of temperate soils

Abstract

Soils developed on the sites of Australian Aboriginal oven mounds along the Murray River in SE Australia, classified as Cumulic Anthroposols under the Australian Soil Classification, are shown to have traits similar to the Terra Preta de Indio of the Amazon basin. Seven such sites were characterised and compared with adjacent soils. The Cumulic Anthroposols contained significantly (p < 0.05) more soil carbon (C), compared to adjacent non-Anthroposols. Solid-state ¹³C NMR spectroscopy showed that the C in the Cumulic Anthroposols was predominantly aromatic, especially at depth, confirming the presence of charcoal. Radiocarbon analysis carried out on charcoal collected from two of these sites showed that it was deposited 650 ± 30 years BP at one site and 1609 ± 34 years BP at the other site, demonstrating its recalcitrance in soil. The charcoal originated from plant material, as shown by SEM, and had high levels of Ca agglomeration on its surfaces. The Cumulic Anthroposols were shown to have altered nutrient status, with total N, P, K and Ca being significantly greater than in the adjacent soils throughout the profile. This was also reflected in the higher mean CEC of 31.2 cmol (+) kg⁻¹ and higher pH by 1.3 units, compared to the adjacent soils. Based on the similarity of these Cumulic Anthroposols with the Terra Preta de Indio of the Amazon, we suggest that these Cumulic Anthroposols can be classified as Terra Preta Australis. The existence of these soils demonstrates that Australian soils, in temperate climates, are capable of storing C in much higher quantities than has been previously recognised, and that this capability is founded on the unique stability and properties of charred organic matter. Furthermore, the addition of charcoal appears to have improved the physical and chemical properties of these soils. Together, this provides important support for the concept of soil amendment with “biochar”, the charred residue produced by pyrolysis of biomass, as a means for sequestering C and enhancing agricultural productivity.

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.