William I. Woods

Amazonian Dark Earths: Wim Sombroek's Vision

1 Amazonian soils are almost universally thought of as extremely forbidding. However, it is now clear that complex societies with large, sedentary populations were present for over a millennium before European contact. Associated with these are tracts of anomalously fertile, dark soils termed terra preta or dark earths. ese soils are presently an important agricultural resource within Amazonia and provide a model for developing long-term future sustainability of food production in tropical environments. e late Dutch soil scientist Wim Sombroek (1934-2003) was instrumental in bringing the signifi cance of these soils to the attention of the world over four decades ago. Wim saw not only the possibilities of improving the lives of small holders throughout the world with simple carbon based soil technologies, but was an early proponent of the positive synergies also achieved in regards to carbon sequestration and global climatic change abatement. Wim’s vision was to form a multidisciplinary group whose members maintained the ideal of open collaboration toward the attainment of shared goals. Always encouraged and o en shaped by Wim, this free association of international scholars termed the Terra Preta Nova Group came together in 2001 and has fl ourished. is eff ort has been defi ned by enormous productivity. Wim who is never far from any of our minds and hearts, would have loved to share the great experience of seeing the fruits of his vision as demonstrated in this volume. William I. Woods Wenceslau G. Teixeira Johannes Lehmann Christoph Steiner Antoinette M.G.A. WinklerPrins Lilian Rebellato Editors Amazonian Dark Earths: Wim Sombroek‘s Vision W oods et al. ds.

The Columbian Encounter and the Little Ice Age: Abrupt Land Use Change, Fire, and Greenhouse Forcing

Pre-Columbian farmers of the Neotropical lowlands numbered an estimated 25 million by 1492, with at least 80 percent living within forest biomes. It is now well established that significant areas of Neotropical forests were cleared and burned to facilitate agricultural activities before the arrival of Europeans. Paleoecological and archaeological evidence shows that demographic pressure on forest resources—facilitated by anthropogenic burning—increased steadily throughout the Late Holocene, peaking when Europeans arrived in the late fifteenth century. The introduction of Old World diseases led to recurrent epidemics and resulted in an unprecedented population crash throughout the Neotropics. The rapid demographic collapse was mostly complete by 1650, by which time it is estimated that about 95 percent of all indigenous inhabitants of the region had perished. We review fire history records from throughout the Neotropical lowlands and report new high-resolution charcoal records and demographic estimates that together support the idea that the Neotropical lowlands went from being a net source of CO2 to the atmosphere before Columbus to a net carbon sink for several centuries following the Columbian encounter. We argue that the regrowth of Neotropical forests following the Columbian encounter led to terrestrial biospheric carbon sequestration on the order of 2 to 5 Pg C, thereby contributing to the well-documented decrease in atmospheric CO2 recorded in Antarctic ice cores from about 1500 through 1750, a trend previously attributed exclusively to decreases in solar irradiance and an increase in global volcanic activity. We conclude that the post-Columbian carbon sequestration event was a significant forcing mechanism of Little Ice Age cooling.

The domestication of Amazonia before European conquest.

During the twentieth century, Amazonia was widely regarded as relatively pristine nature, little impacted by human history. This view remains popular despite mounting evidence of substantial human influence over millennial scales across the region. Here, we review the evidence of an anthropogenic Amazonia in response to claims of sparse populations across broad portions of the region. Amazonia was a major centre of crop domestication, with at least 83 native species containing populations domesticated to some degree. Plant domestication occurs in domesticated landscapes, including highly modified Amazonian dark earths (ADEs) associated with large settled populations and that may cover greater than 0.1% of the region. Populations and food production expanded rapidly within land management systems in the mid-Holocene, and complex societies expanded in resource-rich areas creating domesticated landscapes with profound impacts on local and regional ecology. ADE food production projections support estimates of at least eight million people in 1492. By this time, highly diverse regional systems had developed across Amazonia where subsistence resources were created with plant and landscape domestication, including earthworks. This review argues that the Amazonian anthrome was no less socio-culturally diverse or populous than other tropical forested areas of the world prior to European conquest.

Seasonal bioavailability of sediment-associated heavy metals along the Mississippi river floodplain.

A value of simultaneously extracted metal to acid-volatile sulfide (SEM-AVS) can provide important information regarding metal availability in anaerobic sediment. SEM and AVS concentrations were obtained by the cold-acid purge-and-trap technique during spring and summer at six locations along the Mississippi River floodplain. SEM-AVS values and AVS concentrations did not vary significantly between locations during both seasons. AVS concentrations were significantly greater during summer than spring, resulting in significantly lower SEM-AVS values in summer. Total SEM concentrations did not significantly vary between seasons or specific locations. SEM-AVS values were greater than one at each location during both seasons. Sediment metal toxicity was predicted to be absent for benthic organisms along the river floodplain.

The Quantitative Analysis of Soil Phosphate

Developments in environmental quality testing have revealed the need for a reappraisal of the methods of phosphate determination employed by archaeologists. The results of such a reappraisal are presented with recommendations for the implementation of a new technique of quantitative phosphate determination called sequential fractionation. With this technique three discrete fractions are determined by differential solubility criteria. These fractions closely approximate in amount the major types of inorganic phosphate known to be retained by soils. Sample analyses are presented which indicate that the method can be employed to distinguish between natural and human deposited phosphate and to identify features.

Amazonian Dark Earths: The First Century of Reports

Amazonian dark earths are anthropogenic soils called terra preta de índio in Brazil, created by indigenous people hundreds, even thousands, of years ago (Smith 1980; Woods and McCann 1999). Terra preta proper is a black soil, associated with longenduring Indian settlement sites and is filled with ceramics and other cultural debris. Brownish colored terra mulata, on the other hand, is much more extensive, generally surrounds the black midden soils, contains few artifacts, and apparently is the result of semi-intensive cultivation over long periods. Both forms are much more fertile than the surrounding highly weathered soils, mostly Ferralsols and Acrisols, and have generally sustained this fertility to the present despite the tropical climate and despite frequent or periodic cultivation. This fertility probably is because of high carbon content, which retains nutrients and moisture, and an associated high and persistent microbial activity. The high concentrations of pyrogenic carbon in terra preta come mainly from charcoal from cooking and processing fires and settlement refuse burning, and in terra mulata the carbon probably comes from in-field burning of organic debris. Low intensity “cool” burning, what has been called slash-and-char, resulting in incomplete combustion, can produce carbon in high quantity which can persist in soil for thousands of years. Dated carbon in dark earths is as old as 450 BC (Hilbert 1968; Petersen et al. 2001:100). In contrast, slash and burn shifting cultivation fires today tend to be “hot” fires, set at the end of the dry season, which produce large releases of carbon dioxide to the atmosphere and more ash of brief persistence than charcoal. Denevan (2001:116–119) has argued that in pre-Columbian times the use of stone axes made long-fallow shifting cultivation very inefficient, and as result probably uncommon until the European introduction of metal axes. Previously, soil fertility must have been maintained and improved by frequent composting, mulching, and in-field burning, making semi-permanent cultivation possible with only brief fallowing. Over time these activities could have produced fertile, self-sustaining dark earths. Dark earths may occupy 0.1% to 0.3%, or 6,000 to 18,000 km, of forested lowland Amazonia (Sombroek and Carvalho 2002:130). Because their densities vary

History, Current Knowledge and Future Perspectives of Geoecological Research Concerning the Origin of Amazonian Anthropogenic Dark Earths ( Terra Preta )

Soil degradation is one of the most severe problems of land use in the lowland humid tropics (Zech 1997) largely due to the fact that soil organic matter (SOM) is mineralized rapidly under the optimum growth temperatures for micro-organisms (Tiessen et al. 1994). However, SOM is of particular importance for sustainable agricultural use of the heavily weathered tropical soils. It contributes substantially to nutrient supply, cation exchange capacity (CEC), and to a favorable soil structure (Ross 1993). A loss of SOM after slash-and-burn and other agricultural and pastoral land uses progresses the soil degradation of many tropical soils, resulting in infertile soils after a few years of cultivation. Soil amelioration by application of mineral fertilizers or compost is often unaffordable for poor smallholder farmers or remains ineffective due to the low CEC of the soils or the lack of knowledge about the nutrient release from organic fertilizers (Tiessen et al. 1994).

Phytoliths and Terra Preta: The Hatahara Site Example

In this chapter analyses the results from soils sampled in the Hatahara archaeological site. The first objective of this proposal was to determine if the Hatahara had an agricultural field during the pre-Columbian times and, if validated the agricultural use of the land, to identify the plants that were cultivated in the past. The general site characteristics and location were presented in earlier chapters (see Rebellato et al. Chapter 2 in this volume; Arroyo-Kalin Chapter 3 in this volume). The samples were collected from a ca. 1.5 m column through one of the site’s mounds (M-I; see Fig. 2.2 this volume). The mound is associated with the Paredão phase occupation of the site. The mound was constructed with dark earth soil from the surrounding area that had developed during the previous Manacapuru occupation (see Machado 2005). Consequently, the archaeological materials, including the phytoliths, found within it are not in situ and are from the disturbed context developed by this earthmoving operation ca. 1,000 years ago. Even the terra preta below the mound is not in primary context. It was disturbed by a burial program that occurred during Manacapuru phase c500 years before the construction of the mound. A total of 22 skeletons was recovered from the submound context. So, there are basically two contexts represented by these samples: (1) The redeposited materials from within the mound; and (2) The turbated materials found below the mound. The latter are at least close to their original place of deposition and were sealed by the mound. Within the mound the materials are most likely in inverse stratigraphic/temporal position and come from a much wider area. In spite of the disturbed context for these samples it is felt that the study was available one in that it demonstrated the array of useful plants found in association with the occupations of the Hatahara site. As will be seen the phytolith data derived from this study (one of the very few so far in Amazonia) are in conformance with the stance of that a broadlybased subsistence strategy was operant in the Central Amazon during the preColumbian period.

Towards an Understanding of Amazonian Dark Earths

By way of introduction to the topic and to provide a basis for the context of this volume, we provide the following short discussion. During the last century, a number of researchers looked seriously at development scenarios for various parts of the world and attributed great importance to natural soils and their management or mismanagement. In the case of Amazonia, the extremely poor soils present were cited as the fundamental cause for lack of cultural attainment. In general, soils were used deterministically as an explanatory mechanism when they presumably suited a preconceived notion of development or decline, but little in-depth study was undertaken regarding which soils were actually present on the microscale and how these had been affected by different management strategies over time.

Research on Anthropogenic Dark Earth Soils. Could It Be a Solution for Sustainable Agricultural Development in the Amazon

It has become clear that the economic and cultural growth of humanity is not unlimited, but, to the contrary, it can be seriously constrained by the way resources are administered and managed. The awareness of this perception has made the concept of sustainable development a precondition for economically and socially healthy societies (Hiraoka and Mora 2001). Agricultural systems will have to meet the demands of growing populations, and natural resources will be under increasing pressure (Doran et al. 2002). Changes are happening in our concepts about agricultural development, the use of natural resources, and the stability of the global environment (Doran 2002). The appropriate management of natural resources is unquestionably indispensable in all sectors of agriculture. Governments are investing considerable amounts of resources in research and development projects to provide new products and technologies that may form the basis for sustainable sectoral development.