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This manuscript presents the Raman characterization of the stable carbon materials found in a special type of soil, named Terra Preta de Índio (Indian Dark Earths). The Terras Pretas de Índios have been studied as a potential model for sustainable agriculture in the humid tropics. The stability of organic matter and the long‐term high‐level of ion exchange capacity in these soils are due to unusually high amount of black carbon. Here, we show how Raman spectroscopy can be used to characterize the stable carbon content in the Terras Pretas de Índios (TPI‐carbons). The tangential stretching mode (G band) and the disorder‐induced mode (D band) are analyzed in comparison to laboratory‐produced amorphous carbons at different degree of disorder used here as reference materials. Statistical analysis show predominance of sp2 phase and crystallite sizes within the limit range between nanographite and amorphous carbons, while Raman mapping of a TPI‐carbon grain shows that the surface is more disordered than the grain core. The analysis used here can also differentiate the TPI‐carbon structures from different types of charcoal. Copyright © 2012 John Wiley & Sons, Ltd.

Amazonian Dark Earths (ADE), one of the best-known examples of anthropogenic (man-made) soils, are the result of Amerindian settlements in the pre-Columbian period. ADE are highly variable in terms of their size, shape, depth and physical and chemical make-up. Scholars tend to divide ADE into two categories: terra preta and terra mulata. The former are dark and highly fertile soils replete with ceramic shards, indicating former areas of habitation, while the latter are lighter in colour, less fertile, lacking pottery and thought to be old agricultural fields. While a scientific consensus on the origins of terra preta has existed for several decades, the origins of terra mulata remain enigmatic and contested. We argue that owing to the overlapping and constantly changing boundaries of agricultural and habitational areas, it is unlikely that there exist two clear soil fertility classes. This article examines the hypothesis that rather than two distinct anthrosol categories, ADE sites should exhibit a highly fertile 'core area' , which grades into more subtly modified soils, with a continuum of fertility between them. Using principal components analysis (PCA) and interpolations based on the geographic distribution of the soil samples, we show that ADE along the Middle Madeira, Brazilian Amazon are extremely diverse, but data support more of a gradient between areas of greater and lesser fertility rather than two distinct categories. We also assess local people's perceptions and classifications of anthropogenic and surrounding soils using ethnographic data. Rather than discarding the terra preta-terra mulata opposition however, we suggest abandoning only the idea that they are separate categories, and instead emphasise a continuum, the darker, bluff edge 'central' regions with abundant ceramics are consonant with published descriptions of terra preta, which grade into surrounding areas with lighter, less fertile soils that better fit terra mulata descriptions.

Soils of the lowland tropics in the central Brazilian Amazon are generally highly leached, acidic and nutrient-poor. Charcoal, combined with other soil amendments, might improve fertility but this, in turn, could lead to increased weed problems for agricultural production. This experiment was conducted to assess weed pressure and species composition on plots receiving various inorganic and organic soil amendments, including charcoal. Additions of inorganic fertilizer, compost and chicken manure resulted in increases in weed ground cover of 40, 22 and 53%, respectively, and increases in species richness of 20, 48, and 63%, respectively. When chicken manure was applied, dominance by a few weed species was reduced, such that different species were more evenly represented. Although charcoal additions alone did not significantly affect weed ground cover or species richness, a synergistic effect occurred when both charcoal and inorganic fertilizers were applied. The percentage ground cover of weeds was 45% within plots receiving inorganic fertilizer, 2% within plots receiving charcoal and 66% within plots receiving both amendments. Improvements in the fertility of nutrient-poor soils of the tropics might increase weed pressure and make the development of effective weed management strategies more critical. These effects on weed populations were observed nearly 2.5 years after the addition of charcoal, chicken manure and compost, and > 1 year after the last application of inorganic fertilizer.

The term biochar refers to materials with diverse chemical, physical and physicochemical characteristics that have potential as a soil amendment. The purpose of this study was to investigate the P sorption/desorption properties of various slow biochars and one fast pyrolysis biochar and to determine how a fast pyrolysis biochar influences these properties in a degraded tropical soil. The fast pyrolysis biochar was a mixture of three separate biochars: sawdust, elephant grass and sugar cane leaves. Three other biochars were made by slow pyrolysis from three Amazonian tree species (Lacre, Ingá and Embaúba) at three temperatures of formation (400 °C, 500 °C, 600 °C). Inorganic P was added to develop sorption curves and then desorbed to develop desorption curves for all biochar situations. For the slow pyrolysis, the 600 ºC biochar had a reduced capacity to sorb P (4–10 times less) relative to those biochars formed at 400 °C and 500 °C. Conversely, biochar from Ingá desorbed the most P. The fast pyrolysis biochar, when mixed with degraded tropical mineral soil, decreased the soil's P sorption capacity by 55% presumably because of the high soluble, inorganic P prevalent in this biochar (909 mg P/kg of biochar). Phosphorus desorption from the fast pyrolysis biochar/soil mixture not only exhibited a common desorption curve but also buffered the soil solution at a value of ca. 0.2 mg/L. This study shows the diversity in P chemistry that can be expected when biochar is a soil amendment and suggests the potential to develop biochars with properties to meet specific objectives.

RESUMOCom o objetivo de determinar as características de adsorção de fósforo utilizando-se a isoterma de Langmuir, e suas relações com algumas propriedades físicas e químicas de solos, foi desenvolvido um estudo no Laboratório de Solos do Instituto Nacional de Pesquisas da Amazônia (INPA), com amostras da camada superficial (0 -20 cm) de oito solos do Estado do Amazonas. Grande variação na capacidade máxima de adsorção (CMAP), energia de adsorção e fator capacidade de P máximo (FCP máx PALAVRAS-CHAVE Fixação de P, Solos tropicais, AmazôniaPhosphorus adsorption characteristics in some Central Amazonian soils. ABSTRACT The objective of this paper was to determine the phosphorus adsorption characteristics using the Langmuir isotherm and it is relation with some physical and chemical properties of soils. The study was carried out in the Soil and Plant

Carbon particles containing mineral matter promote soil fertility, helping it to overcome the rather unfavorable climate conditions of the humid tropics. Intriguing examples are the Amazonian Dark Earths, anthropogenic soils also known as "Terra Preta de Índio'' (TPI), in which chemical recalcitrance and stable carbon with millenary mean residence times have been observed. Recently, the presence of calcium and oxygen within TPI-carbon nanoparticles at the nano- and mesoscale ranges has been demonstrated. In this work, we combine density functional theory calculations, scanning transmission electron microscopy, energy dispersive X-ray spectroscopy, Fourier transformed infrared spectroscopy, and high resolution X-ray photoelectron spectroscopy of TPI-carbons to elucidate the chemical arrangements of calcium-oxygen-carbon groups at the molecular level in TPI. The molecular models are based on graphene oxide nanostructures in which calcium cations are strongly adsorbed at the oxide sites. The application of material science techniques to the field of soil science facilitates a new level of understanding, providing insights into the structure and functionality of recalcitrant carbon in soil and its implications for food production and climate change.