Fractions of soil organic matter (SOM) were obtained from three soils using alternative physical fractionation procedures, and evaluated against the requirements of model pools. We compared two‐stage density fractionation (isolating free and intra‐aggregate fractions, before and after dispersion, respectively) with particle‐size separation of dispersed soil. For full comparison, the organomineral fraction residual from density fractionation was also size separated. In standardizing the density‐based method, we found recovery of intra‐aggregate organic matter highly sensitive to separation density as compared with the free. Recovery of the intra‐aggregate was also influenced by dispersion energy. The greatest amount was obtained using a combination of the highest density (1.80 g cm−3) and dispersion energy (1500 J g−1). Analysis by 13C nuclear magnetic resonance (NMR) showed O‐alkyl/alkyl‐C ratios 1.38 to 2.30 times greater in intra‐aggregate organic matter than in the free. Diffuse reflectance Fourier transform infrared spectroscopy (DRIFT) also indicated a greater proportion of aliphatic hydrocarbon, carboxylic anions, and aromatic C in intra‐aggregate organic matter. The findings suggest this fraction comprises more decomposed and transformed organic matter relative to the free. Higher signal/noise ratios in NMR spectra of particle‐size fractions (compared with their organomineral equivalents) were attributed to C in particulate SOM, not removed by prior density separation. Whilst particle‐size fractions confuse particulate SOM with that attached to mineral surfaces, fractions isolated by two‐stage density separation are small in number and display distinct chemical properties. We suggest they provide a sound basis for a model of SOM turnover based on measurable pools.

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O objetivo deste trabalho foi avaliar o desempenho de plantas de cobertura quanto à fitomassa, ao acúmulo e à liberação de nutrientes, durante a entressafra, em um Latossolo Vermelho distroférrico, no Cerrado. O experimento foi realizado em Santo Antônio de Goiás e Rio Verde, GO, de novembro de 2007 a outubro de 2008. O delineamento experimental foi o de blocos ao acaso, em esquema de parcelas subdivididas no tempo, com as plantas de cobertura avaliadas nas parcelas principais e com os períodos de coleta de fitomassa nas subparcelas, com quatro repetições. As espécies avaliadas foram: Urochloa brizantha, U. ruziziensis, Pennisetum glaucum e U. ruziziensis + Cajanus cajan e, como referência, pousio com vegetação espontânea. As épocas de coleta foram seis em Santo Antônio de Goiás e cinco em Rio Verde, a partir da data de dessecação do P. glaucum, aos 60 dias após a semeadura. O P. glaucum apresentou as maiores quantidades de fitomassa seca no início da entressafra, enquanto as outras coberturas apresentaram acúmulos significativos de fitomassa e nutrientes no final da entressafra.

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The 4p1000 Initiative: opportunities, limitations and challenges for implementing soil organic carbon sequestration as a sustainable development strategy

Biochar (carbonized biomass for agricultural use) has been used worldwide as soil amendment and is a technology of particular interest for Brazil, since its "inspiration" is from the historical Terra Preta de Índios (Amazon Dark Earth), and also because Brazil is the world's largest charcoal producer, generating enormous residue quantities in form of fine charcoal and due to the availability of different residual biomasses, mainly from agroindustry (e.g., sugar-cane bagasse; wood and paper-mill wastes; residues from biofuel industries; sewage sludge etc), that can be used for biochar production, making Brazil a key actor in the international scenario in terms of biochar research and utilization). In the last decade, numerous studies on biochar have been carried out and now a vast literature, and excellent reviews, are available. The objective of this paper is therefore to deliver a critical review with some highlights on biochar research, rather than an exhaustive bibliographic review. To this end, some key points considered critical and relevant were selected and the pertinent literature "condensed", with a view to guide future research, rather than analyze trends of the past.