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.

Abstract. The main objective of this study was to assess the impact of biochar rate (0, 8, 16 and 32 Mg ha −1 ) on the water retention capacity (WRC) of a sandy loam Dystric Plinthosol. The applied biochar was a by-product of slow pyrolysis (∼ 450 • C) of eucalyptus wood, milled to pass through a 2000 µm sieve that resulted in a material with an intrinsic porosity ≤ 10 µm and a specific surface area of ∼ 3.2 m 2 g −1 . The biochar was incorporated into the top 15 cm of the soil under an aerobic rice system. Our study focused on both the effects on WRC and rice yields 2 and 3 years after its application. Undisturbed soil samples were collected from 16 plots in two soil layers (5-10 and 15-20 cm). Soil water retention curves were modelled using a nonlinear mixed model which appropriately accounts for uncertainties inherent of spatial variability and repeated measurements taken within a specific soil sample. We found an increase in plant-available water in the upper soil layer proportional to the rate of biochar, with about 0.8 % for each Mg ha −1 biochar amendment 2 and 3 years after its application. The impact of biochar on soil WRC was most likely related to an effect in overall porosity of the sandy loam soil, which was evident from an increase in saturated soil moisture and macro porosity with 0.5 and 1.6 % for each Mg ha −1 of biochar applied, respectively. The increment in soil WRC did not translate into an increase in rice yield, essentially because in both seasons the amount of rainfall during the critical period for rice production exceeded 650 mm. The use of biochar as a soil amendment can be a worthy strategy to guarantee yield stability under short-term water-limited conditions. Our findings raise the importance of assessing the feasibility of very high application rates of biochar and the inclusion of a detailed analysis of its physical and chemical properties as part of future investigations.

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The objective of this work was to evaluate the effect of eucalyptus biochar on the transpiration rate of upland rice 'BRSMG Curinga' as an alternative means to decrease the effect of water stress on plant growth and development. Two-pot experiments were carried out using a completely randomized block design, in a split-plot arrangement, with six replicates. Main plots were water stress (WS) and no-water stress (NWS), and the subplots were biochar doses at 0, 6, 12 and 24% in growing medium (sand). Total transpirable soil water (TTSW), the p factor - defined as the average fraction of TTSW which can be depleted from the root zone before water stress limits growth -, and the normalized transpiration rate (NTR) were determined. Biochar addition increased TTSW and the p factor, and reduced NTR. Consequently, biochar addition was able to change the moisture threshold (p factor) of the growing medium, up to 12% maximum concentration, delaying the point where transpiration declines and affects yield.

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Index terms: Cerrado, climate change, greenhouse gas, N fertilization, nitrous oxide, no-tillage. Fluxos de nitrogênio em feijoeiro irrigado influenciados pela cobertura morta e a fertilização mineralResumo -O objetivo deste trabalho foi medir os fluxos de N 2 O-N e NH 3 -N, ao longo de uma safra de feijoeiro irrigado (Phaseolus vulgaris), influenciados pelo uso ou não de cobertura morta e fertilização mineral. Termos para indexação: Cerrado, mudanças climáticas, gas de efeito estufa, óxido nitroso, plantio direto.

Abstract. The main objective of this study was to assess the impact of biochar rate (0, 8, 16 and 32 t ha−1) on the water retention capacity (WRC) of a sandy Dystric Plinthosol. The applied biochar was a by-product of slow pyrolysis (∼450 °C) of eucalyptus wood, milled to pass through a 2000 μm sieve that resulted in a material with an intrinsic porosity ≤10 μm and a specific surface area of ∼3.2 m2 g−1. The biochar was incorporated into the top 15 cm of the soil under an aerobic rice system. Our study focused on both the effects on WRC and rice yields at 2 and 3 years after application. Undisturbed soil samples were collected from 16 plots in two soil layers (5–10 and 15–20 cm). Soil water retention curves were modelled using a nonlinear mixed model which appropriately accounts for uncertainties inherent of spatial variability and repeated measurements taken within a specific soil sample. We found an increase in plant available water in the upper soil layer proportional to the rate of biochar, with about 0.8% for each t ha−1 of biochar amendment at 2 and 3 years after application. The impact of biochar on soil WRC was most likely related to an increase in overall porosity of the sandy soil, which was evident from an increase in saturated soil moisture and macro porosity with 0.5% and 1.6% for each t ha−1 of biochar applied, respectively. The increment in soil WRC did not translate into an increase in rice yield, essentially because in both seasons the amount of rainfall during critical period for rice production exceeded 650 mm. The use of biochar as a soil amendment can be a worthy strategy to guarantee yield stability under water limited conditions. Our findings raise the importance of assessing the feasibility of very high application rates of biochar and the inclusion of a detailed analysis of its physical and chemical properties as part of future investigations.

-The objective of this work was to investigate the impact of the application of wood biochar, combined with N fertilizations, on N 2 O-N fluxes, nitrogen availability, and water-filled pore space (WFPS) of a clayey Oxisol under rice (wet season) and common bean (dry season) succession. Manual static chambers were used to quantify N 2 O-N fluxes from soil immediately after a single application of wood biochar (32 Mg ha Index terms: cropping systems, gas fluxes, greenhouse gases, nitrate and ammonium, soil amendment, soil porosity.Disponibilidade de nitrogênio, espaço poroso preenchido por água e fluxos de N 2 O-N após aplicação de biochar e fertilização nitrogenada Resumo -O objetivo deste trabalho foi investigar o impacto da aplicação de biochar de madeira, combinada com fertilizações nitrogenadas, nos fluxos de N 2 O-N, na disponibilidade de nitrogênio e no espaço poroso preenchido por água (EPPA), em um Latossolo argiloso sob sucessão com arroz (época chuvosa) e feijão (época seca). Câmaras estáticas manuais foram utilizadas para quantificar os fluxos de N 2 O-N no solo logo após uma única aplicação de biochar (32 Mg ha Termos para indexação: sistemas de cultivo, fluxo de gases, gases de efeito estufa, nitrato e amônio, condicionador de solo, porosidade do solo.