International audienceMankind is actually facing serious issues due to the overexploitation of fossil fuels, biomass, soils, nitrogen, and phosphorus. It is claimed that biochar addition to soil improves C sequestration to prevent CO2 from atmospheric cycling. Biochar addition should also increase soil fertility in a similar way as anthropogenic dark earths of Central Amazonia. Previous studies have shown that biochar stimulates plant growth and increase fertilizer efficiency, especially when biochar is combined with organic fertilizers such as compost. However, little is known about optimum addition amounts and mixture ratios of biochar and compost. Indeed most experiments to mimic Terra preta de Indio focused on biochar alone or biochar in combination with mineral fertilizers. Therefore, we studied optimum biochar and compost amounts and mixture ratios with respect to plant response and soil fertility. We tested the effect of total amount from 0 to 200 Mg/ha, and biochar proportion from 0 % to 50 % biochar, of 18 different compost mixtures on growth of oat (Avena sativa L.) and soil properties in a fully randomized greenhouse study with sandy and loamy soil substrates. We sampled soil substrates before and after plant growth and analyzed plant growth and yield, total organic carbon (TOC), total nitrogen (TN), mineralized nitrogen (Nmin), soil reaction (pH), and electrical conductivity (EC) applying standard procedures. Results show that biomass production was increased with rising biochar and compost amounts. Oat plant height and seed weight was improved only with rising biochar amounts, but not with compost amounts. This could be explained by increase of total organic C and total N but not by plant-available ammonium and nitrate. The positive influence of composted biochar on plant growth and soil properties suggests that composting is a good way to overcome biochar’s inherent nutrient deficiency, making it a suitable technique helping to refine farm-scale nutrient cycles

Abstract:It is claimed that the addition of biochar to soil improves C sequestration, soil fertility and plant growth, especially when combined with organic fertilizers such as compost. However, little is known about agricultural effects of small amounts of composted biochar. This greenhouse study was carried out to examine effects of co-composted biochar on oat (Avena sativa L.) yield in both sandy and loamy soil. The aim of this study was to test whether biochar effects can be observed at very low biochar concentrations. To test a variety of application amounts below 3 Mg biochar ha −1 , we co-composted five different biochar concentrations (0, 3, 5, 10 kg Mg −1 compost). The biochar-containing compost was applied at five application rates (10, 50, 100, 150, 250 Mg ha −1 20 cm −1 ). Effects of compost addition on plant growth, Total Organic Carbon, N tot , pH and soluble nutrients outweighed the effects of the minimal biochar amounts in the composted substrates so that a no effect level of biochar of at least 3 Mg ha −1 could be estimated.

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Besides carbon sequestration and improvement of soil properties, biochar (BC) has increasingly been studied as an amendment to immobilise heavy metals in contaminated soils. In a 2-year experiment, we analysed the effects of poplar BC (P-BC, mixed with compost) and gravel sludge with siderite-bearing material (GSFe) on a Cd-, Pb- and Zn-contaminated soil and on metal concentration in Miscanthus × giganteus shoots under greenhouse and field conditions. In the greenhouse, 1% (m/m) P-BC addition reduced NHNO-extractable Cd, Pb and Zn concentrations by 75, 86 and 92%, respectively, at the end of the study. In the leachates, P-BC (1%) could significantly reduce Cd and Zn in both years. In the field, P-BC (3%) induced a reduction of extractable Cd by 87% whereas a combination of P-BC + GSFe reduced Pb by 82% and Zn by 98% in the first year and by 83 and 96% in the second year. In contrast, the metal immobilisation in the soil was hardly reflected in the shoots of Miscanthus × giganteus which generally showed metal concentrations close to control. While Cd was not influenced in both years, Pb and Zn were slightly reduced. Our study confirmed that Miscanthus is an efficient metal excluder, corroborating its suitability for the production of renewable biomass on metal-contaminated soils.