F. Verheijen, S. Jeffery, A.C. Bastos, M. van der Velde, I. Diafas, 2010
Biochar is defined as “charcoal" (biomass that has been pyrolysed in a zero or low oxygen environment) for which, owing to its inherent properties, scientific consensus exists that application to soil at a specific site is expected to sustainably sequester carbon and concurrently improve soil functions (under current and future management), while avoiding short- and long-term detrimental effects to the wider environment as well as human and animal health."
Biochar is a stable carbon (C) compound created when biomass (feedstock) is heated to temperatures between 300 and 1000ºC, under low (preferably zero) oxygen concentrations. The objective of the biochar concept is to abate the enhanced greenhouse effect by sequestering C in soils, while concurrently improving soil quality. The proposed concept through which biochar application to soils would lead to C sequestration is relatively straightforward. Carbondioxide from the atmosphere is fixed in vegetation through photosynthesis. Biochar is subsequently created through pyrolysis of the plant material thereby potentially increasing its recalcitrance with respect to the original plant material.
The estimated residence time of biochar-carbon is in the range of hundreds to thousands of years while the residence time of carbon in plant material is in the range of decades. Consequently, this would reduce the CO2 release back to the atmosphere if the carbon is indeed persistently stored in the soil. The carbon storage potential of biochar is widely hypothesised, although it is still largely unquantified, particularly when also considering the effects on other greenhouse gasses, and the secondary effects of large-scale biochar deployment.
Concomitant with carbon sequestration, biochar is intended to improve soil properties and soil functioning relevant to agronomic and environmental performance. Hypothesised mechanisms that have been suggested for potential improvement are mainly improved water and nutrient retention (as well as improved soil structure, drainage).
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