Abstract
Bio-char, biomass that has been deliberately charred to slow its rate of decomposition, has been proposed as an amendment with the potential to sequester carbon and improve certain soil properties. Slow pyrolysis (temperature ≤500 °C) and hydrothermal carbonization (low temperature, high pressure) are two efficient methods to produce bio-char with high yield and are applicable to a broad range of feedstocks. Chars made using slow pyrolysis (PC) and hydrothermal carbonization (HTC) of the same feedstock material (corn, C4) differed in physical appearance, chemical properties and decomposition behavior. We added these HTC and PC chars as amendments to three soils with C3-derived organic matter that differed in clay content, pH, and land use (managed spruce forest, unmanaged deciduous forest and agriculture), and compared their impacts on carbon sequestration and net greenhouse gas (CO2, 13CO2, N2O and CH4) emissions. HTC addition (1% w/w) significantly increased CO2 emissions in all three soils (p < 0.001), with much of the extra C derived from HTC decomposition. In contrast, PC addition (1% w/w) had almost no impact on deciduous forest soil and actually decreased CO2 emission from the agricultural soil. HTC treatment resulted in increased CH4 emission from all soils but reduced N2O fluxes in the agricultural and spruce forest soils. PC amendment had no significant effect on CH4 emission, and resulted in intermediate levels of N2O emission (between control and HTC treatments). Although both HTC and PC chars were produced from the same feedstock, PC had markedly higher potential for carbon sequestration than HTC.
Highlights
The annual growth rate of atmospheric CO2 was 1.9 ppm in the past decade (2000e2009), reflecting a continuing, large, imbalance between carbon (C) release to the atmosphere and removal by natural sinks (Peters et al, 2012)
Slow pyrolysis and hydrothermal carbonization are two of the most efficient methods to produce bio-char in terms of carbon yield and utility to a broad range of feedstocks
The hydrothermal carbonization (HTC) method especially is proposed as a useful way to increase soil C sequestration as it uses less energy than pyrolysis char (PC) production and can use unconventional wet biomass sources such as sewage sludge, city wastes, animal and human excreta without requiring additional pretreatment such as drying
Summary
The annual growth rate of atmospheric CO2 was 1.9 ppm in the past decade (2000e2009), reflecting a continuing, large, imbalance between carbon (C) release to the atmosphere and removal by natural sinks (Peters et al, 2012). The concept of bio-char amendment was derived from the study of Amazonian dark earth soils, known as anthroposols. Amendment with bio-char is widely hypothesized to increase carbon storage capacity, this effect is largely unquantified and depends on many factors (Liang et al, 2010). One such factor is the method used for bio-char production. Large differences in the composition of bio-char produced using different methods can result in timescales for persistence in soils, ranging from millennia (Forbes et al, 2006; Liang et al, 2008) to decades (Steinbeiss et al, 2009)
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