Comparing current chemical methods to assess biochar organic carbon in a Mediterranean agricultural soil amended with two different biochars

Raya-Moreno I., Cañizares R., Domene X., Carabassa V., Alcañiz J.M. (2017) Comparing current chemical methods to assess biochar organic carbon in a Mediterranean agricultural soil amended with two different biochars. Science of the Total Environment. 598: 604-618.
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Doi: 10.1016/j.scitotenv.2017.03.168

Resum:

Several methods have been proposed to quantify biochar C recalcitrance but their suitability is questionable. The aims of this work are: i) to compare the suitability of thermal or chemical oxidation and acid hydrolysis methods to quantify biochar C-pool in a biochar-amended soil, and ii) to calculate the biochar content in the soil through a mass balance derived from the obtained data. Two contrasted biochars from pine wood and corn cob remains were incorporated at a rate of 5 Mg C ha− 1 to a sandy loam vineyard soil with neutral pH and low organic carbon content, in field conditions. The analytical methods used to determine the oxidability and hydrolyzation of soil and biochar-C were: i) weight loss-on-ignition (LOI) at three temperatures (375 °C, 550 °C and 950 °C) for the assessment of organic matter, and ii) dry-combustion (TOC), strong (sO) and mild (mO) acid potassium dichromate oxidations, acid hydrolysis (AH) and peroxide oxidation (PO) for the assessment of organic C-pools. mO mainly estimated the easy oxidisable organic fraction of soil. Resistant organic carbon (ROC), estimated as non-hydrolysable organic carbon by AH and as non-oxidisable by mO, led to similar values in control soil (5 g C kg− 1 soil), whereas different ROC values were obtained in soils amended with biochar (6–12 g C kg− 1 soil). The suitability of these different methods as proxies to quantify biochar C was verified through a mass balance observing differences between them. PO removes well native soil organic matter, but also attacks partially biochar's fraction, so an underestimation exists. However, mO leaves intact biochar in the amended soil. Summarising, LOI, TOC and mO were the best proxies for biochar-C quantification, especially the last one, somewhat clarifying the debate on this topic. © 2017 Elsevier B.V.

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Stabilization of metal(loid)s in two contaminated agricultural soils: Comparing biochar to its non-pyrolysed source material

Trakal L., Raya-Moreno I., Mitchell K., Beesley L. (2017) Stabilization of metal(loid)s in two contaminated agricultural soils: Comparing biochar to its non-pyrolysed source material. Chemosphere. 181: 150-159.
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Doi: 10.1016/j.chemosphere.2017.04.064

Resum:

Two metal(loid) contaminated agricultural soils were amended with grape stalk (wine production by-product)-derived biochar as well as its pre-pyrolysed origin material, to investigate their geochemical impacts on As, Cr, Cu and Zn. Detailed physico-chemical evaluation combined with a column leaching test determined the retention of metal(loid)s from soil solution by each amendments. A pot experiment measured metal(loid)s in soil pore water and their uptake to ryegrass when the amendments were mixed into soils at 1 and 5% (w/w). Total Cr and Zn concentrations were reduced furthest in column leachates by the addition of raw material and biochar respectively, compared to the untreated soil; Cr(III) was the predominant specie initially due to rapid acidification of leachates and organic complexation resulting from raw material addition. Loadings of metal(loid)s to the amendments recovered from the post-leached columns were in the order Cu » Zn > Cr ≈ As. In the pot test ryegrass Cr uptake was initiated by the addition of both amendments, compared to the untreated soil, whereas only biochar addition resulted in significant increases in Zn uptake, explained by its significant enhancement of ryegrass biomass yield, especially at 5% dosage; raw material addition significantly decreased biomass yields. Inconsistent relationships between pore water parameters and ryegrass uptake were common to both soils investigated. Therefore, whilst both amendments modified soil metal(loid) geochemistry, their effects differed fundamentally; in environmental risk management terms these results highlight the need to investigate the detailed geochemical response of contaminated soils to diverse organic amendment additions. © 2017 Elsevier Ltd

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