A Procedure for Isolating Soil Organic Matter Fractions Suitable for Modeling

Abstract

Fractions of soil organic matter (SOM) were obtained from three soils using alternative physical fractionation procedures, and evaluated against the requirements of model pools. We compared two‐stage density fractionation (isolating free and intra‐aggregate fractions, before and after dispersion, respectively) with particle‐size separation of dispersed soil. For full comparison, the organomineral fraction residual from density fractionation was also size separated. In standardizing the density‐based method, we found recovery of intra‐aggregate organic matter highly sensitive to separation density as compared with the free. Recovery of the intra‐aggregate was also influenced by dispersion energy. The greatest amount was obtained using a combination of the highest density (1.80 g cm−3) and dispersion energy (1500 J g−1). Analysis by 13C nuclear magnetic resonance (NMR) showed O‐alkyl/alkyl‐C ratios 1.38 to 2.30 times greater in intra‐aggregate organic matter than in the free. Diffuse reflectance Fourier transform infrared spectroscopy (DRIFT) also indicated a greater proportion of aliphatic hydrocarbon, carboxylic anions, and aromatic C in intra‐aggregate organic matter. The findings suggest this fraction comprises more decomposed and transformed organic matter relative to the free. Higher signal/noise ratios in NMR spectra of particle‐size fractions (compared with their organomineral equivalents) were attributed to C in particulate SOM, not removed by prior density separation. Whilst particle‐size fractions confuse particulate SOM with that attached to mineral surfaces, fractions isolated by two‐stage density separation are small in number and display distinct chemical properties. We suggest they provide a sound basis for a model of SOM turnover based on measurable pools.

Organic Matter Fractions in a Quartzipsamment Under Cultivation of Irrigated Mango in the Lower São Francisco Valley Region, Brazil

“…After that, the samples were centrifuged at 630 g for 15 min and the supernatant was passed through a sieve with 100 mesh (0.149 mm) for collection of the light organic matter fraction (LF). As the procedure adopted involves breakup of the aggregates through continuous shaking, before separation, the isolated LF is constituted by the inter-and intra-aggregate compartment (Sohi et al, 2001). The organic matter associated with the soil mineral components and that settled to the bottom of the centrifuge tube was considered the heavy fraction (WF).…”

Eficiência De Soluções Densas No Fracionamento Físico Da Matéria Orgânica Do Solo

“…Embora o uso corrente do NaI e do PTS no fracionamento físico densimétrico (Sohi et al, 2001;Freixo et al, 2002b;Diekow et al, 2005;Basile-Doelsch et al, 2009;Hofmockel et al, 2011;Potes et al, 2012), poucos estudos têm testado a eficiência de soluções e densidades elaboradas com o uso desses produtos quanto à quantificação das frações da MO. Golchin et al (1994b) preconizaram o uso de solução de PTS 1,6 kg dm -3 , tendo obtidos resultados com predominância da FLL sobre a FLO.…”

“…Entretanto, o uso de PTS 1,8 e 2,0 kg dm -3 resultou em aumento do rendimento de C da FLO, sendo superior ao rendimento obtido para a FLL nos estudos de Golchin et al (1994a). Sohi et al (2001), avaliando o efeito da densidade da solução no rendimento de C, propuseram a utilização de densidade de 1,8 kg dm -3 como ideal para a solução de NaI. Freixo et al (2002a,b), utilizando NaI 1,8 kg dm -3 , obtiveram rendimento de C na FLO inferiores aos obtidos para a FLL, similar ao ocorrido em estudos desenvolvidos por Roscoe et al (2001).…”

Soil fertility, organic carbon and fractions of the organic matter at different distances from eucalyptus stumps

“…The free light fraction (FLF) of the organic matter was separated by means of soil flotation in NaI (1.8 kg L -1 ) (Sohi et al, 2001); the C concentration in the fraction was determined by dicromatometry. The Table 1 soil humic substances were fractioned based on their differential solubility in alkaline or acidic medium according to Swift (1996), resulting in fulvic acid fraction (FAF), humic acid fraction (HAF), and humin fractions (HF).…”