Abstract
Physical fractionation methods used in soil organic matter (SOM) research commonly include density-based procedures with heavy liquids to separate SOM pools with varying turnover rates and functions. Once separated, the heavy SOM pools are often thoroughly rinsed with water to wash off any residues of the heavy liquids. Using four soils with contrasting properties, we investigated the effects of using either sodium polytungstate (SPT) or sodium iodide (NaI), two of the most commonly used heavy liquids, on the distribution of organic carbon (C) and total nitrogen (N) in free light, intra-aggregate light, and mineral-associated heavy SOM pools isolated by a common fractionation scheme. We also determined the effects of washing the mineral-associated heavy SOM fractions on the recovery of organic C and total N after separation. Because of its smaller viscosity compared to that of NaI, SPT consistently yielded greater intra-aggregate and smaller mineral-associated soil organic C contents. We also confirm that some commercial SPT products, such as the one used here, can contaminate organo-mineral heavy pools with N during density-based fractionation procedures. We do not recommend the repeated washing of heavy fractions separated with Na-based heavy liquids, as this can mobilize SOM.
Highlights
Soil organic matter (SOM) is key to terrestrial ecosystems, contributing to the support of natural vegetation and agricultural production, filtering and holding water, and storing carbon (C)[1,2]
This scheme is intended to separate three fractions: a free light soil organic matter (SOM) fraction not physically disconnected from microorganisms and enzymes; a occluded light SOM fraction located within aggregates, which forms a physical barrier that limits O2 diffusion and the accessibility of microorganisms and enzymes; and a heavy fraction consisting of SOM intimately associated with minerals, which decreases microbial and enzymatic capacity to decompose organic substrates[10]
The free SOM fraction is isolated by an initial density separation, and the intra-aggregate SOM is separated from the mineral-associated pool by a second density separation after ultrasonic disruption of stable aggregates[10]
Summary
Soil organic matter (SOM) is key to terrestrial ecosystems, contributing to the support of natural vegetation and agricultural production, filtering and holding water, and storing carbon (C)[1,2]. Among the large number of physical fractionation methods available in the literature (see Poeplau et al.[9] for a recent comparative description of many of them), the density-based separation scheme developed by Golchin et al.[10] is one of the most commonly used and has constituted the basis for the development of other fractionation methods[6,11,12,13,14,15,16,17] This scheme is intended to separate three fractions: a free light SOM fraction not physically disconnected from microorganisms and enzymes; a occluded light SOM fraction located within aggregates, which forms a physical barrier that limits O2 diffusion and the accessibility of microorganisms and enzymes; and a heavy fraction consisting of SOM intimately associated with minerals, which decreases microbial and enzymatic capacity to decompose organic substrates[10]. The final purpose of this study was to provide general recommendations to prevent misinterpretation of fractionation results in SOM research
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