Density fractions versus size separates: does physical fractionation isolate functional soil compartments?

C. Moni,B. Zeller,M. Kleber,D. Derrien,P.-J. Hatton

doi:10.5194/bg-9-5181-2012

Abstract

Abstract. Physical fractionation is a widely used methodology to study soil organic matter (SOM) dynamics, but concerns have been raised that the available fractionation methods do not well describe functional SOM pools. In this study we explore whether physical fractionation techniques isolate soil compartments in a meaningful and functionally relevant way for the investigation of litter-derived nitrogen dynamics at the decadal timescale. We do so by performing aggregate density fractionation (ADF) and particle size-density fractionation (PSDF) on mineral soil samples from two European beech forests a decade after application of 15N labelled litter. Both density and size-based fractionation methods suggested that litter-derived nitrogen became increasingly associated with the mineral phase as decomposition progressed, within aggregates and onto mineral surfaces. However, scientists investigating specific aspects of litter-derived nitrogen dynamics are pointed towards ADF when adsorption and aggregation processes are of interest, whereas PSDF is the superior tool to research the fate of particulate organic matter (POM). Some methodological caveats were observed mainly for the PSDF procedure, the most important one being that fine fractions isolated after sonication can not be linked to any defined decomposition pathway or protective mechanism. This also implies that historical assumptions about the "adsorbed" state of carbon associated with fine fractions need to be re-evaluated. Finally, this work demonstrates that establishing a comprehensive picture of whole soil OM dynamics requires a combination of both methodologies and we offer a suggestion for an efficient combination of the density and size-based approaches.

Highlights

Physical fractionation methods have been frequently employed in soil research because they can isolate such subsets of soil materials that are important to assess the accessibility of soil organic matter (SOM) and the bioavailability or sequestration of limiting nutriments such as nitrogen (Balesdent, 1996; Gregorich et al, 2006; von Lutzow et al, 2007)
We set out to test whether physical fractionation would allow us to identify functional subunits of the soil fabric, regarding litter-derived N dynamics
We further showed that both fractionation procedures yielded complementary information about N dynamics

Summary

Introduction

Physical fractionation methods have been frequently employed in soil research because they can isolate such subsets of soil materials that are important to assess the accessibility of soil organic matter (SOM) and the bioavailability or sequestration of limiting nutriments such as nitrogen (Balesdent, 1996; Gregorich et al, 2006; von Lutzow et al, 2007). All physical fractionation protocols involve various degrees of soil dispersion, followed by density and/or size separation to isolate pools of SOM based on their size and degree of organo-mineral interaction (Torn et al, 2009). Depending on the extent of force used, dispersion generates two types of products: individual particles and complex aggregates.

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Discussion

Conclusion