Contrasting properties of soil organic matter fractions isolated by different physical separation methodologies

Abstract

Physical soil organic matter (SOM) fractions provide increased insight into the biogeochemical functioning of soils. Several fractionation methodologies have been developed to separate particulate (POM) from mineral-associated organic matter (MAOM) fractions either by particle size, particle density, or some combination of these two properties. The proliferation of approaches to separation has led to ambiguity regarding what these methodologically defined fractions should be understood to conceptually represent, hindering robust data synthesis analyses and model development. Here we attempt to identify chemical similarities and differences amongst POM and MAOM fractions separated by different physical approaches, with the aim of guiding fractionation choices for future research. We obtained soils from 11 farms across the United States with variable parent materials, textures, and pHs, and fractionated them using 4 methods. We tested two single-step methods: a size fractionation (53 µm cutoff) and a density separation (1.85 g cm−3 cutoff), and two multi-step methods: a combined size and density separation after full dispersion, which isolates a third pool of organic matter associated with coarse, heavy particles (coarse heavy associated organic matter; CHAOM), and a combined method with delayed dispersion to separate the free POM from the occluded POM + CHAOM fraction. We analyzed all fractions for C and N concentrations, their isotopic composition, and for their chemical composition via mid-infrared spectroscopy. We found that MAOM fractions tended to be very homogenous in character regardless of fractionation schemes, while POM varied widely. In particular, to POM isolated by density floatation was distinct in both isotopic and spectroscopic signature compared to POM isolated by size fractionation alone. Indeed, our results indicated that POM isolated by size is a composite fraction of light POM and CHAOM, and does not map well onto a conceptual pool. We found the CHAOM fraction to be more similar to MAOM in terms of chemical and spectroscopic composition, indicating that one-step density separations may be the most effective means of isolating contrasting SOM pools in a time and cost-efficient matter. However, our analysis highlights that POM, CHAOM and MAOM exhibit distinct features, and the three fractions are worth separating via a combined size-density fractionation when possible.