Mineral Associated Organic Matter (MAOM) and Particulate Organic Matter (POM) Size Fractionation Protocol v1

Purpose: The aim of this protocol is to fractionate soils based on particle size to recover mineral-associated organic matter (MAOM) and particulate organic matter (POM) fractions. In the MAOM/POM fractionation procedure, soils are dispersed in sodium hexametaphosphate (HMP) to disrupt microaggregates, and subsequently separated by size into sand (> 53 um) and silt + clay (<53 um) fractions. This is a simple size-based fractionation approach to isolate particulate organic matter (POM) and mineral-associated organic matter (MAOM) fractions, recommended in a comprehensive methods review by Poeplau et al. (2018) and adapted from Sanderman et al. (2013). Depending on your interests in the MAOM fraction, an additional density or chemical fractionation step may be paired with this approach to further isolate putatively stable pools of SOM (see Poeplau et al., 2018 and data on corresponding turnover times associated with different fractions). Studying these fractions is particularly exciting because they represent two contrasting but complementary aspects of soil carbon dynamics. POM, consisting of plant and microbial residues, is a highly active pool that turns over rapidly and provides a direct window into how fresh carbon inputs are cycled through the soil. By contrast, MAOM forms when organic compounds bind tightly to mineral surfaces, creating one of the most stable long-term carbon reservoirs on Earth. Together, they capture both the dynamic and the persistent sides of soil organic matter: the short-term energy fueling microbial processes and the long-term storage that underpins soil’s role in climate regulation.