As we appreciate the importance of separating soil organic matter carbon (C) and nitrogen (N) pools in physical fractions distinct for the mechanisms of and controls on formation, persistence and mineralization, we face the challenges of a lack of harmonization in terminology and procedures to separate those fractions, as well as a lack of quantitative data on C and N recoveries from different fractionation methods, and on the spatial-temporal variability of the distribution of C and N across SOM physical fractions. We will discuss these challenges, propose a nomenclature and protocols to harmonize SOM fractionation work, and present some existing data on the spatial-temporal variability of soil organic C and N across physical fractions.
