Can secular stable soil organic carbon be isolated? An assessment of Zimmermann fractionation using a long-term bare fallow.

The “42 plots of Versailles” site is a long-term bare fallow established in 1928. Over the course of almost 100 years, these plots have been carefully maintained without vegetation and enriched annually with various fertilizers and amendments (16 types x 2 replicate plots + 10 control plots), and sampled throughout the period. In particular, the control plots offer the opportunity to use elemental analysis to monitor the kinetics of soil organic carbon (SOC) evolution in the absence of carbon inputs.

Zimmermann fractionation (Zimmermann et al., 2007, DOI : 10.1111/j.1365-2389.2006.00855.x) is a granulo-densimetric separation protocol that separates SOC into 5 fractions: dissolved organic carbon (DOC), coarse particles > 63 µm consisting of particulate organic matter (POM) and “heavy” coarse matter containing sand and aggregates (S+A), and fine particles < 63 µm consisting of oxidation-sensitive fine fraction (sSOC) and oxidation-resistant organic matter (rSOC).

These 5 fractions are expected to have distinct mean residence times; in particular, the rSOC fraction is seen as a stable fraction, with aged carbon whose quantity changes little or not at all over time; conversely, the POM fraction is composed of very labile carbon. When applying this fractionation to control samples from the 42 plots at different times, we therefore expect to see strong variations in the size of the labile compartments, and on the contrary very little variation in the stable compartments.

In this study, we compare the results of this fractionation on 5 control plots at various dates (notably at the start of the experiment in 1929; at the present in 2021, which is the latest sampling date; and intermediate dates), in order to verify whether Zimmermann fractionation is indeed capable of separating SOC fractions with very distinct kinetics.