Magnetic separation reveals overestimation of soil organic matter due to undecomposed particulate residues

Abstract. Soil organic matter (SOM) is a complex mixture of organic compounds derived from the decomposition of plant and animal residues. Only after undergoing microbial transformation and forming stable associations with minerals can it be considered “true” SOM, rather than a simple mechanical accumulation of carbon-containing substances. According to current understanding, particulate organic matter (POM) comprises both undecomposed and partially decomposed organic residues. Of these, the undecomposed fraction does not qualify as SOM in the strict sense. However, conventional analytical methods cannot fully distinguish fine particulate residues from soil matrices, leading to an overestimation of POM-derived carbon content. The extent and persistence of this “false increase” in SOM due to “disguised” POM remains poorly understood. In this study, straw and biochar were magnetized via chemical co-precipitation and applied to soils. The incompletely decomposed magnetized residues in the soil were separated using magnetic separation at various time points, enabling more accurate tracking of SOM dynamics. Five treatments were established: blank control (CK), untreated straw (CS), untreated biochar with carbon input equivalent to untreated straw (Bc), magnetized straw with carbon input equivalent to untreated straw (MCS), and magnetized biochar with carbon input equal to untreated straw (MBc). The results showed that after the application of organic materials into the soil, the recovery rate of magnetized straw residues declined continuously, reaching 54.55 % after 360 d, whereas biochar remained largely stable at 92.48 %. In CS and Bc treatments, the organic carbon content of POM fractions and their proportion in the total SOM were consistently higher than in CK, particularly during early incubation. However, this was attributable to overestimation from incompletely decomposed residues. In contrast, MCS-D and MBc-D treatments (after magnetic residue removal) showed minimal deviation from CK, confirming the contribution of incomplete decomposition to SOM overestimation. On day 30, the apparent increase in the particulate organic carbon (POC) content reached 63.48 % for CS and 58.99 % for Bc. Over time, the overestimation in the CS treatment declined to 15.34 % by day 360, whereas the overestimation in the Bc treatment remained largely unchanged, with a 53.71 % increase persisting. These findings highlight the potential for SOM overestimation when POM fractions are fully included without accounting for undecomposed inputs, particularly the long-term persistence of recalcitrant organic materials, which may introduce systematic bias in global SOM quantification.