Rehabilitating degraded calcareous soil into a metal repository: A focus on particulate organic matter storage

Soil particulate organic matter (POM) is crucial for regulating trace metal dynamics by acting as either a sink or source. This study examined cadmium (Cd) storage pathways in POM within calcareous soil that has been degraded by over 50 years of wastewater irrigation. We conducted a microcosm experiment combined with soil incubation and crop cultivation to examine the effects of manure application (2.5 %, 5 %, and 10 %) coupled with recalcification (25 %, 50 %, and 100 % of soil CaCO₃ deficit relative to its background level). The results showed that the POM mass percentage and Cd mass load in POM increased as POM size decreased, with 56.93 % of Cd (on average) concentrated in the smallest fraction (0.053–0.25 mm). Manure application increased the absolute mass of all POM sizes (−3.53 % to 45.14 %), although their mass percentages remained stable. Critically, 5 % and 10 % manure significantly elevated the Cd enrichment factor in POMs from <1 (control: 0.39–0.92) to 4.81–12.23 and 2.86–5.24, respectively. However, higher recalcification (50 % and 100 %) diminished manure-induced enrichment. The combination of 5 % manure with low recalcification (25 %) resulted in the lowest Cd content in wheat grains. Soil carbon component and redundancy analyses indicated that Cd enrichment in POM was favored by an organic carbon content >20 g kg⁻¹ and an inorganic/organic carbon ratio < 1. These findings demonstrate that applying 5 % manure coupled with 25 % recalcification is optimal for utilizing POM as a metal repository in degraded calcareous soils.