Plant litter inputs shape microbial residue carbon and metal bioavailability in zinc smelting slag: ¹³C-traced carbon partitioning in a metal-stressed substrate.

Zinc smelting slag is depleted in organic carbon but enriched in potentially toxic metals, creating dual constraints for ecological restoration. However, how plant litter carbon is partitioned among microbial- and plant-derived pools and whether these responses are accompanied by element-specific changes in metal bioavailability remains unclear. Here, we conducted a 90-day decomposition experiment with 13C-labeled ryegrass leaf and root litter in ryegrass-conditioned slag, combining 13C tracing with organic carbon fractionation, phospholipid fatty acid (PLFA) and amino-sugar biomarkers, lignin phenol analysis, metagenomic profiling, and DTPA-extractable metal measurements. Litter-derived carbon was recovered in dissolved, particulate, microbial biomass, and operational mineral-associated organic carbon (MAOC; <53 μm fraction) pools, with leaf litter generally supporting greater microbial incorporation than root litter. Bacterial PLFAs dominated the living microbial biomass profile, whereas fungal-derived residue carbon exceeded bacterial-derived residue carbon, with fungal-to-bacterial ratios of 2.10-2.96. Detectable litter-derived 13C in microbial residues was recovered predominantly in the fungal-derived component, contrasting with the bacteria-dominated living biomass. 13C-labeled lignin phenols provided complementary evidence for short-term retention of litter-derived structural carbon. Amino sugar metabolism-related gene abundances covaried with microbial residue carbon and organic carbon pools. Carbon responses were accompanied by element-specific changes in metal extractability: DTPA-extractable Pb generally decreased, whereas localized increases in DTPA-extractable Cu, Cd, and Zn occurred under leaf litter addition in selected particle-size fractions. Together, these findings indicate early litter carbon retention in both microbial residue and plant-derived structural pools and a potential short-term carbon-metal trade-off relevant to litter-assisted slag restoration.