Nonlinear loss of recalcitrant organic carbon in saltmarsh sediments following Spartina alterniflora eradication: Evidence from microbial metabolic shifts

Large-scale eradication of the invasive Spartina alterniflora on China’s coasts is implemented to restore wetland ecosystems, yet it introduces a critical yet overlooked environmental trade-off: massive plant residue input may destabilize sediment organic carbon (SOC), the core part of coastal blue carbon. While carbon loss from biomass removal is recognized, the impact on the recalcitrant organic carbon (ROC) pool remains unknown. Here, we conducted field measurement in a subtropical mangrove wetland in Southeast China with a 60-day incubation experiment across five plant residue-addition gradients, using 13 C isotope tracing and metagenomics to quantify ROC loss and to explore its microbial mechanisms. Field observations post-eradication showed the ROC to SOC ratio decreased by 11%–19%. Incubation experiments provided the direct evidence of nonlinear, dose-dependent ROC loss, with low-dose inputs inducing disproportionately greater loss. All residue addition treatments showed net ROC loss (12%–383%), whereas only the lowest addition (0.5 kg dry residue m −2 ) caused net SOC loss (63 %) compared with control group. Metagenomic analysis further showed that the lower-dose treatment (1 kg m −2 ) was enriched in MAGs with broader predicted capacities for complex-carbon utilization, whereas the high-dose treatment (4 kg m −2 ) showed greater potential for plant-derived carbohydrate utilization. This study establishes a mechanistic link between invasive plant eradication and nonlinear ROC loss, providing a critical perspective for optimizing coastal blue carbon management during ecological restoration.