Organ type governs macrophyte decomposition while inundation status drives the decoupling of carbon, nitrogen, and...

Macrophyte-derived organic carbon potentially contributes 66-85% of carbon turnover in lake wetlands. However, the carbon and nutrient fate of its debris decomposition remain elusive, particularly under extreme hydrological regimes. Here, we conducted a 150-day aquatic mesocosm experiment simulating the decomposition of Phragmites australis debris to explore how organ-specific structural traits (leaves, stems, and roots) interact with inundation conditions (Floating, Submerged) to affect debris decomposition and aquatic C release, as well as nutrient dynamics. Our results demonstrate that debris decomposition is associated with organ-specific structural traits rather than inundation conditions. Notably, roots (70.3%-82.7% mass loss) decomposed at a rate comparable to that of leaves (74.2%-80.2% mass loss). Conversely, stems (38.6%-52.4% mass loss), characterized by recalcitrant lignin matrices, degraded at approximately half the foliar rate. Correspondingly, the rapid initial leaching of aboveground tissues generated aquatic carbon peaks within the first 15 days, reaching up to 684 mg/L for dissolved organic carbon and 72 mg/L for particulate organic carbon. Furthermore, inundation conditions drove a stoichiometric decoupling among carbon, nitrogen, and phosphorus release. While the submerged condition significantly decelerated structural carbon losses, it significantly facilitated nitrogen losses and near-complete phosphorus leaching (lost 88.2%-95.3%). These findings elucidate that current models basing aboveground aquatic carbon inputs on foliar decomposition or treating fibrous root pools as generalized, homogeneous carbon pools potentially miscalculate overall debris decomposition and C release in littoral wetlands. Our findings provide an empirical basis for balancing carbon preservation against potential nitrogen and phosphorus leaching risk in lake wetland management.