Microbial communities and litter quality co-mediate seagrass litter decomposition under eutrophication

The decomposition of recalcitrant organic matter (ROM) in seagrass litter plays a crucial role in mediating blue carbon magnitude. However, the mechanisms underlying ROM decomposition in seagrass litter under eutrophication, driven by anthropogenic pressures, remain poorly understood. Here, we investigated the effects of eutrophication on the decomposition of two types of seagrass litter (leaf and sheath) through laboratory experiments adding nitrogen and macroalgae. We found that macroalgae addition inhibited ROM breakdown in leaves and sheaths. Nitrogen addition enhanced ROM decomposition in leaves by 4% while suppressing it in sheaths compared to the treatments without nitrogen addition. Additionally, the combined addition of nitrogen and macroalgae led to a more substantial loss of sheath ROM (35-39%) compared with macroalgae alone. The opposite response of ROM breakdown in leaf and sheath under nitrogen addition were co-regulated by litter quality and bacteria taxa (i. e. , Flavobacteriales, Actinomarinales, and Phycisphaerales). In the presence of macroalgae, fungi, including Lecythophora luteoviridis, Tyrannosorus hystrioides, and Pseudeurotiumₒvale, were found to play a significant role in the decomposition of leaf ROM, but not in sheath decomposition. These findings highlight the negative effects of eutrophication on litter ROM sequestration and the important role that litter quality and microbe play. Combined addition of macroalgae and nitrogen enhanced decomposition of recalcitrant organic matter in sheath, whereas it was meditated by litter quality and microbes, according to laboratory experiments spanning 116 days with two seagrass litters.