Abstract: Standing litter is an important stage of litter decomposition and a key component of carbon cycle in wetland ecosystems. Dissolved organic carbon (DOC) is the product of litter decomposition, playing a crucial role in ecosystem carbon cycling. Most previous studies have focused on the concentration and quality of DOC in leaf litter decomposed on soil surface. However, the DOC concentration and quality of leaf and non-leaf organs (e.g., sheath and culm) in standing litter are largely unknown. In this study, the concentration and quality of DOC in litter (i.e., leaf, culm, and  sheath) of Phragmites australis were measured in the air and on the soil surface in the Yellow River Delta, China. The results showed that: (1) the concentration of DOC in standing litter was higher than or equal to that of litter on soil surface, with the highest DOC concentration of standing litter being 1.64 times of that on soil surface. The E₂/E₃ ratios of DOC in standing litter were higher than that of litter on soil surface, whereas the C∶C ratios of DOC in standing litter were lower than those on the soil surface. (2) The concentration of DOC in leaf litter was higher than that in nonleaf litter, which was 1.28-2.49 and 1.02-1.57 times of those in stem and sheath litter, respectively. Nevertheless, the SUVA₂₅₄ values and C∶C ratios of DOC in leaf litter were greater than those in non-leaf litter, whereas the E₂/E₃ ratios of DOC in leaf litter were lower than those in non-leaf litter. Taken together, standing litter of Phragmites australis had higher concentration and quality of DOC than litters on the soil surface. Furthermore, leaf litter had higher DOC concentration and lower DOC quality than non-leaf litter. Our results highlight that standing litter, especially non-leaf litter, plays a non-negligible role in litter decomposition. More attention should be paid to the decomposition of standing leaf and non-leaf litter in order to deeply understand carbon cycling of coastal wetlands, which is of great significance to accurately evaluate carbon budget of coastal wetlands and its impact on global climate change.

Key words: standing litter, dissolved organic carbon, Phragmites australis, coastal wetland, Yellow River Delta.