Abstract: Litter decomposition plays a vital role in governing ecological processes of wetland ecosystems. To investigate the variations of litter quality and nutrients during decomposition varying with tidal regime, we conducted manipulative experiments with different species of Suaeda salsa and Spartina alterniflora, at four sites along a tidal gradient, to test the decomposition rates, by putting the litter of each plant into the decomposition bags. Our results showed that losses of litter mass were related to litter species, soil salinity, soil water content and tidal disturbance, revealing that the tidal disturbance can be an important force to influence the retention and decomposition. We found that S. alterniflora litter decomposed faster than S. salsa litter at the sites with strong tidal disturbance, but for the sites with weak tidal disturbance, a converse trend was found that S. salsa decomposed faster than the S. alterniflora. The decomposition rates of them ranged from 0.00134 d^-1 to 0.00234 d^-1. We also found that the decomposition varies with time. After 270 days of ending experiments, the mass losses of S. alterniflora and S. salsa litter exceeded 30%, showing that the losses at the sites with stronger tidal disturbance were larger than those at sites with weak tidal disturbance. The variations of N and C/N of S. salsa and S. alterniflora litter differed significantly among sites (N: F₇₁=3.280, P<0.05; C/N: F₇₁=3.571, P<0.05), expect for C content (F₇₁=0.856, P>0.05). The average contents of C, N and C/N of S. salsa litter were 40.81±1.15%, 0.51±0.08% and 82.22±13.18, respectively, while the average contents of C, N and C/N of S. alterniflora litter were 31.72±2.37%, 0.39±0.05% and 82.59±14.91, respectively. Both litters showed a net release of carbon and nitrogen. The average releasing amounts of C and N were 36.9% and 55.8%, respectively in S. salsa litter and the average release amount of C and N were 53.1% and 47.1%, respectively in S. alterniflora litter. The net releasing amount of carbon at sites with strong tidal disturbance was larger than that of sites with weak tidal disturbance. Our result suggests that decomposition of coastal vegetation litter provided nutrient element (e.g. C and N) for coastal wetland ecosystems, in the Yellow River Delta. Therefore, our study highlights that tidal regime should be regarded as an important environmental factor in regulating biogeochemical cycles and carbon accumulation, consequently altering the ecosystem functioning.

Key words: tidal gradient, litter decomposition, carbon and nitrogen, Yellow River Estuary