Potential effects of tidal flat variations on decomposition and nutrient dynamics of Phragmites australis, Suaeda salsa and Suaeda glauca litter in newly created marshes of the Yellow River estuary, China

Salinity gradient and tidal flooding are two key factors in the newly created marshes of the Yellow River estuary as they are closely correlated with the accretion or erosion of tidal flat. To investigate the potential effects of tidal flat variations on decomposition and nutrient dynamics of Phragmites australis, Suaeda salsa and Suaeda glauca litters in the Yellow River estuary, the natural salinity and flooding gradients in the newly created tidal flat [high marsh (river bank), middle marsh and low marsh] were applied to simulate the variations of salinity and tidal flooding in the three typical halophyte communities. Results showed that the mass losses and decomposition rates of the three halophyte litters differed significantly along salinity and flooding gradients (p < 0.05) and high decomposition rates were generally observed in high salinity and moisture. The responses of halophyte decomposition to salinity and flooding gradients also significantly differed among species (p < 0.05). In high or middle salinity and moisture, the decomposition rates of different halophytes presented P. australis > S. glauca > S. salsa, while in low salinity and moisture, the order demonstrated S. glauca > S. salsa > P. australis. The C and N concentrations in the three halophytes along salinity and flooding gradients showed different variations, and, with the increasing of salinity and moisture, there was a tendency that the N in P. australis and S. salsa shifted from accumulation to release while those in S. glauca demonstrated strong release at all times. This study also indicated that salinity, tidal inundation and C/N ratios were three important factors affecting the differences in decomposition rates and C and N release patterns of the three halophytes. If the salinity and flooding frequency in P. australis and S. salsa communities were greatly increased, the decomposition rates would increase by 61.88-205.48% and 103.93%, and the C release after 571 days would increase by 40.23-83.49% and 30.12%, respectively; if the salinity and flooding frequency in S. glauca community were significantly decreased, the decomposition rates would decrease by 42.79-44.78% and the C release would decrease by 23.78-27.93%. Differently, if the salinity and flooding frequency in S. glauca community were greatly decreased, the N release would at least decrease by 67.46%; if the salinity and flooding frequency in P. australis and S. salsa communities were significantly increased, the N release only would occur in high salinity and flooding frequency. In recent years, the coastal marsh in the Yellow River estuary has shifted from erosion to accretion with the long-term implementation of "Flow-sediment Regulation Project", and our results provided valuable information for predicting the possible influences of tidal flat variations on decomposition and nutrient dynamics of the three halophytes in future. (C) 2016 Elsevier B.V. All rights reserved.