The Difference of Litter Decay, Litter- and Sediment-Associated Hydrolytic Enzymes between Brackish and Freshwater Tidal Marshes

While an increase in saltwater intrusion into historically freshwater environments will undoubtedly affect ecosystem functioning, the resultant impacts on litter decay and hydrolytic enzyme activities remain poorly understood. Here, we examined the decay of shichito matgrass stem litter, the associated loss of total carbon (TC) and total nitrogen (TN), and the activity of litter- and sediment-associated hydrolytic enzymes in two subtropical tidal marshes (one brackish and one freshwater) in southeastern China. Results obtained after 202 days of incubation revealed that there was no significant difference in litter mass loss between the brackish and freshwater marshes during both the standing phase (aerially decomposing litter) and the late phase of sediment-surface decomposition (slow decomposition of relatively refractory materials) (p > 0.05). The loss of TC and TN from litter was most rapid during the first month of the sediment-surface phase, and the loss rates were significantly greater in the brackish marsh (mass loss 61.63%, TC released 50.55%, and TN released 36.40%) than in the freshwater marsh (mass loss 43.28%, TC released 46.31%, and TN released 41.31%). Moreover, we found significantly higher sediment-associated β-glucosidase activities in brackish marsh sediments during the sediment-surface phase, and it correlated significantly with litter mass loss. The enhanced β-glucosidase activities stimulated decomposition in the early sediment-surface phase in brackish marsh sediments; however, a decrease in decomposition caused by salinity and tidal SO₄²⁻ levels at the last phase ultimately offset the previous difference. Our findings provide insight into the potential consequences of modest saltwater intrusion; these may not alter the rates of litter decomposition, but could change the dominant pathway.