Pulsed vs. chronic salinization effects on microbial-mediated leaf litter decomposition in fresh waters

Freshwater salinization is a growing worldwide threat affecting the structure and functioning of aquatic systems. Increased salinity frequently occurs as result of human activities that determine chronic or, less commonly studied, pulsed contamination of streams and rivers. Here we compared the effects of both patterns of salt addition (i.e. continuous vs. 2 consecutive days/week exposure), within the same ionic concentrations (1, 4 or 6 g l(-1) NaCl), on microbial-mediated litter decomposition and associated microbial endpoints. Mass loss of oak leaf litter was consistently depressed by salinization with stronger effects at the highest concentrations, despite the pattern of salt addition. At higher (>= 4 g l(-1)) salt concentrations, chronic salt exposure was more deleterious than pulsed inputs to microbial activity, reproductive outputs and fungal richness, despite no effects on fungal biomass. Overall, results suggest that the effective salt concentration to alter fungal activity should be greater in pulsed (vs. chronic) salinization, as intervals between salt pulses seem to facilitate a total or partial recovery of microbial functions. We suggest that, along with the concentration, attention should be devoted to the dynamics of salt contamination when analyzing the consequences of salinization in streams.