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.

Automated summary

Freshwater salinization is a growing threat that affects aquatic systems worldwide. Human activities contribute to the contamination of streams and rivers, resulting in chronic or pulsed salt additions. This study compared the effects of continuous and pulsed salt addition on microbial-mediated litter decomposition and associated microbial endpoints. The results showed that chronic salt exposure had a greater impact on microbial activity, reproductive outputs, and fungal richness than pulsed inputs, despite no effect on fungal biomass. The findings suggest that the effective salt concentration to alter fungal activity may be higher in pulsed salinization.