Salt Pulse Effects on Leaf Litter Decomposition in a Manipulated Stream: The Importance of the Conditioning Status

Anthropogenic salinization constitutes a still understudied growing global threat impacting the biodiversity and the functioning of stream ecosystems. In this study, we evaluated the consequences of salt pulses in a manipulated mountain stream (central Portugal) with a salinized and a reference reach, assessing microbial decomposition of conditioned and nonconditioned chestnut (Castanea sativa) leaves, immediately after a period of NaCl exposure (daily short-term pulses for 7 days; salinization period) and 4 days after salt contamination cessation (recovery period). Conditioned leaves consistently presented higher mass loss, fungal biomass, and respiration rates than nonconditioned leaves. Leaf conditioning status modulated the deleterious effects of salinization, being more pervasive on the decomposition process of conditioned leaves. The depressing effect on mass loss and associated parameters promoted by daily pulses of salt on these leaves was extended after the cessation of salt contamination. This suggests salt legacy effects on already established microbial communities promoted by structural changes and/or mycelial physiological adjustments. In opposition, no effects of salinization were observed in nonconditioned leaves in either period. This may result from a potentially higher salt tolerance of the pioneer species that may also take advantage of the low basal salt concentration between salt pulses and/or of further incorporation of fungal species provided from upstream. Such continuous fungal imprint may result in redundant dissimilar (salinization period) or similar (recovery period) fungal decomposing communities able to determine balanced mass loss between sides in each period. Globally, results point to the importance of considering leaf litter quality and salt exposure timing in relation to leaf litter pulses when evaluating the consequences and delineating protection measures for streams facing discrete salt contamination.