Effects of salinity on leaf breakdown: Dryland salinity versus salinity from a coalmine

Abstract

Salinization of freshwater ecosystems as a result of human activities represents a global threat for ecosystems' integrity. Whether different sources of salinity with their differing ionic compositions lead to variable effects in ecosystem functioning is unknown. Therefore, the present study assessed the impact of dryland- (50μS/cm to 11,000μS/cm) and coalmine-induced (100μS/cm to 2400μS/cm) salinization on the leaf litter breakdown, with focus on microorganisms as main decomposer, in two catchments in New South Wales, Australia. The breakdown of Eucalyptus camaldulensis leaves decreased with increasing salinity by up to a factor of three. Coalmine salinity, which is characterised by a higher share of bicarbonates, had a slightly but consistently higher breakdown rate at a given salinity relative to dryland salinity, which is characterised by ionic proportions similar to sea water. Complementary laboratory experiments supported the stimulatory impact of sodium bicarbonates on leaf breakdown when compared to sodium chloride or artificial sea salt. Furthermore, microbial inoculum from a high salinity site (11,000μS/cm) yielded lower leaf breakdown at lower salinity relative to inoculum from a low salinity site (50μS/cm). Conversely, inoculum from the high salinity site was less sensitive towards increasing salinity levels relative to inoculum from the low salinity site. The effects of the different inoculum were the same regardless of salt source (sodium bicarbonate, sodium chloride and artificial sea salt). Finally, the microorganism-mediated leaf litter breakdown was most efficient at intermediate salinity levels (≈500μS/cm). The present study thus points to severe implications of increasing salinity intensities on the ecosystem function of leaf litter breakdown, while the underlying processes need further scrutiny.

Multiple riparian–stream connections are predicted to change in response to salinization

“…We review how elevated common salt concentrations could change key detrital linkages in and across the riparian-stream interface, although most pathways have not been tested and even fewer linkages among pathways have been established. Three main pathways (figure 1) predicted to change with salinization are: pathway 1 (PW1) salt runoff changes microbial decomposer [41,62] and macroinvertebrate detritivore [50,69] and algae [70] performance that could lead to changes in the composition and processing rates of detrital pools; pathway 2 (PW2) riparian plant salt uptake [71], altered litter chemistry [72] and litterfall for riparian and aquatic detritivores and their subsequent enrichment (e.g. senesced leaves, insects or corpses) could stimulate decomposition rates and production of DOM and FPOM; and pathway 3 (PW3) direct consumption of salts in salinized soils could increase riparian detritivore growth [51], decomposition and DOM production [73].…”

“…Mounting evidence suggests that even relatively low-level increases at similar concentrations across different salt types can alter macroinvertebrate performance and has even been measured as changes in macroinvertebrate communities and associated traits [114,115]. Therefore, low-level salt increases to relatively low concentrations not only directly affect macroinvertebrate community structure indicative of impaired aquatic life [116,117], but rises could also lead to altered detrital processing [50,98,118].…”

“…[69,[119][120][121][122][123]), and/or by measuring salts in stream observational field studies (e.g. [14,50,114,124,125]). These studies demonstrate four principle results for how salinization could alter detrital processing.…”

“…leaves, wood and FPOM) is often the dominant energy source for aquatic detritivores. These same detritivores are sensitive to oxygen and ion changes; thus, detrital-based ecosystems are predicted to exhibit measurable changes from salinization [50,[61][62][63]]. If increased low-level salinization changes the quantity and quality of detritus by reducing fungal and bacterial enzymatic activity and production, or by changing microbial community identity, then changes would probably occur in the macroinvertebrate detritivore consumption and assimilation [64].…”

Are fungal strains from salinized streams adapted to salt-rich conditions?

“…Considering the importance of resource quality for conidia settlement [65], intensity of fungal interactions [39], mycelial invasiveness [66] and degradative potential and growth [3], it seems reasonable to assume that the rates of the proposed strategies might be dependent on the traits (stoichiometry included; [67]) of the available leaf material provided by the riparian areas. This seems particularly relevant for fungal communities from salinized streams that may have lower assimilation capacity of phosphorus from the environment [21,68]; in this case, leaf litter quality will likely have a crucial role in compensating for such limitation and guaranteeing mycelial growth [67,69].…”