Do fungi need salt licks? No evidence for fungal contribution to the Sodium Ecosystem Respiration Hypothesis based on lab and field experiments in Southern Ecuador

Though typically discussed in the context of salinity, a shortage of sodium in tropical forests has been demonstrated in recent years, inhibiting soil biota with impacts on decomposition processes (Sodium Ecosystem Respiration hypothesis). So far, sodium limitation has been shown for soil fauna, whereas a potential role in saprotrophic fungi remains unknown. We tested the hypothesis that fungi have sodium demands at low levels resulting in reduced activity in tropical soils, analyzing fungal responses to sodium availability. Despite careful experimentation and the usage of different fungal strains we did not find evidence for fungal sodium demands using controlled nutrient media, not even in the absence of potassium. Likewise, there was no significant fungal response to sodium additions in litter samples, though decomposition rates were positively affected. These findings suggest that sodium is not essential for osmoregulation of tested fungal strains, which are hence not affected by sodium limitation.