Too much of a good thing: Evidence of sodium stress in an inland subtropical riparian detrital system

Global salinization is impacting both terrestrial and freshwater systems. Riparia link these systems and are likely impacted by increased sodium, but impacts are understudied. The sodium ecosystem respiration hypothesis posits that increased sodium under sodium-limitation should stimulate decomposition. We tested this hypothesis in an inland subtropical riparian system by adding 0.067% NaCl every 2 weeks to 1 m2 plots (n = 10) or just H2O (n = 10) and measuring decomposition rates of red maple litter across 9 months. Additionally, we measured leaf sodium content of 4 riparian plant species. Contrary to our predictions, decomposition was 21% slower in sodium addition plots than in controls, but invertebrate decomposer communities did not differ between treatments. Plants had 1.4- to 4-fold higher leaf sodium content after 9 months, but soil sodium content did not differ. This is the first evidence of sodium stress for mesic inland terrestrial ecosystems. Slower decomposition supports the Sodium Subsidy-Stress hypothesis and extends the sodium ecosystem respiration to include sodium-stress impacts on ecosystem functions when sodium occurs in excess of optimal sodium levels. Changes to decomposition and leaf sodium content in riparia demonstrate sodium can impact both green and brown food webs and ultimately affect exchange across terrestrial-aquatic boundaries. •Sodium Subsidy-Stress hypothesis predicts reduced function with scarce or excess Na.•Low-level NaCl addition in riparia slowed decomposition rates by 21% after 9 months.•Added NaCl did not alter detrital community structure, suggesting microbial stress.•NaCl addition did not change soil chemistry but increased riparian plant Na content.•Na-driven changes in decomposition and plant chemistry could change riparian C flux.