Sodium as a subsidy in the spring: evidence for a phenology of sodium limitation

Understanding the factors that mediate carbon (C) cycling is increasingly important as anthropogenic activities and climate change alter ecosystems. Decomposition rates mediate C cycling and are in part regulated by sodium (Na) where Na is limiting up to some threshold after which Na becomes stressful and reduces decomposition rates (i.e., the Sodium Subsidy-Stress hypothesis). An overlooked pathway by which decomposers encounter increased salts like NaCl is through plants, which often take up Na in proportion to soil concentrations. Here we tested the hypothesis that Na addition through litter (detritus) and water and their interaction would impact detrital processing and leachate chemistry. Laboratory riparian soil mesocosms received either artificial litter (100% cellulose sponges) soaked in 0.05% NaCl (NaCl_L) or just H₂O (H₂O_L: control) and half of each litter treatment received weekly additions of 150 ml of either 0.05% NaCl water (NaCl_W) or just H₂O (H₂O_W: control). After 8 weeks decomposition was higher in NaCl addition treatments (both NaCl_L and NaCl_W and their combo) than controls (H₂O_L + H₂O_W) but reflected a unimodal relationship where the saltiest treatment (NaCl_L + NaCl_W) was only marginally higher than controls indicating a subsidy-stress response. Previous studies in this system found that Na addition in either water or litter decreased decomposition. However, differences may reflect a phenology of Na demand where Na-limitation increases in the spring (this study). These results indicate that our understanding of how Na impacts detrital processes, C cycling, and aquatic-terrestrial linkages necessitates incorporation of temporal dynamics.