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

Abstract

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 (NaClL) or just H2O (H2OL: control) and half of each litter treatment received weekly additions of 150 ml of either 0.05% NaCl water (NaClW) or just H2O (H2OW: control). After 8 weeks decomposition was higher in NaCl addition treatments (both NaClL and NaClW and their combo) than controls (H2OL + H2OW) but reflected a unimodal relationship where the saltiest treatment (NaClL + NaClW) 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.

Low sodium availability in hydroponically manipulated host plants promotes cannibalism in a lepidopteran herbivore

“…We manipulated tissue sodium of common sunflowers ( Helianthus annuus, Asteraceae) to test whether plant sodium concentration would influence an herbivore's cannibalistic behaviour and survival. We hypothesized that: (1) cannibalism would be higher at lower plant sodium concentrations, owing to sodium deficiency at low dietary levels; and (2) larval survival would suffer under plant sodium extremes, owing to physiological stress at the very lowest and highest levels, following a subsidy-stress response 12 , 47 . To test these predictions, we used the bordered patch butterfly, Chlosyne lacinia (Lepidoptera: Nymphalidae), a highly variable species with an extensive geographic range from Oklahoma, USA, to northern Argentina 48 – 52 (Fig.…”