Short communicationFirst evidence that the sodium ecosystem respiration (SER) hypothesis may also hold for a coastal tropical rainforest

- First evidence that the SER hypothesis may also hold for coastal tropical forests.
- Standard organic material with high sodium content was highly palatable.
- Higher decomposition when macro-invertebrates were excluded.

The sodium (Na) respiration hypothesis (SER) states that low Na availability limits the activity and density of soil organisms and therefore plays an important role in decomposition processes of tropical forest soils. Support for this hypothesis was found in several studies reporting higher decomposition rates attributed to higher densities of macro-invertebrates after sodium chloride (NaCl) additions. However, it was also suggested that high doses of NaCl might make organic material unpalatable or even toxic for microbes. Most of these studies were conducted in inland tropical rainforests, where atmospheric deposition of Na is low compared to coastal areas and therefore Na limitation may be more severe. We assessed how treating standardized organic material with different NaCl concentrations affects decomposition rates in a coastal primary rainforest on the island of Borneo. For this purpose, we established a multi-factorial short-term (7 days) decomposition experiment in which we incubated cotton cloths (standard organic material). The cloths were treated with 0%, 0.5%, 1%. 2.5% or 5% NaCl solutions and either provided access for all soil decomposer communities (no mesh) or excluded macro-invertebrates >2 mm (metal mesh). The cloths were placed in the mineral soil and on top of the litter layer. We found higher decomposition in the mineral soil compared to the top of the litter layer. Decomposition rates of cloths treated with 5% NaCl were significantly higher compared to all other cloths, which provides first evidence that the SER might also hold for coastal systems. In addition, we found higher decomposition for the meshed than the non-meshed cloths suggesting that macro-invertebrates either “grazed” on the smaller invertebrates/microbes or that the smaller soil fauna was more efficient in exploiting NaCl when macro-invertebrates were absent. Overall our findings suggest that Na may also control the activity of soil organisms in coastal tropical forest ecosystems.