Infra-red spectroscopy reveals chemical interactions driving water availability for enzyme activities in litters of typical Mediterranean tree species

- Chemical interactions driving water availability in litters have been explored.
- Sorption isotherms are similar in Pinus halepensis and Quercus pubescens litters.
- NaCl addition provokes a shift in sorption isotherms above water activity of 0.75.
- FTIR-ATR reveals that Na+ ions are bound to cellulose.
- Lipase activities are not affected by a decrease in water availability.

In Mediterranean ecosystems, water is one of the main drivers of the microbial activities that support organic matter turnover in soils or litters. In addition to drought stress, coastal areas are subject to osmotic stress linked to sea spray exposure. Here we explored i) how water availability, characterized by water activity aw, is impacted by adding NaCI to litter, ii) the chemical interactions between water, NaCl and the litter matrix and iii) whether microbial activities (using lipase as a model) are affected under these conditions. Litters of two vegetal species typical of the Mediterranean area (Quercus pubescens and Pinus halepensis) were subjected to FTIR-ATR (Fourier-Transformed Infra-Red - Attenuated Total Reflectance) spectroscopy for chemical characterization. Q. pubescens and P. halepensis litters were characterized by cutin and aromatics respectively. Sorption isotherms were identical for both species litters; when NaCl was added, a shift in isotherm shape was observed at aw ranging from 0.75 to 1. FTIR also discriminated samples with and without added NaCl and revealed that cellulose is probably the polymer in interactions with ions. Very interestingly, no differences were found between lipase hydrolytic activities with and without added NaCl: salt addition had no effect on these activities.