Changes in soil organic matter molecular structure after five-years mimicking climate change scenarios in a Mediterranean savannah

Mediterranean savannahs (dehesas) are agro-sylvo-pastoral systems with a marked seasonality, with severe summer drought and favourable rainy spring and autumn. These conditions are forecasted to become more extreme due to the ongoing global climate change. Under such conditions, it is key to understand soil organic matter (SOM) dynamics at a molecular level. Here, analytical pyrolysis (Py-GC/MS) combined with chemometric statistical approaches was used for the molecular characterization of SOM in a five-years field manipulative experiment of single and combined rainfall exclusion (drought) and increased temperature (warming). The results indicate that SOM molecular composi-tion in dehesas is mainly determined by the effect of the tree canopy. After only five years of the climatic experiment, the differences caused by the warming, drought and the combination of warming+drought forced climate scenarios became statistically significant with respect to the untreated controls, notably in the open pasture habitat. The climatic treatments mimicking foreseen climate changes affected mainly the lignocellulose dynamics, but also other SOM com-pounds (alkanes, fatty acids, isoprenoids and nitrogen compounds) pointing to accelerated humification processes and SOM degradation when soils are under warmer and dryer conditions. Therefore, it is expected that, in the short term, the foreseen climate change scenarios will exert changes in the Mediterranean savannah SOM molecular structure and in its dynamic.

Automated summary

The molecular composition of soil organic matter (SOM) in Mediterranean savannahs is mainly determined by the effect of the tree canopy. After a five-year field manipulative experiment, the differences caused by drought, warming, and the combination of both became statistically significant with respect to the untreated controls, particularly in the open pasture habitat. The predicted climate change scenarios mainly affected lignocellulose dynamics, as well as other SOM compounds, pointing to accelerated humification processes and SOM degradation under warmer and drier conditions.