Both altitude and vegetation affect temperature sensitivity of soil organic matter decomposition in Mediterranean high mountain soils

• Soil organic C and microbial activity decreased with altitude under grassland.
• Q10 of soil microbial respiration increased with altitude under grassland.
• Microbial activity was lower in soils under shrubs.
• Q10 of microbial respiration and soil organic C contents were higher under shrubs.
• Altitude and vegetation led differences in organic matter decomposition.

The aim of this work was to study the sensibility to warming of soil organic matter (SOM) decomposition in Mediterranean high mountain areas. Thus, we investigated the effects of temperature, C availability and vegetation in a Mediterranean high-mountain area in relation to SOM decomposition patterns. Along an altitudinal gradient (from 2100 to 2380 m a.s.l.) in Central Spain mountains, we assessed the altitudinal shifts in soil organic C (SOC), soil nitrogen (N), dissolved organic carbon (DOC), microbial biomass C (MBC), microbial respiration, microbial respiration sensitivity to temperature (Q10) and C availability index (CAI). Furthermore, we tested the differences in SOM decomposition rates between grasslands and shrub vegetation. SOC, DOC, N content, MBC, microbial respiration and CAI decreased, while Q10 increased with increasing altitude. In the grassland, MBC and microbial respiration were positively correlated to SOM. Q10 was positively correlated to pH and negatively correlated to substrate-induced microbial respiration. Soils below shrubs showed lower microbial respiration rates, lower CAI, and higher Q10 than soils below grassland. However MBC, DOC and soil N content were higher below shrubs. The results suggest that a rise in temperature would enhance SOM decomposition rates in grasslands more dramatically at higher altitudes, since they are more sensitive to temperature increases. The SOC accretion observed below shrubs may be due to the lower respiration rate of soil microorganisms, possibly determined by lower C substrate availability below shrubs. This result suggests a higher recalcitrance of shrub litter compared to grassland litter. Nevertheless, SOC in shrubland may be released at a higher rate due to its higher temperature sensitivity.