Temperature sensitivity of forest soil organic matter decomposition along two elevation gradients

The temperature sensitivity of the soil organic matter (SOM) decomposition along the slopes of mountain forests in Austria and Spain was analyzed. High-altitude forest soils store large quantities of organic carbon (C) and are particularly vulnerable to global warming if the decomposition of the SOM is more temperature sensitive than at lower altitude. Mineral soil and O-layer material was incubated in the laboratory at temperatures increasing from 5 degrees C to (20 degrees C) 25 degrees C. The temperature sensitivity (Q(10)) was determined by fitting different temperature response functions to the measured CO2 efflux. Bulk soil and density fractions were analyzed for organic C and nitrogen (N) contents. C and N stocks along the elevation gradients were estimated. Q(10) over the whole incubation temperature range varied between 1.5 and 2.5 but did not show any altitudinal trends for O-layer material and mineral soils along both gradients. Besides that, Q(10) generally increased with decreasing soil temperatures. SOM decomposition at higher elevation forests will be more responsive to global warming because it will be affected in a more sensitive (cooler) temperature range compared to lower elevation sites. This effect was modeled by the Lloyd and Taylor function and Gaussian but not by the frequently used exponential temperature function. Both soil C and N contents increased with increasing altitude. Density fractionation showed deviating altitudinal C and N patterns of labile and recalcitrant SOM pools along the Spanish gradient. Soil C stocks along both gradients did not resemble the trend in C contents and were determined by other site-specific factors. This, and significantly low C and N contents and stocks of a site that was used as a forest pasture, indicates that both forest management and land use can play equally important roles in the development of soil C as climatic factors.