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Mountain soils stock large quantities of carbon as particulate organic matter that may be highly vulnerable to climate change. To explore potential shifts in soil organic matter (SOM) form and stability under climate change (warming and reduced precipitations), we studied the dynamics of SOM pools of a mountain grassland in the Swiss Jura as part of a climate manipulation experiment. The climate manipulation (elevational soil transplantation) was set up in October 2009 and simulated two realistic climate change scenarios. After 4 years of manipulation, we performed SOM physical fractionation to extract SOM fractions corresponding to specific turnover rates, in winter and in summer. Soil organic matter fraction chemistry was studied with ultraviolet, 3D fluorescence, and mid-infrared spectroscopies. The most labile SOM fractions showed high intra-annual dynamics (amounts and chemistry) mediated via the seasonal changes of fresh plant debris inputs and confirming their high contribution to the microbial loop. Our climate change manipulation modified the chemical differences between free and intra-aggregate organic matter, suggesting a modification of soil macro-aggregates dynamics. Interestingly, the 4-year climate manipulation affected directly the SOM dynamics, with a decrease in organic C bulk soil content, resulting from significant C-losses in the mineral-associated SOM fraction (MAOM), the most stable form of SOM. This SOC decrease was associated with a decrease in clay content, above- and belowground plants biomass, soil microbial biomass and activity. The combination of these climate changes effects on the plant-soil system could have led to increase C-losses from the MAOM fraction through clay-SOM washing out and DOC leaching in this subalpine grassland. [ABSTRACT FROM AUTHOR]

Mountain ecosystems are currently experiencing the strongest climatic warming and the largest changes in land-use during the last millennia. The impacts of these changes on soils and their roles in the cycling of carbon and nutrients are, however, largely unknown. Here, we define mountain soils as soils from mountainous areas with cool summers and cold winters and thus, soils from ecosystems that are influenced by snow and ice and where biogeochemical processes are limited by temperature. Because climatic conditions, soil properties, plant species and productivity vary at a small scale in mountains, they provide a unique natural but a seldom used laboratory to study soil processes. In this special issue, we compile different studies on soils from European mountains, reaching from the functioning of mountain soils along natural climatic gradients to responses of greenhouse gas fluxes from mountain soils to experimental warming, soil frost and changes in precipitation. [ABSTRACT FROM AUTHOR]

We investigated the response of soil macroaggregate dynamics to soil temperature modification along a spatial gradient located on a forested north-facing slope in the southern French Alps, simulating long-term adjustment of soil–plant interactions to absence or occurrence of soil frost. Soil macroaggregate (>250 μm) content of Ah horizons was strongly depleted (72%) in colder plots affected by freeze-thaw events, compared to 96% in warmer and frost-free plots ( p < 0.05). A visual assessment of soil macroaggregation showed that physical processes were the main drivers of soil macroaggregation in colder plots, with 66% of the 5–12.5 mm fraction and the whole 3.15–5 mm fraction. Conversely, we found a balanced contribution of biological and physical aggregation pathways in warmer plots. All identified macroaggregate types could be classified, depending on their organic matter (OM) quality, using principal component analyses of their near infrared spectra. Such spectral classifications indicated temporal changes in OM quality of macroaggregates, from formation to colonization by fine roots, suggesting ecosystem-specific ontogenic trajectories for soil macroaggregation. Further physico-chemical characterizations of soil macroaggregates and Ah horizons showed that soil organic carbon content in the Ah horizon was constant along the gradient, whereas soil erodibility was reduced in warmer soils, which prevented the occurrence of fragile macroaggregates formed by freeze-thaw events. Our study thus suggests changes in the erodibility of mountain forest soils under changing climate. Soil erodibility could be affected either positively under warmer conditions, or negatively, under increased soil frost. [ABSTRACT FROM AUTHOR]