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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]

Abstract: We examine the effect of natural differences in soil temperature at two locations distant by approximately 500 m in the frozen scree of La Plagne en Chartreuse (Savoy, France). We determined humus properties, soil organisms and biologically mediated soil-forming processes in screes from a low part dominated by an open pine forest (OF, 1100 m asl) and in a top part covered by a dense pine forest (DF, 1200 m asl). Our results show a soil temperature increase of about 3.8 °C in the DF plot during June to October, but similar humus morphology at both plots. Soil pH was lower, and C/N ratio higher in the DF plot. Soil faunal composition differed between DF and OF plots for some taxa. While the abundance of the Oribatid mites was higher, abundance of Collembola was significantly lower in the DF plot. As earthworm activity was scarce at the site, Oribatid mites and Collembola presumably play an important role in the humus microstructure which was confirmed by the analysis of the humus micro-aggregation. To date, changes in the pedofaunal communities did not alter the humus form which may be explained by the complexity of biotic soil interactions and the high functional redundancy of the soil fauna. [Copyright &y& Elsevier]