Soil macroaggregate dynamics in a mountain spatial climate gradient

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.