Hillslope Morphological Features Modulate Soil Microbial Communities and Chestnut Ink Disease via Clay and Water Redistribution

Ink disease, caused by the soil-borne pathogens Phytophthora cambivora and Phytophthora cinnamomi, is threatening sweet chestnut (Castanea sativa) groves across Europe. This study investigates whether the morphology of soil and related properties influence the development of ink disease throughout the entire soil profile. In a C. sativa stand in Northern Italy, soil pits were excavated near symptomatic (INK1, 978 m a.s.l.) and healthy trees (INK2, 988 m a.s.l.; INK3, 998 m a.s.l.) along an altitudinal transect. The slope gradients at these sites were 3%, 9%, and 30%, respectively. Soils were classified as Luvisols. Results suggest that the lower slope position and gentler gradient of INK1 may have facilitated the downslope transport of clay and water from INK2 and INK3, leading to increased clay accumulation throughout the INK1 soil profiles. This, in turn, enhanced saturated hydraulic conductivity (Ks) and the wilting point (WP), favoring water retention in deeper horizons, where Phytophthora DNA was detected. Indeed, INK1 had a higher WP (14.9%) compared to INK2 (11.7%) and INK3 (8.2%), and exhibited the highest Ks (25.1%), significantly exceeding values in INK2 (4.6%) and INK3 (6.5%). The presence of the pathogen in INK1 appeared to affect microbial functionality, as indicated by the dominance of contact (~20%) and medium-distance ectomycorrhizal (~60%) exploration types over long-distance ones. Overall, our findings highlighted the key role of soil processes, particularly clay and water redistribution, in shaping microbial communities and soil-borne pathogen dynamics through their influence on edaphic properties.