Soil health and microbial diversity in fruit chestnut groves affected by ink disease

Soil is one of the most diverse and complex natural systems at global scale and is involved in several reactions of formation, development, and breakdown of chemicals, many of these depending on soil microorganisms.

According to the combination of soil forming factors, soil properties show a large horizontal and vertical variability, which can have a huge impact on soil microbial communities. However, relatively few investigations have been carried out to associate the changes in soil microbial community with the variations of soil properties across the genetic horizons and along depth.

In Italy, chestnut groves used to be key sources of products in mountain regions. Recent socio-economic changes have led to the progressive abandonment of this land use, producing degradation of the landscape and increased hydrogeologic risk. Now, programs for the restoration of this activity collide with outbreaks of illnesses such as ink disease, which is caused by Oomycetes present in the soil. In this framework, the physicochemical and biological features of soils play a crucial role in the distribution and assessment of the risk and severity of this disease.

The objective of this study was to correlate the diversity and eco-functionality of the microbial communities with soil properties and health indicators along depth and across a transect including chestnut trees with and without symptoms of ink disease.

The study area was a fruit chestnut grove located in the Apennine mountains south of Bologna, Italy. Soil profiles were opened along a transect ranging from a chestnut tree showing ink disease symptoms (INK1) to two subsequent chestnut trees with no visible symptoms (INK2 and INK3). In each profile, the horizons were described and sampled; to assess spatial variability, three minipits were opened around each profile, and their horizons were also described and sampled. Each horizon was also sampled in sterility. The samples were then analyzed for physicochemical and biological parameters, and total DNA was extracted to perform a taxonomic analysis.

Results showed that some physicochemical parameters, while presenting a trend with depth, also presented a trend with distance from the diseased tree: pH and base saturation decreased near this tree, while C:N and C:P increased, as well as water-extractable organic carbon. The taxonomic analysis showed that, while no substantial variation was detected in the bacterial composition of INK2 and INK3, INK1 showed a higher prevalence of phyla involved in the organic matter cycle (Acidobacteriota and Proteobacteria). Regarding the fungal population, in INK2 and INK3 the main trophic group was soil saprotrophs, while in INK1 the most frequent were short- and medium-distance ectomycorrhizae, which often indicate plant stress. Shannon index showed that bacterial diversity increased with depth while fungal diversity decreased. In both cases the profile near the diseased tree presented the least diversity. No difference was detected between the profiles in soil health indicators, such as organic matter content and microbial biomass and its activity, and these were not correlated with microbial diversity. These results suggest that taxonomic analysis of soil microorganisms could integrate traditional indicators in assessing soil and ecosystem health.