Degree of soil disturbance affects success of microbiome restoration by artificial humic substances: a plant health perspective

Soils are crucial for biogeochemical elemental cycles and, on a more anthropocentric note, for agriculture. The ongoing degradation of agricultural soils, including horticultural substrates, has large scale implications for crops, humans and the surrounding ecosystems (OneHealth). One critical aspect of soil degradation is the subsequent loss of functional microbial diversity, which is essential for soil and plant health. Thus, the maintenance and manipulation of those microbial players is a key interest of sustainable farming and the European Union (The Mission 'A Soil Deal for Europe'; SPIN-FERT: Grant agreement ID: 101157265, DOI: https://doi.org/10.3030/101157265).

To understand the role of artificial humic substances on plant health, plant performance and soil microbiomes we grew tomato seedlings along a soil disturbance gradient. Each substrate was treated with artificial humic substance and/or Rhizoctonia solani AG-4, a fungal soilborne plant pathogen, to infer potential mechanisms of plant growth enhancement and disease resistance. We hypothesize that humic substances increase soil microbial diversity and disease resistance of tomato seedlings.

Plant height and microbial diversity were observed to be highest in undisturbed soil and were further increased by the addition of humic substance and decreased by the presence of R. solani. Disease incidence was noticeably lower under humic substance amendment except for the most disturbed soil. Both treatments caused the microbial communities of the soil and rhizosphere to shift, with β-diversity clustering the most complete and distinct after the disturbance recovery and revealing several plant and soil health associated taxa to be enriched through humic substance addition.

By altering the soil microbiome composition, the plant is offered a wider selection of microorganisms to recruit from while the fungal pathogen is met with a more diverse battery of potential antagonists. Our findings may contribute to more effective manipulation of the microbial aspects of agriculture to promote and improve healthy produce.