Microbiome interventions combined with artificial humic acid treatments for restoring soil bacterial diversity.

Soil health is under threat worldwide and technologies for soil restoration are urgently needed. Here we study the effect of artificial humic acids (A-HA) and soil transplants fo one restoration of depleted soil microbiomes. We used a controlled microcosm experiment with gradients of microbial diversity, with and without A-HA, across three soil types. Microbial abundance, diversity, and composition were assessed using qPCR and 16S rRNA gene amplicon sequencing, complemented by metabolomic profiling of water-extractable compounds and the growth characterization of bacterial isolates. A-HA treatment had a stronger effect on bacterial richness and community structure in degraded than in the original soil. Soil microbiome transplants could partially regenerate microbial abundances and increased bacterial richness and diversity in the degraded soils. Interestingly, the combination of A-HAs with addition of 10% soil transplants yielded the best restoration effect. The effect of individual as well as combined treatments strongly depended on the composition of the native soil microbiome. From a mechanistic point of view, A-HA treatment inhibited fast-growing bacteria, which allowed slow-growing bacteria to recover. By combined treatment, depending on the soil type and its native soil microbiome, we can synergistically restore the soil microbiome to resemble its original composition.