Agroecological Soil Management of an Organic Apple Orchard: Impact of Flowering Living Mulches on Soil Nutrients and Bacterial Activity Indices

The introduction of living mulches into an orchard can be considered an agroecological practice that can provide several ecosystem services related to integrated crop protection, also in relation to the impact on soil microbiome. In this study, the introduction in an organic apple orchard of two plant mixtures designed as multifunctional living mulches to reduce weed competition and increase shelter for beneficial arthropods was evaluated in relation to their impact on soil nutrient content and bacterial activity indices. One mixture was composed of Trifolium repens (20%) and Festuca ovina (80%), the second made of 40 different plant species including legumes, flowering species and grasses. Both living mulches increased N-nitrate levels in spring, and the two-component plant mixture was also able to improve P and K levels in soil at the same time, in comparison to the natural cover (control). The two mixtures induced an increase in bacterial activity in the beginning (40 plant species mix) or middle of the growing season (two-component plant mix), without major effects on bacterial biodiversity at the phyla level, showing a high share of Proteobacteria and Actinobacteriota among treatments. Nevertheless, both plant mixtures modified the phenotypic profile of the bacterial population, measured with the Biolog method, of different classes of C sources including carbohydrates, amino acids and carboxylic acid. The results are pointing to possible benefits of the practice on soil microbial activity, which will have to be confirmed by longer studies.