In-depth insight into the short-term effect of floor management practice on young apple trees development and soil microbial biodiversity and activity

PLOS One | https://doi.org/10.1371/journal.pone.0329979 August 7, 2025 10 / 15 Discussion Both living mulch mixtures affected negatively, particularly during the first vegetative season, the initial growth of the apples trees compared to the control method of tree row management based on herbicide application and following natural cover. This effect impacted on the overall 2-year growth, particularly when considering the trunk diameter. Unexpectedly, such impact resulted more pronounced for Mix1, composed by a legume (T. repens) and a grass (F. ovina). The major rationale for testing this mixture was to combine a small growing grass with a N-fixing legu- minous that would have reduced the competition for soil nutrients and provided nitrogen supporting the tree growth [33]. The soil nutrient level observed confirmed the correctness of the approach since the content of nitrate nitrogen was higher than in the control. However, this was not paralleled by the nitrogen content of the apple trees leaves, which was at the same level of the control. The association of T. repens with apple trees in a rhizobox system also resulted in trunk diameter growth reduction, but without negative effect on shoot growth (weight) and a higher root development [34]. In case of Mix2, which contained a lower proportion of T. repens, resulted in a limited negative impact on tree growth parameters, but still a N-leaf content similar to control and Mix1, though both N forms in the soil were higher

than control only in summer. A possible explanation of this effect could be the presence of more plant species in Mix2, including P. lanceolata, which was reported to suppress soil N mineralization through exudation of various second- ary metabolites interacting with the soil microbiome thereby reducing ammonia monooxygenase and nitrifiers activity [35,36]. This interaction may also explain the increase in ammonia levels and the almost no change of N-NO3 levels observed at different stages of the growing season in Mix2. A meta-analysis considering the impact of living mulches containing leguminous and grasses under various pedo-climatic conditions, showed they increased soil nutrients, but this was much less observed when the cover of the soil was only for a short term [37]. The negative effect of living mulches based on white clover on the tree growth after the orchard establishment was observed also under pedo- climatic conditions similar to those of the current study, but such negative impact was less evident when blue fescue Fig 5. Venn’s Diagram visualizing significant changes in C-sources use according to changed bacterial activity between two living mulches mixtures when compared to the control, depending on the season. https://doi.org/10.1371/journal.pone.0329979.g005