Beneath the apple trees - Exploring soil microbial properties under Malus domestica concerning various land management practices

The presented study evaluated the impact of six land management practices on soil bacterial and fungal communities under apple trees: green field belts, forests, gardens with trees and ornamental plants, gardens with farm animals, and uncultivated and cultivated orchards. The study explored soil microbial properties including the enzymatic activity of dehydrogenases (tested by colorimetric reaction with 2,3,5-triphenyl-tetrazolium chloride), metabolic profile (using Biolog™ ECO Plates), metataxonomy structure (Next Generation Sequencing using Illumina®), followed by physicochemical properties (pH, N, P, K, microelements concentrations, organic matter, and C_org content). The hypothesis was that different land management practices would influence soil microbial properties, with cultivated orchards expected to show significantly lower dehydrogenases activity, and higher substrate-based respiratory than biomass response, within substrate stress occurrence, when testing metabolic profiles, but also different composition and lower relative abundances of specific microbial taxa and different biomarker genera, compared to other treatments. We aimed to identify practices promoting diverse microbial substrate-based metabolic and taxa diversity responses, with a focus on Bacillus and Trichoderma abundances, which are potential biological agents against fungal pathogens. As expected the presented research revealed significant statistical variations in microbial communities among different land management practices in soil beneath apple trees. It was accordingly noted that cultivated orchards, but also green belts, clearly exhibited reduced microbial activity (3.59 and 4.76 TPF kg⁻¹ d⁻¹, respectively) compared to gardens and uncultivated orchards (12.08 and 9.89 TPF kg⁻¹ d⁻¹). Cultivated orchards notably showed higher respiration levels and substrate stress compared, especially to forests and other land management practices represented by a clear separation of observed according to Sneath's criteria in cluster analysis. Different land management practices induce unique stress responses in microbial communities: forests struggled with B-Methyl-d-Glucoside, gardens with Serine and Putrescine, cultivated orchards with d-Glucosaminic Acid and Cyclodextrin, and bounds with 2-Hydroxy-Benzoic Acid. Substantial differences were also observed in the relative abundance of the top ten bacterial and fungal orders, and biomarker genera representatives. In cultivated orchards, there was a significant decrease in the relative abundance of many bacterial taxa such as e.g. Rhizobiales, Burkholderiales, Vivinamibacterales, and fungal taxa including Eurotiales, and Saccharomycetales. Notably, no significant differences were noted for Bacillus abundance among tested management practices. Forests favored Trichoderma abundance the most among tested practices (relative abundance 0.05 %). In turn, Trichoderma representatives were revealed as biomarker genera in gardens with animals. Williamsia representatives, as found in uncultivated orchards were suggested to be a biomarker of less disturbance, resulting from area restoration. Overall, the study discussed how different land management practices influence soil microbial communities and their functional roles, emphasizing the potential impacts of use on soil health and biodiversity within its implications. The most important recommendation bullet points:

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  using Williamsia representatives as a soil biomarker microorganism to indicate successful area restoration processes in apple orchards,

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  using d-Glucosaminic Acid metabolic stress test to reveal early difficulties in controlling fungal pathogens in soil microbial communities,

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  promoting diverse plant covers and reasonably reduced agrochemical inputs in apple orchards for enhancing soil microbial resilience,

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  preserving habitats of wild apple trees in forests or green belts, as these environments exhibit reduced metabolic stress and support the occurrence of fungi like Trichoderma, in the context of locations from which such isolates should be sought for further biocontrol use.