Effects of combined application of amino acids and potassium fulvic acid on bayberry (Myrica rubra)fruit quality and soil microorganisms

【Objective】The intensification of protected cultivation systems has brought about new challenges for perennial fruit production, including soil acidification, nutrient imbalance, and organic matter depletion. Chinese bayberry (Myrica rubra), a high-value fruit crop cultivated in southern China, experiences particularly severe quality deterioration when grown under continuous monoculture systems. Additionally, conventional fertilization practices often lead to low nutrient use efficiency and environmental pollution. This study aimed to develop an integrated nutrient management strategy that combines the biostimulant properties of amino acids with the soil conditioning capabilities of humic substances. Specifically, we investigated the synergistic effects of amino acid water- soluble fertilizer and mineralsourced potassium fulvate, applied through drip irrigation, on fruit quality enhancement, improvement of soil physicochemical properties, and restructuring of bacterial communities in greenhouse-cultivated Dongkui bayberry. This research provides scientific evidence for developing precision fertigation protocols that simultaneously meet productivity, quality, and sustainability goals in intensive fruit production systems.【Methods】A field experiment was conducted using 22-year-old Dongkui bayberry trees under greenhouse conditions. Two treatments were established: a control (CK) receiving clear water, and a treatment (PHA) receiving a combination of amino acid fertilizer and mineral potassium fulvate via drip irrigation. PHA treatment was applied at two critical growth stages—fruit expansion and pre-color transition—using specific commercial formulations diluted 500-fold (amino acid fertilizers) and 1500-fold (potassium fulvate), with 30 kg applied per tree each time. Fruit quality parameters, including single fruit weight, and the contents of total soluble solids (TSS), vitamin C (Vc), anthocyanin (AC), total soluble sugar (TS), and titratable acid (TA), were measured at fruit maturity. Soil samples collected from the 0-20 cm rhizosphere layer were analyzed for pH, organic matter (OM), available nitrogen (HN), available phosphorus (AP), available potassium (AK), exchangeable calcium (Ca) and exchangeable magnesium (Mg). Bacterial community composition and diversity were assessed via high-throughput sequencing of the 16S rRNA V4-V5 region. Statistical analyses included t-tests, correlation analysis, and calculation of diversity indices (Chao1, Shannon).【Results】The PHA treatment demonstrated significant improvements across multiple measured parameters. Vegetative growth was markedly enhanced: spring shoot length increased by 25.26% compared with CK, while no significant differences were observed in shoot diameter, leaf width, leaf length, or leaf thickness. All fruit quality parameters showed comprehensive improvement after PHA application: single fruit weight increased by 17.16%, soluble solids content rose by 5.33%, vitamin C concentration increased by 47.57%, anthocyanin content increased dramatically by 128.65%, and sugar-acid ratio improved by 73.01%, while titratable acid content decreased significantly by 37.21%. Soil physicochemical properties underwent substantial improvement with PHA treatment. Soil pH increased by 10.37%, organic matter content more than doubled with a 101.37% increase, available potassium showed the most dramatic improvement with a 155.09% increase, exchangeable calcium increased by 37.21% , and exchangeable magnesium rose remarkably by 180.00% . Although available phosphorus and alkali-hydrolyzable nitrogen showed modest increases of 10.98% and 8.66% respectively, these changes did not reach statistical significance. Microbiological analyses revealed profound changes in soil bacterial communities. Alpha diversity indices increased significantly under PHA, with the Chao1 index rising by 12.15% and the Shannon index increasing by 4.18%. Beta diversity analysis via PCoA revealed a clear separation between PHA and CK communities, indicating substantial structural reorganization of the bacterial assemblages. At the phylum level, PHA treatment significantly increased the relative abundance of Verrucomicrobiota (276.04% ), Patescibacteria (274.73%), and Gemmatimonadota (128.59%), while reducing that of SAR324_clade (Marine group B) by 47.91%, Bacillota by 26.70%, and Actinomycetota by 22.42%. At the genus level, Gemmatimonas (146.23%), Gaiella (58.75%), and Bryobacter (14.82%) were notably enriched, whereas Candidatus Koribacter, Moorella, Acidothermus, and Desulfofundulus exhibited significant reductions. Correlation analysis revealed intricate relationships among soil properties, microbial communities, and fruit quality. Soil organic matter and available potassium showed exceptionally strong positive correlations with anthocyanin content (r = 0.999 and 0.997, respectively) and sugar-acid ratio (r = 0.990 and 0.996, respectively). Key bacterial genera showed significant associations with soil parameters: Gaiella showed a perfect correlation with exchangeable calcium (r = 1.000) and a strong correlation with organic matter (r = 0.982), while Gemmatimonas was strongly correlated with organic matter (r = 0.928) and pH (r = 0.833). Both genera showed significant positive correlations with anthocyanin content and soluble sugar content (r = 0.886- 0.943). Negative correlations were detected between fruit quality parameters and several bacterial taxa: Acidothermus and Candidatus Nitrosoarchaeum, in particular, exhibited strong negative correlations with multiple quality indices.【Conclusion】The integrated drip fertigation system combining amino acid water-soluble fertilizer and mineral-derived potassium fulvate delivers comprehensive benefits for greenhouse bayberry production. This method simultaneously resolves multiple constraints in intensive cultivation systems: it significantly improves soil physicochemical properties, enhances bacterial diversity, restructures microbial community composition, and ultimately improves fruit quality attributes. This treatment effectively alleviates soil acidification, promotes organic matter accumulation, improves potassium availability, and optimizes calcium and magnesium nutrition. The enrichment of specific bacterial taxa including Gemmatimonas, Gaiella, and Bryobacter indicates that these groups may act as key mediators in the soil-plant quality pathway, likely functioning through their involvement in nutrient cycling, organic matter decomposition, and plant growth promotion. The strong correlations observed among soil parameters, microbial communities, and fruit quality indicators provide compelling evidence for a microbiome-mediated quality enhancement mechanism. This integrated fertilization strategy represents a viable approach for the sustainable intensification of bayberry production, and offers a practical solution for reconciling productivity, quality, and environmental goals in protected cultivation systems.