Combined effects of super absorbent polymers and microbial fertilizers Co-application on apple production in the Loess Plateau

Super absorbent polymers (SAP) and microbial fertilizers (MF) are pivotal in ameliorating soil physicochemical properties, regulating plant growth, and enhancing fruit quality. However, the combined effects of their co-application on apple plants remain unclear. To address this, a two-year field experiment was conducted on the Loess Plateau to evaluate different co-application ratios of SAP and MF. Results showed that the combined application improved soil properties, with higher contents of available nitrogen and organic matter, better moisture retention and lower soluble salt content. These improvements were accompanied by significant increases in chlorophyll content, net photosynthetic rate (Pn), water use efficiency (WUE), hundred-leaf weight, and fruit yield. Fruit quality was also enhanced, as evidenced by higher single fruit weight, vitamin C (VC), and soluble solids contents in apple fruits. Comprehensive evaluation identified 180 g SAP combined with 750 g MF per plant (P3M3) as the optimal treatment. Transcriptomic analysis revealed that P3M3 upregulated genes involved in nitrogen metabolism and arginine biosynthesis, consistent with elevated free amino acid levels in the roots. 16S rRNA sequencing further showed that P3M3 restructured the bacterial communities of the rhizosphere and bulk soil without reducing their diversity, enriching Actinobacteriota and diazotrophic genera including Frankia and Bradyrhizobium, and enhancing the predicted potential for nitrogen fixation. Overall, our findings reveal the associations among soil properties, bacterial communities, and root metabolism under SAP and MF co-application, providing a theoretical and practical framework for resource-efficient fruit production.