The decline of the Korean fir has long been a serious problem for which no clear cause has been found . Most previous studies have reported a link between the tree decline and environmental factors, including typhoons , droughts , excess moisture [3, 11], and other climatic changes [12, 13]. Although soil microbiota plays an essential role in the forest ecosystem processes , the rhizosphere microbiome of Korean firs remains unknown. In addition, plant-associated soil microbial communities maintain soil health by stimulating plant growth, supplying nutrients, and increasing resistance to biological and abiotic stresses, such as climate change . Thus, understanding the rhizosphere microbiota of healthy and dead plants of Korean firs and their association with edaphic factors will advance our knowledge about the contribution of microbial communities to resistance to abiotic stresses for such endangered tree species. It will also help in designing effective alternative strategies for conserving Korean firs. To the best of our knowledge, this study is the first report about the rhizosphere microbial structure of Korean firs. In addition, this study reports the association between microbiome and soil physicochemical features.
PCoA beta diversity and relative abundance analysis in our study showed that significant differences were observed between the two groups in the fungal community but not in the bacterial community. This result suggests that the fungal community assembly is comparatively more sensitive than the bacterial community to the stress-induced changes of the fir tree rhizosphere, emphasizing fungi as a good bioindicator of habitat transition . In addition, the bacterial community response to abiotic factors in our study was similar to that reported in previous studies . Bacteria have high plasticity to environmental stresses, which supports the idea that soil bacterial communities differ in their vulnerability to stresses  and host health status doesn’t affect members of soil microbiota equally . In addition to the direct impact of environmental stresses on microbial communities, the effect of changes in plant root exudates following abiotic stress on the microbial community cannot be ruled out .
The fungal diversity of the DKF soil sample was generally slightly higher than that of the HKF sample, which may be attributed to woody plants or crops manufacturing exudates to control an environment suitable for self-growth, which regulates symbiotic soil microorganisms through chemical interaction . Alternatively, dead plant roots in DKF soil could create a conducive environment for different fungi groups, especially necrotroph microbes . The proportion of symbiotic fungal genera in DKF soil was reduced, which might be attributed to the interaction between plant-derived metabolites and mycobiome . Our study showed that although nonsignificant changes in bacterial diversity between the two groups were observed, fungal diversity appears to be comparatively more influenced by stress-induced rhizosphere changes in the Korean firs. However, more research is needed to confirm the evidence of such relationships.
Differential abundance, LEfSe, and co-occurrence network analysis at genus level between HKF and DKF showed that ectomycorrhizas (ECMs), including Russula, Sebacina, and Hydnotrya, had the highest abundance in the HKF group. Russula was also found to be a keystone taxa in the HKF network. Russula was found to be one of the most essential and abundant ECM fungi having a symbiotic relationship with diverse higher plants in the mountain rain forest, which conforms to the findings of previous studies [22–25]. These keystone taxa in HKF play essential roles in the fungal communities of the soil environment. Interestingly, Kohout et al.  highlighted that ECMs, including Russula, mostly drove the rapid dynamics in the fungal community composition. In addition to Russula, a previous study has reported Sebacina as the commonly observed ECM in the forest ecosystem . Hydnotrya has also been reported as one of the most dominant ECM symbiont fungal genera , which may be because of the presence of rhizodeposits from the live fir . In contrast, there was a decrease in the abundance of such ECMs in dead plant roots , which might be attributed to the lack of plant-derived active carbon inputs, including exudates from the dead plant roots . Co-occurrence network analysis also showed that HKF network, which has keystone taxa of known plant symbionts, is more stable and complex, and it has more efficient global connectivity than the DKF network, which conforms to previous studies that plants supply high nutrients to microbes, leading to more complex microbial networks .
Our soil physicochemical properties and correlation analysis indicate that discriminative symbiotic fungal taxa had a linear relationship with soil features. In DKF soil, the percentage of sand that has a large particle size and small surface area was relatively higher compared with HKF soil (Fig. 1). DKF also had reduced CEC, K+, and Na+ exchangeable cations, which have low adsorption strength to soil particles. A previous study had emphasized that ECM fungi can increase CEC, K+, and Na+ exchangeable cations in soil , which aligns with our findings that symbiotic fungi strongly correlate with soil nutrients (Fig. 5). Potassium and sodium are important nutrients for the forest ecosystem and defend against many biotic and abiotic stresses [32, 33].
The soil of Jeju island is well known for its high phosphate absorption coefficient (Table S2) in Korea, which may be due to high ashes from volcano eruption . Alternatively, although phosphate (Pi) is one of the important macronutrients for plant growth and survival , it has a strong affinity to bind to soil minerals and becomes unavailable to plants. In our results, high levels of Avail P in HKF soil may be partly attributed to the release of soluble phosphorus owing to dephosphorylation of the soil microbiome . In line with this, a previous study had emphasized that Sebacina, which has a strong positive linear relationship with phosphate, has an acid phosphatase enzyme .
The high abundance of saprotrophs, such as Ascocoryne, Umbelopsis, and Hypochnicium, in the DKF group supports the fact that saprotrophs are found at very low abundance in actively growing plant roots , and they mainly feed on nonliving organic matter, such as dead plant roots . Trichoderma spp., a key genus specializing in the breakdown of complex compounds, such as lignin and cellulose , which commonly arise after plant death , were likewise abundant in DKF soil. Collectively, these findings indicate a reduction in the abundance of symbiotic fungi in the rhizosphere of the declined tree population and an increase in saprotrophic fungi. Similar to previous studies, we found that different plant saprotrophic fungal genera act as network keystones in the DKF group . Nevertheless, pathogenic fungal genera were not found to be as abundant in DKF soil as in HKF soil, which is consistent with previous findings that the survival of microbes, including ECMs, is related to soil nutrient availability.