Shifts in rhizosphere soil microorganisms of dominant plants' response to climate change profoundly impact mountain soil ecosystem multifunctionality; relatively little is known about the relationship between them and how they depend on long-term environmental drivers. Here, we conducted analyses of rhizosphere microbial altitudinal pattern, community assembly, and co-occurrence network of 6 dominant plants in six typical vegetation zones ranging from 1350 to 2900 m (a.s.l.) in Helan Mountains by absolute quantitative sequencing technology, and finally related the microbiomes to root zone soil multifunctionality ('soil multifunctionality' hereafter), the environmental dependence of the relationship was explored. It was found that the altitudinal pattern of rhizosphere soil bacterial and fungal diversities differed significantly. Higher co-occurrence and more potential interactions of Stipa breviflora and Carex coninux were found at the lowest and highest altitudes. Bacterial α diversity, the identity of some dominant bacterial and fungal taxa, had significant positive or negative effects on soil multifunctionality. The effect sizes of positive effects of microbial diversity on soil multifunctionality were greater than those of negative effects. These results indicated that the balance of positive and negative effects of microbes determines the impact of microbial diversity on soil multifunctionality. As the number of microbes at the phylum level increases, there will be a net gain in soil multifunctionality. Our study reveals that geographical and climatic factors can directly or modulate the effects of soil properties on rhizosphere microbial diversity, thereby affecting the driving effect of microbial diversity on soil multifunctionality, and points to the rhizosphere bacterial diversity rather than the fungi being strongly associated with soil multifunctionality. This work has important ecological implications for predicting how multiple environment-plant-soil-microorganisms interactions in mountain ecosystems will respond to future climate change.

C. korshinskii is one of the most widely-planted sand-fixing legumes in northwest China and exploring its rhizosphere microbiome is of great ecological importance. However, the effect of long-term sand fixation on the composition, diversity, and underlying functions of microbes in the C. korshinskii rhizosphere in dryland ecosystems remain unclear. Here, we performed high-throughput sequencing using a 16S rRNA (absolute quantification) and bacterial functional annotation of prokaryotic taxa (FAPROTAX) analysis and an ITS (relative quantification) and fungal functional guild (FUNGuild) analysis to investigate the C. korshinskii rhizosphere microbiome and metabolic functional groups at different sand-fixing ages (six years, CK6; twelve years, CK12; and eighteen years, CK18) and determined the physicochemical properties of the rhizosphere soil. Results showed that the key bacterial taxa of the rhizosphere were significantly more abundant in CK18 than in CK12 and CK6 at the phylum-class-genus level, and that fungal Glomeromycota was also significantly more abundant in the CK18 rhizosphere compared to CK12 and CK6. Among these bacterial taxa, the enrichment effect of key, functional, genus-level species of bacteria was the most obvious, including Rhizobium, Ensifer, Neorhizobium, Mesorhizobium, Streptomyces, Sphingomonas, and Flavobacterium, which are N-fixing and/or phosphate-solubilizing groups. The significant improvement seen in the physicochemical properties of the CK18 rhizosphere soil, including the higher total nitrogen (TN), available nitrogen (AN), pH, electrical conductivity (EC), higher N:P ratio, and lower C:N ratio, all demonstrated the relationship between the rhizosphere microbes and soil carbon (C) and nitrogen (N) cycling. A redundancy analysis (RDA) of different taxonomic levels indicated a close positive relationship between rhizosphere microbes and AN. In addition, the functional groups of the C. korshinskii rhizosphere bacteria were closely related to soil AN and were mainly composed of chemoheterotrophy and aerobic chemoheterotrophy. A Spearman correlation analysis revealed that these functional groups were mainly identified from bacterial Actinobacteria, Proteobacteria, Verrucomicrobia, Bacteroidetes, and fungal Glomeromycota. Our study provides evidence that the rhizosphere microbes of C. korshinskii are closely related to the accumulation of N in the restoration of desert ecosystems, and that the ecological functional processes they are involved in mainly involve C and N cycles, which play an important role in desertification reversal.

Desertification leads to the extreme fragility of ecosystems and seriously threatens ecosystem functioning in desert areas. The planting of xerophytes, especially leguminous shrubs, is an effective and common means to reverse desertification. Soil microorganisms play a crucial role in nutrient cycling and energy flow in ecosystems. However, the effects of introducing leguminous shrubs on soil microbial diversity and the relevant mechanisms are not clear. Here, we employed the high-throughput absolute quantification 16S rRNA sequencing method to analyze the diversity of soil bacteria in sand-fixing areas of mixed shrublands with three combinations of shrubs, i.e., C. korshinskii × Corethrodendron scoparium (CaKCoS), C. korshinskii × Calligonum mongolicum (CaKCaM), and C. scoparium × C. mongolicum (CoSCaM), in the south of the Mu Us Sandy Land, China. This area suffered from moving dunes 20 years ago, but after introducing these shrubs to fix the dunes, the ecosystem was restored. Additionally, the effects of soil physicochemical properties on soil bacterial composition and diversity were analyzed with redundancy analysis (RDA) and structural equation modeling (SEM). It was found that the Shannon index of soil bacteria in CaKCoS was significantly higher than that in CaKCaM and CoSCaM, and the abundance of the dominant phyla, including Actinobacteria, Proteobacteria, Acidobacteria, Chloroflexi, Planctomycetes, Thaumarchaeota, Armatimonadetes, candidate_division_WPS-1, and Nitrospirae, increased significantly in CaKCoS and CaKCaM compared to that in CoSCaM. RDA showed that the majority of soil properties, such as total nitrogen (TN), available potassium (AK), N:P ratio, soil moisture (SM), and available phosphorus (AP), were important soil environmental factors affecting the abundance of the dominant phyla, and RDA1 and RDA2 accounted for 56.66% and 2.35% of the total variation, respectively. SEM showed that the soil bacterial α-diversity was positively affected by the soil organic carbon (SOC), N:P ratio, and total phosphorus (TP). Moreover, CaKCoS had higher SM, total carbon (TC), total potassium (TK), and AP than CaKCaM and CoSCaM. Collectively, these results highlight a conceptual framework in which the combination of leguminous shrubs can effectively drive soil bacterial diversity by improving soil physicochemical properties and maintaining ecosystem functioning during desertification reversal. Graphical Abstract

The soil microbiome plays a vital role in maintaining soil nutrient levels and ecological stoichiometry balance. However, the relationships between rhizosphere microbiomes and soil ecological stoichiometric characteristics, including organic carbon (SC), total nitrogen (SN), total phosphorus (SP), and their ratios, remain poorly understood. Here, we used a temperate mountain ecosystem as a natural laboratory along a ~ 2190 m elevational gradient spanning a desert steppe-alpine meadow transition. We investigated rhizosphere microbiomes from 20 dominant plant populations across 17 sites by integrating environmental factors, microbial community structure, functional genes, microbial biomass, and ectorhizosphere soil stoichiometric characteristics. Ectorhizosphere soil stoichiometric characteristics were significantly associated with microbial biomass stoichiometric characteristics, rhizosphere community composition, and C-, N-, and P-cycling genes, with functional genes emerging as the strongest predictors. Structural equation modeling further identified the composition and diversity of functional genes as key drivers of soil stoichiometric characteristics. Geographic and edaphic factors exerted primarily direct effects, whereas climatic influences were indirect and mediated through the rhizosphere microbiome. These findings highlight the rhizosphere microbiome as a critical biological filter linking climate to soil nutrient stoichiometry at the root-soil interface.

Astragalus melilotoides Pall. 1776 is a perennial leguminous forage, widely distributed in northern China, with cold, drought and disease resistance characteristics. Here, we determined the complete chloroplast (cp) genome sequence of A. melilotoides. It was 123,827 bp in length and 36.97% GC content with IR loss. The cp genome contained 110 complete genes, including 76 protein-coding genes, 30 tRNA genes, and four rRNA genes. A maximum likelihood (ML) phylogenetic tree revealed that A. melilotoides was related to A. americanus, A. gummifer, A. mongholicus, A. nakaianus, A. mongholicus var. nakaianus, and A. membranaceus var. membranaceus. The cp genome analysis of A. melilotoides will provide a reference for the phylogenetic study of Astragalus in the future.