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Free‐living N2 fixation (FNF) is an important source of bioavailable nitrogen for forests. Though increasing plant species diversity (PSD) benefits soil nitrogen accumulation, how it impacts FNF rate has not been explored.Forty‐five plots covering a PSD gradient were selected in a subtropical karst forest, southwest China. FNF rates in leaf litter and soil were measured using acetylene reduction assay calibrated with a 15N2 fixation method. Diazotrophic community, litter and soil properties were determined as well.The nifH gene abundance was not significantly altered in litter but was significantly decreased in soil by higher PSD. Diazotrophic community's Shannon diversity was significantly increased by higher PSD in both litter and soil. Increasing PSD inhibited litter FNF rate via aggravating phosphorus limitation and decreasing the relative abundance of Betaproteobacteria and Deltaproteobacteria. For the mineral soil horizon, increasing PSD suppressed FNF rate via decreasing nifH gene abundance caused by elevated N availability and lowered phosphorus and iron availability or through decreasing the relative abundance of Deltaproteobacteria but increasing that of Betaproteobacteria.Our findings, for the first time, reveal the microbial and abiotic controls on FNF in litter and soil in response to PSD, and hence benefit the prediction of nitrogen input via biological N2 fixation under changes in PSD. Read the free Plain Language Summary for this article on the Journal blog. [ABSTRACT FROM AUTHOR]

In China, the total area of rocky desertification covers 1.01x107 hm², accounting for 22.3% of the karst area. Nowadays, rocky desertification has become one of the three major land degradation problems in our country, which has seriously affected our economic development and environmental governance. Naturally, rock weathering in the karst area is so long that it takes from a few decades to a hundred years to form a centimeter think of red soil. Because the karst area is rich in calcium and barren in soil, ecological restoration of this area is urgent in both rocky desertification control and karst carbon sink. Carbonic Anhydrase (CA), one of the fastest catalyzing enzymes, can promote the weathering of carbonate rocks by accelerating the hydration reaction of CO2, thus providing soil matrices for plant colonization. At the same time, CA can also be used to absorb CO2 in the atmosphere to fix the carbon source, thereby participating in the carbon cycle process of karst dynamic system. The CA-producing microorganisms are considered to have good application prospects in the restoration of degraded karst habitats. However, for the target strains isolated from favorable habitat, their weak adaptability to the extreme environment may constrain them from exerting their expected effects. In order to select the strain with high CA activity in the extremely degraded karst habitat, CA-producing strains were isolated from the samples of severely degraded rocky karst habitats in this study. The study area is located in Hechi City, Guangxi Zhuang (23°41'--25°37' N, 106°34'-109°09' E). It has a subtropical monsoon climate, with an average annual temperature of 16.9-21.5 °C. The area is typically developed with karst, scattered with thin soil and highly exposed with rocks. From September to October 2021, four different habitat samples were collected, including lichen in the area with extreme rocky desertification weathered materials under moss, soil under moss, and weathered materials under moss in karst native forest. The source of isolation was a mixture of the four different habitat samples, 1g of each. CA-producing strains were isolated and screened from karst rocky habitats by inoculating suspension of isolation source in calcium carbonate medium with a sprayer. The field emission scanning electron microscope was used for morphological identification of the strain. The physiological and biochemical characteristics of the strain were confirmed by carbohydrate decomposition test, V-P test, methyl red test, citrate utilization test, nitrate reduction test, starch hydrolysis test and contact enzyme test. Strains were identified by 16S rRNA sequence analysis. The CA activity of single strain and mixed strain was determined by electrode method. The sequence alignment was performed in the National Center for Biotechnology Information Database. In MEGA6, Neighbor-Joining method is used to construct the phylogenetic tree. SPSS software was used to analyze whether there were significant differences in CA activity among different strains/microflora. The results show that six CA-producing strains were isolated by calcium carbonate medium, namely, Flavobacterium resistens, Delftia acidovorans, Stenotrophomonas rhizophila, Pseudomonas oleovorans, Agrobacterium cavarae and Bacillus albus. The CA activity of Stenotrophomonas rhizophila was the highest, up to 4.27 U½mL-1½OD600-1, while that of Bacillus albus was only 0.46 U½mL-1½OD600-1, which indicated that although CA commonly existed in prokaryotes, the activity of different species of bacteria varied greatly. Mixed culturing of Stenotrophomonas rhizophila whose CA activity is high with other strains led to a decrease in CA activity, indicating the great potential application of Stenotrophomonas rhizophila in the extremely oligotrophic environment. The results provide a new bacterial source for ecological restoration of extremely degraded karst habitats. However, all the CA activity decreased when the isolated Stenotrophomonas rhizophila strain N6 with high CA activity is mixed with other 5 strains. Because of the complex living environment, the competition of indigenous microbial community will reduce the survival efficiency of the added microorganisms, make it difficult to form a stable community structure, and then reduce the repair effect. In restoration of degraded karst habitat, whether there is a kind of dominant bacteria that is symbiotic with Stenotrophomonas rhizophila N6 to jointly promote carbonate dissolution is a research focus. Therefore, future studies should identify the core species that drive the restoration of degraded habitats, and explore the relationship between the isolated and culturable CA dominant bacteria and the core species. These study focuses will promote the research and development of biological fertilizer for the rapid restoration of degraded karst habitats. [ABSTRACT FROM AUTHOR]

Considering the crucial role of soil diazotrophs and root arbuscular mycorrhizal fungi (AMF) in soil nutrient cycling during ecosystem restoration, diazotroph and AMF communities may be determined by slope position. However, the effect of slope position on diazotroph and AMF abundance, diversity, and community composition of karst ecosystems remains unknown. In this study, soil diazotrophs and root AMF characteristics on varying slope positions were assessed in a karst shrub ecosystem. The results displayed that the abundance of soil diazotrophs and root AMF diversity were significantly affected by slope position. Diazotroph abundance accompanied by soil nutrient and plant richness was higher on the lower slopes than the upper slopes, whereas root AMF diversity displayed the opposite trend. The soil diazotroph and root AMF community composition differed among the upper, middle, and lower slopes. The dominant taxa of soil diazotrophs and root AMF at the order level were Rhizobiales and Glomerales, respectively. Moreover, the diazotroph order of Nostocales and the AMF order of Paraglomerales were richer on the upper slopes than on the lower slopes. The slope position directly affected the plant diversity and soil nutrient distribution, indirectly affecting the diazotroph and AMF communities. Increased available nitrogen on the lower slope caused great diazotroph abundance by stimulating plant growth with sufficient carbohydrates. However, low soil nutrients and plant diversity but high plant root biomass induced more root AMF diversity on the upper slope than on the lower slope. Therefore, this study expands the knowledge of soil diazotroph and root AMF ecological functions along different slope positions during vegetation recovery for the successive stages of grass and shrub in the karst region. [ABSTRACT FROM AUTHOR]

Sugarcane is the most important sugar crop. Sugarcane smut is one of the major diseases, which could reduce sugarcane yield and quality and seriously threaten the sustainable and healthy development of sugar industry. Microbial control of sugarcane smut is a rapidly emerging green biocontrol technology, with advantage to increase environmental compatibility and soil fertility. In this review, we briefly described the characteristics of Sporisorium scitamineum which causes sugarcane smut, synthesized the mechanisms underlying the infection of sugarcane by S. scitamineum, and presented the research status of microbial controls of sugarcane smut via the application of bio-organic fertilizers and biopesticides. We then reviewed the mechanisms underlying the suppression of sugarcane smut by microorganisms through competition with pathogens for nutrients and ecological niches, secreting antagonistic substances, and improving plant resistance. It is notable that there are still some problems in the application of microbial control technologies, including poor colonization ability and unstable biocontrol efficiency. Finally, the major directions of future research on the biocontrol of sugarcane smut were proposed from the perspective of improving the biocontrol efficiency. This review would benefit the microbial control of sugarcane smut and the healthy development of sugar industry. [ABSTRACT FROM AUTHOR]