nifH Gene Sequencing Reveals the Effects of Successive Monoculture on the Soil Diazotrophic Microbial Community in Casuarina equisetifolia Plantations

Effects of continuous monoculture on rhizosphere soil nutrients, growth, physiological characteristics, hormone metabolome of Casuarina equisetifolia and their interaction analysis

Continuous planting is unavoidable in agricultural production, but continuous planting affects plant growth and physiological characteristics. In this study, we analyzed rhizosphere soil nutrients, physiological characteristics, hormone metabolome changes and their interactions of Casuarina equisetifolia (C. equisetifolia) with the increase of continuous planting number. The results found that C. equisetifolia root was significantly inhibited, the plant height was dwarfed and the biomass was significantly reduced as continuous planting number increased. Secondly, continuous planting caused a decrease in the rhizosphere soil nutrient transformation capacity, and a significant decrease in the total soil nutrient and available nutrient content. Analysis of physiological indexes showed that continuous planting resulted in a decrease in nitrogen, phosphorus, and potassium content, a decrease in the activity of physiological indexes of resistance, and a decrease in photosynthetic capacity of C. equisetifolia leaves. Hormone metabolome analysis showed that continuous planting critically affected the accumulation of five characteristic hormones in C. equisetifolia leaves, in which salicylic acid 2-O-β-glucoside (SAG), 2-oxindole-3-acetic acid (OxIAA), trans-zeatin-O-glucoside (tZOG) and gibberellin A3 (GA3) content decreased significantly while abscisic acid (ABA) content increased significantly. In conclusion, continuous planting lowered the rhizosphere soil nutrient transformation capacity of C. equisetifolia, lowered the soil available nutrient content, inhibited their root growth, and hindered the nutrient uptake and transportation by the root, thus led to the decrease of the nutrient accumulation capacity in the leaves of C. equisetifolia, and the decrease of SAG, OxIAA, and tZOG, GA3 synthesis ability decreased, ABA accumulated in large quantities, C. equisetifolia resistance and photosynthesis ability decreased, and their growth was impeded. This study provides insights for the effective management of continuous planting in the cultivation of C. equisetifolia.

Effect of continuous planting on Casuarina equisetifolia rhizosphere soil physicochemical indexes, microbial functional diversity and metabolites

Continuous planting has a severe impact on the growth of Casuarina equisetifolia. In this study, the effects of three different long-term monocultures (one, two and three replanting) on the physicochemical indexes, microbial functional diversity, and soil metabolomics were analyzed in C. equisetifolia rhizosphere soil. The results showed that rhizosphere soil organic matter content, cation exchange capacity, total and available nitrogen, total and available phosphorus, and total and available potassium contents significantly decreased with the increasing number of continuous plantings. The evaluation of microbial functional diversity revealed a reduction in the number of soil microorganisms that rely on carbohydrates for carbon sources and an increase in soil microorganisms that used phenolic acid, carboxylic acid, fatty acid, and amines as carbon sources. Soil metabolomics analysis showed a significant decrease in soil carbohydrate content and a significant accumulation of autotoxic acid, amine, and lipid in the C. equisetifolia rhizosphere soil. Consequently, the growth of C. equisetifolia could hinder total nutrient content and their availability. Thus, valuable insights for managing the cultivation of C. equisetifolia and soil remediation were provided.

Metagenomics-based exploration of key soil microorganisms contributing to continuously planted Casuarina equisetifolia growth inhibition and their interactions with soil nutrient transformation

Casuarina equisetifolia (C. equisetifolia) is an economically important forest tree species, often cultivated in continuous monoculture as a coastal protection forest. Continuous planting has gradually affected growth and severely restricted the sustainable development of the C. equisetifolia industry. In this study, we analyzed the effects of continuous planting on C. equisetifolia growth and explored the rhizosphere soil microecological mechanism from a metagenomic perspective. The results showed that continuous planting resulted in dwarfing, shorter root length, and reduced C. equisetifolia seedling root system. Metagenomics analysis showed that 10 key characteristic microorganisms, mainly Actinoallomurus, Actinomadura, and Mycobacterium, were responsible for continuously planted C. equisetifolia trees. Quantitative analysis showed that the number of microorganisms in these three genera decreased significantly with the increase of continuous planting. Gene function analysis showed that continuous planting led to the weakening of the environmental information processing-signal transduction ability of soil characteristic microorganisms, and the decrease of C. equisetifolia trees against stress. Reduced capacity for metabolism, genetic information processing-replication and repair resulted in reduced microbial propagation and reduced microbial quantity in the rhizosphere soil of C. equisetifolia trees. Secondly, amino acid metabolism, carbohydrate metabolism, glycan biosynthesis and metabolism, lipid metabolism, metabolism of cofactors and vitamins were all significantly reduced, resulting in a decrease in the ability of the soil to synthesize and metabolize carbon and nitrogen. These reduced capacities further led to reduced soil microbial quantity, microbial carbon and nitrogen, microbial respiration intensity, reduced soil enzyme nutrient cycling and resistance-related enzyme activities, a significant reduction in available nutrient content of rhizosphere soils, a reduction in the ion exchange capacity, and an impediment to C. equisetifolia growth. This study provides an important basis for the management of continuously planted C. equisetifolia plantations.

Global diversity and distribution of nitrogen-fixing bacteria in the soil

Our knowledge of microbial biogeography has advanced in recent years, yet we lack knowledge of the global diversity of some important functional groups. Here, we used environmental DNA from 327 globally collected soil samples to investigate the biodiversity patterns of nitrogen-fixing bacteria by focusing on the nifH gene but also amplifying the general prokaryotic 16S SSU region. Globally, N-fixing prokaryotic communities are driven mainly by climatic conditions, with most groups being positively correlated with stable hot or seasonally humid climates. Among soil parameters, pH, but also soil N content were most often shown to correlate with the diversity of N-fixer groups. However, specific groups of N-fixing prokaryotes show contrasting responses to the same variables, notably in Cyanobacteria that were negatively correlated with stable hot climates, and showed a U-shaped correlation with soil pH, contrary to other N-fixers. Also, the non-N-fixing prokaryotic community composition was differentially correlated with the diversity and abundance of N-fixer groups, showing the often-neglected impact of biotic interactions among bacteria.

Depth-dependent influence of biochar application on the abundance and community structure of diazotrophic under sugarcane growth

Despite progress in understanding diazotrophic distribution in surface soils, few studies have investigated the distribution of diazotrophic bacteria in deeper soil layers. Here, we leveraged high-throughput sequencing (HTS) of nifH genes obtained to assess the influence of biochar amended soil (BC) and control (CK), and soil depths (0–20, 20–40 and 40–60 cm) on diazotrophic abundance and community structures, soil enzyme activities and physio-chemical properties. Multivariate ANOVA analysis revealed that soil depth had profound impact on majority of the soil parameters measured than fertilization. Although soil physio-chemical properties, enzymes activities, diazotrophic genera and enriched operational taxonomic units (OTUs) were significantly influenced across the entire soil profiles, we also observed that BC amended soil significantly increased cane stalk height and weight, nitrate (NO₃^-), ammonium (NH₄⁺), organic matter (OM), total carbon (TC) and available potassium (AK), and enhanced diazotrophic genera in soil depth 0–20 cm compared to CK treatment. Soil TC, total nitrogen (TN), OM and NH₄⁺ were the major impact factors shifting diazotrophic community structures in soil depth 0–20 cm. Overall, these results were more pronounced in 0–20 cm soil depth in BC than CK treatment.