Physicochemical Properties and Nitrogen-Fixing Bacterial Dataset of Six Casuarina equisetifolia Cultivars (Xingwang, Fada, Qiantou, Ping’an, Ruyifeng, and Jixianglong) in Chihu State Protective Forest Farm, Hui’an County, Fujian Province, China

Three biological replicate plots (2 m × 2 m) were established for each cultivar. Within each plot, three to four soil subsamples were collected from the 0–30 cm soil layer using an S-shaped sampling strategy and then mixed into one composite sample. A total of 18 soil samples were obtained.Each soil sample was divided into three portions:stored at −80 °C for DNA extraction and nitrogen-fixing bacterial functional gene analysis;stored at 4 °C for soil enzyme activity determination;air-dried and sieved for soil physicochemical analyses.(2) Determination of soil physicochemical propertiesThe measured variables included pH, salinity, moisture content, bulk density, soil organic matter, soil particle-size distribution (clay, silt, and sand), N-acetyl-β-D-glucosaminidase (NAG), and leucine aminopeptidase (LAP).The main instruments used were a pH meter (IQ Instruments, USA), a salinity meter (Eutech Instruments Salt-6, USA), a laser particle size analyzer (Mastersizer 2000, Malvern, UK), and a portable conductivity meter (2265FS, Spectrum, USA).Soil organic matter was determined using the loss-on-ignition method at 550 °C. NAG and LAP activities were determined using a fluorometric assay.(3) Determination and processing of nitrogen-fixing bacterial dataTotal soil DNA was extracted using the OMEGA Soil DNA Kit.

The nifH gene, a functional marker gene for nitrogen fixation, was amplified by PCR and sequenced using the Illumina NovaSeq 6000 platform.After quality control and sequence processing, the following datasets were obtained:nitrogen-fixing bacterial abundance (nifH gene abundance);alpha diversity indices (Chao1, Observed species, Shannon, and Simpson);genus-level relative abundance of nitrogen-fixing bacteria.Nitrogen-fixing bacterial abundance was quantified by quantitative real-time PCR (qPCR).2.

Dataset ContentsThe dataset contains one Excel file with six worksheets.Sheet 1: Environmental factorsNumber of records: 18.Contents: Soil bulk density (g·cm-3), moisture content (%), soil organic matter (%), salinity, and pH.Row labels: Sample IDs (cultivar abbreviation + replicate number, e.g., XW1).Column labels: Soil environmental variables.Sheet 2: Soil textureNumber of records: 18.Contents: Percentages of clay, silt, and sand (%).Sheet 3: Soil enzyme activities (NAG and LAP)Number of records: 18.Contents:N-acetyl-β-D-glucosaminidase (NAG)Leucine aminopeptidase (LAP)Units: nmol·g-1·h-1.Sheet 4: Nitrogen-fixing bacterial abundanceNumber of records: 18.Contents: Absolute abundance of the nifH gene.Unit: copies·g-1 soil.Sheet 5: Microbial alpha diversityNumber of records: 18.Contents:• Chao1• Observed species• Shannon• SimpsonThese indices were used to characterize the richness and diversity of nitrogen-fixing bacterial communities.Sheet 6: Genus-level relative abundance of nitrogen-fixing bacteriaNumber of records: 18 samples and 11 dominant genera.Contents: Relative abundance of nitrogen-fixing bacterial genera across all samples.The dominant genera include Bradyrhizobium, Desulfuromonas, Pelomonas, Burkholderia, Azospirillum, Ideonella, Halorhodospira, Citrifermentans, Mesorhizobium, Geobacter, and Others.3.

Data Uncertainty and Error SourcesPotential sources of uncertainty mainly include soil spatial heterogeneity, instrumental errors during physicochemical and enzyme activity measurements, experimental errors introduced during DNA extraction and high-throughput sequencing, and natural variation among biological replicates.To minimize these uncertainties, three biological replicates were established for each C. equisetifolia cultivar, and standardized sampling, laboratory procedures, and data processing workflows were adopted to ensure the reliability and reproducibility of the dataset.