Data on physical, chemical and biological indicators of soil at different rainfall gradients in the loess hilly area

Soil water content (SWC) was determined using the oven-drying method; soil bulk density (BD) was measured by the cutting ring method, and maximum water-holding capacity (MWHC), capillary water-holding capacity (CWHC), and capillary porosity (CP) were calculated accordingly; soil pH was measured with a PHS-320 high-precision intelligent pH meter at a soil-to-water ratio of 2.5:1; electrical conductivity (EC) was determined using a DDS-608 multifunctional conductivity meter at a soil-to-water ratio of 5:1; soil chemical properties were analyzed according to "Soil and Agricultural Chemistry Analysis" (Bao, 2000), with soil organic matter (SOM) measured by the potassium dichromate oxidation-external heating method; total nitrogen (TN) and total phosphorus (TP) were analyzed using a fully automated discrete chemical analyzer (Germany); alkaline hydrolysis nitrogen (AHN) was determined by the alkaline diffusion method; available potassium (AK) was measured by NH4OAc extraction-flame photometry; available phosphorus (AP) was determined by 0.5 mol/L NaHCO3 extraction-molybdenum blue colorimetry; ammonium nitrogen (AN) and nitrate nitrogen (NN) were extracted using the KCl exchange method and determined by indophenol blue colorimetry; alkaline phosphatase (ALP) activity was measured using disodium phenyl phosphate colorimetry; urease activity was determined by indophenol blue colorimetry; catalase (CAT) activity was analyzed by spectrophotometry; soil nematodes were extracted using the Baermann funnel method and shipped on dry ice to Paisennuo Biotechnology Co., Ltd. for DNA extraction and identification the next day; the company performed DNA extraction using the FastDNA™ Spin Kit for Soil (MP Biomedicals, Solon, OH, USA, Cat.116560200), with primers NF1 (5'-GGTGGTGCATGGCCGTTCTTAGTT-3') and 18Sr2b (5'TACAAAGGGCAGGGACGT AAT-3') used for amplification to ensure accuracy and efficiency (Quast et al., 2012); the amplification program included pre-denaturation for 3 min, 30 cycles (98°C denaturation for 30 s, 55°C annealing for 30 s, 72°C extension for 45 s), followed by a final extension at 72°C for 5 min; PCR products were recovered using 2% agarose gel electrophoresis, purified, and eluted, with detection by 2% agarose electrophoresis; finally, paired-end sequencing of community DNA fragments was performed on the Illumina platform at Paisennuo Company; high-throughput sequencing data were processed by converting unrarefied OTU sequences into relative abundances at the genus level, enabling the calculation of the Shannon index, Simpson index, Pielou index, and Chao1 index.