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.

土壤含水量（Soil water content, SWC）采用烘干法测定；土壤容重（soil bulk density, BD）采用环刀法测定，最大持水量（maximum water-holding capacity, MWHC）、毛管持水量（capillary water-holding capacity, CWHC）和毛管孔隙度（capillary porosity, CP）据此计算得到。土壤pH值采用PHS-320型高精度智能pH计，以土水比2.5:1进行测定；电导率（electrical conductivity, EC）采用DDS-608型多功能电导率仪，以土水比5:1进行测定。土壤化学性质参照《土壤农业化学分析》（鲍士旦，2000）进行分析，其中土壤有机质（soil organic matter, SOM）采用重铬酸钾氧化-外加热法测定；全氮（total nitrogen, TN）和全磷（total phosphorus, TP）采用德国产全自动离散化学分析仪进行分析；碱解氮（alkaline hydrolysis nitrogen, AHN）采用碱扩散法测定；速效钾（available potassium, AK）采用NH4OAc浸提-火焰光度法测定；速效磷（available phosphorus, AP）采用0.5 mol/L NaHCO3浸提-钼蓝比色法测定；铵态氮（ammonium nitrogen, AN）和硝态氮（nitrate nitrogen, NN）采用KCl交换法浸提，靛酚蓝比色法测定；碱性磷酸酶（alkaline phosphatase, ALP）活性采用磷酸苯二钠比色法测定；脲酶活性采用靛酚蓝比色法测定；过氧化氢酶（catalase, CAT）活性采用分光光度法分析。土壤线虫采用贝尔曼漏斗法（Baermann funnel method）提取，次日以干冰运输至派森诺生物科技有限公司（Paisennuo Biotechnology Co., Ltd.）进行DNA提取与鉴定；该公司采用FastDNA™ Spin Kit for Soil（美国俄亥俄州索伦市MP Biomedicals公司，货号116560200）完成DNA提取，使用引物NF1（5'-GGTGGTGCATGGCCGTTCTTAGTT-3'）和18Sr2b（5'-TACAAAGGGCAGGGACGTAAT-3'）进行扩增以保证扩增准确性与效率（Quast等，2012）；扩增程序包括：预变性3 min，30个循环（98℃变性30 s、55℃退火30 s、72℃延伸45 s），最终72℃延伸5 min；PCR产物采用2%琼脂糖凝胶电泳回收、纯化并洗脱，经2%琼脂糖电泳检测；随后在派森诺公司的Illumina平台上完成群落DNA片段的双端测序。高通量测序数据处理环节：将未抽平的OTU序列转换为属水平相对丰度，据此计算香农指数（Shannon index）、辛普森指数（Simpson index）、皮卢指数（Pielou index）及Chao1指数。