2.1 Location and materials
The experimental field was located at Dafeng port (33°14′ N, 120° 44′ E), Dafeng district, Yancheng city, on the east coast of Jiangsu province.
The naturally saline experimental field (pH 7.13-8.16) was divided based on different salt content (g salt kg-1 soil) into high salinity (H, 4.1-5.0), moderate salinity (M, 2.9-3.9) and low salinity (L, 1.5-2.0). Different salinity levels had the same initial soil properties, and the soil (silty clay loam) was contained 65% silt and 18% clay.
The grass was mowed before plowing using a conventional moldboard plow. We selected Helianthus tuberosus L. cv. NY-1 (Su-Jian Jerusalem artichoke 200901) as a test material. NY-1 was planted with 0.6 m row spacing and 0.4 m intra-row spacing. No fertilizer and no irrigation was used during the 8-year period. Five replicate plots (5 × 5 m each) were planted in each area (Shao et al., 2019).
Soil was sampled in each replicate plot as follows: control soil (no Jerusalem artichoke planting, no human influence), rhizosphere soil (attached to the root system of Jerusalem artichoke) and bulk soil (away from Jerusalem artichoke roots). One composite sample (containing five soil cores) was taken from each replicate plot by the five-point sampling method (depth of 0-20 cm) using a soil auger (6 cm diameter). All soil samples were packaged in separate sterile plastic bags; a portion of each soil sample was wrapped in tin foil, labeled, and snap-frozen in liquid nitrogen for subsequent DNA extraction and molecular analysis. The remaining soil was air-dried and passed through a 0.15-mm sievs, then stored in a refrigerator at 4℃ for soil biological and chemical analyses (Lauber, Zhou, Gordon, Knight & Fierer, 2010; Tatangelo, Franzetti, Gandolfi, Bestetti & Ambrosini, 2014).
In addition, undisturbed soil columns were collected for the determination of root channels below the plants as well as in the inter-row positions. The sampling tools were rigid PVC pipes (inner diameter of 11 cm, length of 40 cm, wall thickness of 0.2 cm) with a sharp edge at one end for driving into the soil profile. The extracted soil columns were wrapped in plastic for transport to the lab.