2.2 Sample collection
The field experiment was set up in a randomized complete block design with three replications for each planting density. The fertilizer rates used in this trial were N 120 kg/ha, P 34.9 kg/ha and K 0. The area of the plot was 2 m2 (1 m × 2 m) with a 0.5 m border row. The two quinoa transplanting densities were 40 × 25 cm (10 plants/m2) and 20 × 7.5 cm (65 plants/m2), which were denoted low-density (LD) and high-density (HD), respectively. The experimental variety tested was “Yan Suli NO.1”, a quinoa material selected and bred at Xinyang Agricultural Experiment Station, Yancheng City, Jiangsu Province. Sowing of quinoa was done on September 15, 2020; on October 15, quinoa seedlings of similar height and number of leaves were selected for transplanting.
On November 25, 2020, during the quinoa fruit development (Sosa-Zuniga et al., 2017), soil samples were randomly taken at three points in an ”S” mode at one depth (0-10 cm) using soil auger (6 cm diameter) in each of the three replicate plots in each treatment. We collected rhizosphere soil (soil attached to the plant root system) (Shao et al., 2018), non-rhizosphere soil (soil away from plant root system), and blank control soils (from the undisturbed and unplanted area adjacent to the experiment). The soil samples were mixed and placed in separate sterile plastic bags. At the same time, about 10 g of each soil sample was wrapped in tinfoil, labeled, snap-frozen in liquid nitrogen, and returned to the laboratory in an ultra-low temperature refrigerator at -80°C for subsequent analysis of soil microorganisms. The remaining soil samples were air-dried, passed through 0.15-mm (for biological analyses) or 0.05-mm sieves (for chemical analyses) and stored at room temperature.
On November 26, 2020, during the quinoa fruit development (Sosa-Zuniga et al., 2017), six quinoa plants were randomly taken from each plot, divided into four parts (roots, stem, leaves, and panicle), dried in an oven at 75°C, and weighed. A portion of the fresh root system was rinsed with deionized water, blotted with sterile filter paper, placed in plastic centrifuge tubes, labeled, snap-frozen in liquid nitrogen, and stored in an ultra-low temperature refrigerator at -80°C before being returned to the laboratory for subsequent metabolomic analysis.
At plant maturity on January 5, 2021, six randomly taken panicles from each plot were weighed to estimate total yield per plot.