2.2. Soil collection and analyses

We collected soil samples from 81 pairs of uninvaded versus invaded plots in southern China to allow them to represent diverse soil regimes. Field soil is fit for addressing PSF in natural conditions, and has been widely used in previous studies (Brinkman, Van der Putten, Bakker, & Verhoeven, 2010; Heinze et al., 2016; van der Putten et al., 1993; Rutten, Prati, Hemp, & Fischer, 2016). In each plot, five soil samples were taken from the rhizospheres of Solidago (i.e., invaded plots) or from the top 10 cm of the soil profile in native plant communities (i.e., uninvaded plots), and then composited as a soil sample. Each soil sample was further divided into two portions: one for determining the following abiotic properties and soil microbes, and the other for a bioassay experiment.

For soil abiotic properties, pH was determined in a soil solution of 1:2.5 (soil:distilled water) using a pH meter (Sartorius PB‐10 meter); organic carbon (OC) was determined using the potassium dichromate oxidation method; total nitrogen (TN) was determined using the Kjeldahl apparatus (FOSS 2200); available phosphorus (AP) was determined using a UV‐2550 ultraviolet spectrophotometer; and ammonia (NH₄‐N) and nitrate (NO₃‐N) were determined using a continuous flow analyzer (Dong, Sun, et al., 2015). Soil texture was determined using a laser particle size analyzer (Mastersizer, 2000).

For soil microbes, we employed phospholipid fatty acid (PLFA) analysis (Dong, Yu, et al., 2015). The fatty acids chosen to represent fungi were 18:2ω6,9c and 16:1ω5c, to represent bacteria were i14:0, 14:0, i15:0, a15:0, 15:0, a16:0, i16:0, 16:0, 16:1ω7c, 16:1ω9c, i17:0, a17:0, 17:0, cyl7:0, 18:0, 18:1ω5c, 18:1ω7c, and cyl9:0, and to represent actinomyces were 10Me16:0, 10Me18:0, and 10Me20:0. The ratio of fungi to bacteria was calculated (Bossio & Scow, 1998; Larsen & Bodker, 2001).