Study site description, sampling design and plant growth response
measurement
During summer 2015 we conducted a field study at the University of
Alberta’s Mattheis Ranch, located in southern Alberta and at the
northern edge of the Great Plains (50°53′29.9″N 111°55′07.4″W). The
vegetation was a semi‐arid mixed grassland dominated by Koeleria
macrantha (Ledeb.) Schult, Hesperostipa comata (Trin. & Rupr.)
Barkworth, Pascopyron smithii (Rydb.) Á Löve and Bromus
inermis Leyss. (Shorthouse, 2010).
To evaluate plant-soil feedbacks in this system, we sampled 49
10-cm-diameter soil cores in 10 different plots, for a total of 490 soil
samples. Details on the soil sampling methods can be found in Chagnonet al. (2018). These soils were used as living inocula in a
greenhouse experiment, where Bromus or Koeleria were grown
in a 9:1 mixture of sterile soil and living inoculum. The sterile soil
consisted of a 1:1 mixture of a sandy soil collected at our study site,
and fine sand. Using such a design, any growth response by Bromusor Koeleria was expected to be caused largely by soil biota
(Brinkman et al. , 2010). After 15 weeks of growth, plants were
harvested and total (shoot + root) dry mass was used as a measure of
plant growth response to soil inocula (i.e., PSF). The remaining methods
described below are new to this paper, and were not part of the original
publication (Chagnon et al. , 2018).
To identify soil microorganisms potentially generating plant growth
responses in our experiment, we combined the data generated from the
previously described greenhouse experiment with targeted soil DNA
sequencing. Of our 490 initial soil living inocula, we selected 230
inocula that would also be analyzed for microbial community structure,
representing 23 samples in each of our 10 plots. The 23 inocula were
chosen in a way that maximized their spatial dispersion in all four
quadrants of the plot (Fig. S1). The field soil subsamples had been kept
at -20C until DNA extraction, which was within 2 months after soil
collection.