2.2 | Phase I: Training Phase
In the training phase of the experiment individuals are grown in potting mix to condition (or train) soils for use in the feedback phase. In February 2020 four shallow trays were filled with sterilized sand. Sand was steam sterilized in a pressurized autoclave at 121 ºC for ~60 min, cooled and then sterilized for an additional cycle (e.g. Crawford & Knight, 2017). Each tray was sown with a monoculture of A. gerardi , S. scoparium, B. inermis , orP. smithii . All seeds were purchased from OPN Seed, Ohio, USA. In early March 2020, 30 seedlings (mean grass height ~ 5 cm) of each species were transplanted into 5.6 L pots of common potting mix (120 pots total). Plants were grown in a greenhouse and received auxiliary lighting in the evening hours to promote growth. Ten randomly selected pots of each grass species were harvested in mid-June following ~16 weeks of growth. In addition, we randomly selected ten pots from a pool of ~18-month-old redcedars that had been growing in 5.6 L pots in the same greenhouse for the previous ten-months. Grass and tree samples were clipped at the root collar and aboveground biomass was dried in a 65 ºC oven and weighed. Training soils were separated from root materials manually by running material through a 2 mm sieve. Half of the soil (> 2 L) collected from each sample was set-aside for sterilization in an autoclave. Each pot was processed individually, and all materials used in processing were sterilized with an alcohol solution in-between each sample. This procedure was established to prevent the transfer of soil particles and microbes between samples.