Phase II: Community response to soil legacy and drought
Bulk soil was collected at the Heishiding nature reserve and sterilized by gamma-irradiation at 25kGy. Gamma irradiation has been shown to effectively sterilize soil with minimal impacts on other soil properties (Berns et al. 2008). Mesocosms (21cm diameter, 17cm height) were filled with 70% (by volume) sterilized bulk soil, 10% conditioned soil and 20% sterilized bulk soil. This approach allowed us to reduce the impacts of differences in soil abiotic properties on plant growth, and explore the role of soil microorganisms (Smith-Ramesh & Reynolds 2017). Two-week-old seedlings were transplanted into mesocosms on May 13th 2020, and the plant community compositions were consistent with those that provided soil inoculum in Phase I. Plants that failed during the first week were replaced. During the first three weeks, all mesocosms again received 600 ml water every four days.
On June 2nd 2020, soil moisture treatments were established. The water amounts used in ambient and drought treatments was consistent with those in Phase I. There were four soil moisture regimes over Phase I and II, namely, ambient treatments in both phases (A-A), drought treatments in both phases (D-D), ambient in Phase I + drought in Phase II (A-D) and drought in Phase I + ambient in Phase II (D-A) (Fig. 1). In total, there were 5 plant richness levels × 2 soil moistures in Phase I × 2 soil moistures in Phase II × 12 replicates = 240 mesocosms. During the period of experiment, volumetric soil moisture content at 5 cm depth was monitored regularly every two weeks by GS-3 soil moisture probe (Decagon Devices, Inc. WA). Over the period of Phase II, the soil moisture treatments caused significantly different soil moisture contents among mesocosms (Table S2, Fig. S1b). We randomized the mesocosms every two weeks. Plants were harvested from August 5th to 11th 2020 when all species were in their reproductive stages. Aboveground biomass was sorted by species. Belowground biomass was harvested at the mesocosm level, because root systems cannot be clearly sorted out to individual species. Plant materials were oven-dried at 60 ℃ for 48 h and weighted to determine biomass.