Phase I: Conditioning soil
We performed a soil conditioning phase of the experiment to obtain specified soil communities of different plant communities at the Heishiding nature reserve in Fengkai (23.46° N, 111.90° E). Plant communities with different species richness levels were established in the collected field soil and subjected to ambient or drought treatments. Five plant diversity treatments were established, including monoculture, two, four, six and twelve species mixtures. Each of the twelve species used in this experiment were planted as monocultures, and each monoculture was replicated two times. For the two, four, or six species mixtures, species compositions were determined using separate random draws from the twelve species pool (Table S1), and each drawn species composition was replicated twice. In total, this experiment consisted of 5 plant diversity levels × 2 moisture treatments × 12 replicates = 120 plant communities. Each community consisted of twelve individual plants, and all plant species had equal density in mixtures.
Seeds were first surface sterilized by rinsing in 75% ethanol for 2 minutes and then germinated in sterilized vermiculite. On April 28th 2019, twelve two-week-old seedlings were transplanted into each mesocosm (24 cm diameter, 19 cm height), and plants that failed during the first week were replanted. All mesocosms were regularly watered with 600 ml every four days for the first six weeks to avoid drought stress. On June 15th, half of the mesocosms continued to be watered regularly (i.e., ambient treatment), and the remaining ones received one third of water amount of the ambient treatment (the drought treatment). Mesocosm location was randomized monthly to avoid position effects.
Volumetric soil moisture content at 5 cm depth was monitored by GS-3 soil moisture probe (Decagon Devices, Inc. WA) monthly to make sure soil moisture treatments were effective. Over the period of Phase I, the soil moisture treatments caused significantly different soil moisture contents among mesocosms (Table S2, Fig. S1a). Plants were grown for 18 weeks. In the beginning of September 2019 when all species were in their reproductive stages, plants were removed in all mesocosms. Soil was thoroughly homogenized for each mesocosm, and a subsample was collected and stored at -80 oC for soil microbial DNA sequencing. For the twelve species treatment, we randomly selected five mesocosms for microbial analyses in each moisture treatment. A subsample of 30% (by volume) mesocosm soil was stored at -20 ℃ as soil inoculum prior to Phase II.