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.