add greenhouse experiment assay and protocol

assays/GreenhouseExperiment/protocols/GreenhouseExperimentProtocol.md

+## Greenhouse experiment
+
+*Lj* Gifu and *At* Col-0 were grown for 5 weeks in CAS soil (batch 15 from January 2020) in 7 × 7 cm pots alongside unplanted control pots under short-day conditions. Pots were watered with sterile water from the bottom as needed. Root, rhizosphere and soil samples were harvested and processed as described previously (Thiergart, T. et al., 2020). In total, 15, 13 and eight replicates were sampled for Col-0, Gifu and unplanted controls, respectively. DNA was isolated from those samples using the MP Biomedicals FastDNA Spin Kit for Soil.
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