## 4 Discussion
### Domain Archaea  
In terms of Archaea, the phyla Euryarchaeota and Crenarchaeota showed a pattern of higher AB/RA in the rhizosphere microbiome than that in the bulk soil microbiome, and a lower AB/RA in the watering group B (referring to 24 h watering time point) microbiome than that of group A (referring to 0 h watering time point) (Table S18). Phylum Euryarchaeota is known for its ability to produce methane and survive moderate and extreme environmental conditions, while Crenarchaeota is a thermophilic and acidophilic archaeal phylum that colonizes plant roots and mediates influx of important minerals and has possible roles in root biology [44]. AB of the mycorrhizal population, e.g., genus _Rhizophagus_, was proven to correlate with those of euryarchaeotal and crenarchaeotal populations, due to their proven direct association (Figure 8) [44]. Nonetheless, archaeal phyla Euryarchaeota and Crenarchaeota showed higher AB/RA in the rhizosphere microbiomein the present study, while eukaryotic genus _Rhizophagus_ showed contrary results (Tables S18 and S20, respectively). There is no clear explanation to the high AB/RA of genus _Rhizophagus_ in the bulk soil, except that the high AB/RA in the bulk soil was diluted by its high AB/RA 24 h after watering. Moreover, members of the genus _Rhizophagus_ not only associate with microbes in the bulk soil, but also with microbes in the rhizosphere microbiome as a bridge between these microbes (or pathogens) and the plant root (Figure 8). As the two highly abundant archaeal phyla can tolerate adverse environmental conditions, it is expected that watering results in higher AB of the other less water stress-tolerant taxa, which, in turn, might reduce the RA of the two phyla at the 24 h watering time point when water is available.  
The genus _Candidatus Nitroscosmicus_ and its species _Candidatus Nitroscosmicus oleophilus_ of the phylum Thaumarchaeota showed higher AB/RA in the plant rhizosphere microbiome than that in the bulk soil (Table S18). The microbe(s) can carry out both ammonia oxidation and nitrogen fixation, but a recent report indicated that the rate of the first action is much lower than that of the other mesophilic ammonia-oxidizing archaea (AOA) [45]. Moreover, ammonia oxidation only takes place upon increase in temperature [46]. This might result in a lower rate of ammonia oxidation than that of nitrogen fixation. In addition, species of the genus _Nitroscosmicus_ encode beta-1,2-mannosidase, which is a member of the CAZy family involved in the degradation of plant cell wall mannans and the production of sugars. Sugars, in turn, act as substrates for the pyrroloquinoline quinone (PQQ)-dependent glucose/sorbosone dehydrogenases that are further transported into the plant cell to promote central carbon metabolism [47]. This information justifies the high AB/RA of this genus in the rhizosphere microbiome.  
The genera _Nitrososphaera_ (along with its two species _Candidatus Nitrososphaera gargensis_ and _Candidatus Nitrososphaera evergladenis_) and _Candidatus Methanoperedens_ of the phyla Nitrososphaerotis and Euryarchaeota, respectively, showed lower AB/RA in the watering group B microbiome (Table S18). The genus _Nitrososphaera_ usually participates in oxidizing ammonia (AOA) in the ocean, while _Candidatus Methanoperedens_ participates in two contradicting actions, e.g., oxidizing ammonia to produce nitrate and anaerobically oxidize methane (AOM) to reduce nitrate level. Again, it seems that the direction of the reaction depends on the culture which these microbes inhabit. As they usually favor salty water (ocean), non-salty water does not promote their growth. In the present study, they seem to instantly respond. Their response, similar to heat shock, to watering is to reduce their growth rate at the 24 h time point; then, they recover their growth rate afterwards, e.g., after 48 h of watering.