If you make a artificial environment having high concentration of CO2 and controls the amount of CO2 release in chamber so you can study the effect on crop as well as soil nutrition. For this purpose i feel open top chamber is suitable. Some studies related to this was conducted in India. You can search Dr Pratap Bhhattachharya articles for detailed information.

Mohammad Ibrahim , it is a very nice question , and indeed the most talked about in the context of global warming . Global warming/climate change surely brings distinct changes in soil microbial diversity . Infact , compositional , structural and functional diversity of microbes within the rhizosphere undergo colossal reorientation in response to global warming . This is one of the prime reasons for unprecedented loss in plant diversity due to such micro- level changes in growing environment of the plants . What popularly  we call loss in diversity due to  biotic change  in site characteristics. Many of the  biosphere reserves are so stable simply because of microbial population of the soil environment .  But , unfortunately , very little has  been done in this context .  Lets start concentrating on this important issue , better than never .  

The impacts of extreme climate events extend more to soil microorganisms.

In considering this going by your question, as other colleagues have stressed, it depends on the region or the area you are at researching because in a more arid environment, a peculiar event of soil faunal extinction will occur quite different from other regions without aridity. 

Also when you have identified the important climate event of that area, you establish and itemize close variables as you have mentioned that will form the elements of the research that could help you generate your hypothesis. Then, you go by experimental approach and survey on soil ecosystem depending on your scope. But whichever way, soil micro biology design must be adopted to help you arrive at an empirical conclusion. Thanks