As for soil-C it depends on the status of the soil-C, whether it is decaying with large amount of potentially available-C. In the case of a steady and undisturbed system much of the C will be in undissolved status and hence the changes to total-C following a rainfall will be minimal.
As for the pH, again most soil pH is buffered against any fluctuations unless the rainfall contains acidic components.
Thank you. But what can be the reason of increase in pH after rain?
Just to add some inputs to Muhammad's initial response and to those of others. The impact of rainfall on soil physical and chemical characteristics depend on several factors, such as the chemical/physical states of the soil in question, land cover/soil cover, soil temperature variations across the field, composition of the rainwater (whether or not it is acidic or alkaline), soil water characteristics, etc. More specifically, changes in soil C and pH values would vary across the field even within 1m, thus, it can not be concluded that these suggested variations would be potentially the same or assumed to be same across the field. The changes would be different either more or less and that would depend on how randomized the data you collect are. Also, external factors such as wind speed and direction could influence the uniformity of rainfall distribution across the land.
Short term displacement of oxygen from the pore spaces and switch to anaerobic fermentation processes will infuse CO2 and organic acids into the pore space waters. Speciation of the contributing acids would be useful to use for following this ( ion exchange chromatography, and micro titrimetry). As the air reenters the pore space, a rapid growth of organisms will also give rise to metabolic CO2. Stable isotope C should reflect some of these metabolic changes and pool dilutions/ enrichments.
So when will you measure? The answers by Sid and Jenkins give the overall idea what can happen. Rain will influence the redox conditions, which are variatble in space and time. You would need to be more precise on what you think is the organic matter in your system, and define the time scale you are interested in.
You may wish to decide the extent of detail of your study. Do you wish to assess C dynamics in detail and pinpoint C changes by biological process (microbial interaction) and physical process (e.g. leaching)? As Michel mentioned determination of a suitable timescale is important as well.
Also, be mindful of the chemical processes. As colleagues mentioned the redox reactions. Again, it depends on how detailed the study is. Regardless of the extent of the study, what is fundamental to soil pH changes is soil's buffer capacity against pH changes. For example, organic acids produced by microbial activity may not change soil pH markedly in well buffered soils, meaning, acid production may not necessarily translate into corresponding pH changes. Equally, a small amount of weak acids could change pH in soils with low buffer capacity. You may wish to consider a range of soil types with different buffer capacity. Generally, well weathered tropical soils (e.g. kaolinites) are less buffered against pH changes.
Please try and keep the variables low. Otherwise, it will be a very complex project. However, if it is a very detailed work, complexity is complementary.
@Muhammad Shaaban Could you elaborate on that? In clay/organic soils one would expect the redox potential to drop after rain, and also pH to drop / not rise due to biological processes releasing protons. What is your experience with dry soils?
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