Conservation tillage results in decreased pH and increased cation exchange capacity. Please check the following links for more details.

http://www.sciencedirect.com/science/article/pii/S2095633915300630

http://www.rroij.com/open-access/effects-of-conservation-agriculture-production-system-caps-on-soil-310-314.pdf.php?aid=40158

Dear Mrs. Ebrahimian,

one of the major parameters for getting a more or less precise idea of the CEC is a precise determination of the clay mineral content and the type of organic matter. I am dealing mainly with inorganic material and would like to direct your thoughs also in terms of pH to the clay mineral content. It makes a difference whether you have a clay mineral composition enriched in kaolinite-group phyllosilicates, Al-enriched chlorite or minerals of the smectite group. Moreover carbonate and phosphate also play a significant role and can be taken as geo-acidometers. As long as these major components (there are also Fe minerals and heavy minerals) are not well determined it is hard to predict what is going on in the soil.

Best regards

Harald G.Dill

Conservation tillage in Aridisol under arid environment in India for 8 years resulted an decrease in soil pH by 0.3 unit , unfortunately we never estimated CEC of the soils in the experiment.

The soil PH should be slightly acid neutral to be able to maintain optimum levels of exchangeable bases and micronutrients.

Conservation tillage cannot affect clay content within our time frame, but it can affect organics and they can affect the surface properties of both the clays and oxides.  

Dear Dr. Elnaz,

I agree with Dr. Paul Milham. Due to retention of crop residues, organic carbon content of the soil under conservation tillage improves. We recorded this observation in our experiment conducted under the Indo-Gangetic Plain of West Bengal, India. Our experimental site is located at 88o 26' E (longitude) and 22o 45'N (latitude) and at 9 m above MSL. We did not observe any remarkable effect on the soil clay content (our soil has 18% clay), and noted slight drop in pH of the near-neutral soil (pH 7.1).

Dear Elnaz Ebrahimian

I understand that conservation tillage will tend to lower the soil pH in arid region soils due to decreased aeration, meanwhile the practice of CT will tend to enhance OM due to decreased oxidation. There might not be any change in clay contents. Hence I think there will be no big change in soil CEC. Any change in soil textural components needs centuries of time. But in some canal irrigated areas in Pakistan, there is significant increase in silt and clay contents in the head sections of distributries and water courses that has been termed as "Flood Coatings" but has resulted in a period of about 100 years.

I agree with above answers, adoption of conservation tillage along with residues could leads to increase in CEC by decreasing pH of the soil in general. It also depends on the soil conditions and climatic conditions may vary the extent of exchange of cations.

Some of the  fascinating answers by our  colleagues, making the whole discussion worth participating . Cation exchange capacity  of soil is bound to increase under conservation tillage ,especially in arid/semi arid regions ,  mostly due to improvement in surface properties of soil particles, even the surface area of sand particles could improve. We have observed this  process in Vertisols having  predominantly smectite clyas , but separated sands were also observed black in coloration due to organic carbon coatings.