Xin,  your question is very good. But we need to look at then answer of this question on the lines , what is the nature and properties of a given soil type. Take a sandy soil with available water content of 5-6% against clay soil with available water content of 15-20% . And accordingly the bulk density vis-a-vis prosity will also vary.  So under these two contrasting soil conditions , in which case nitrification will be potentially higher. I would say in clay soils because of the porosity and oxygen trapping ability of the soil . What a potential nitrification rate for clay soil is optimum , will be the upper threshold limit for a sandy soil. I hope my answer satisfies you .   

Hi Anoop, so you think the for clay soil, the optimal water content may be higher than the sandy soil? In terms of my results, the clay showed the highest nitrification at 80% WFPS. Maybe I should have a look at soil texture. Thank you very much.

Thats what I am saying Xin. You are endorsing whst I am saying.

Sandy soils are greatly limited by inability to retain nutrients and water. 

A clay soil has much greater ability to retain nutrients and water but is very able to provide aeration.

Loams offer the best of both worlds and soil organic matter can improve both of their performance.

Sandy soil is most challenged in soil organic carbon conversion which is a function of clay and calcium content. 

In relation to nitrification the two bacteria involved are Nitrosomas and Nitrobacter both of which are aerobic. As pore space is occupied with water aeration will decrease their activity. Under anaerobic conditions the facultative anaerobic bacteria can use Nitrate as as energy source in the process of denitrification which releases the Nitrogen as a gas into the atmosphere. When Nitrogen is in Nitrate is can be leached and when anaerobic condition it is volatilizes. The more stable species of Nitrogen is ammonium which will attach to the clay minerals and can be exchanged with necessarily being leached or volatilized immediately. Nitrogen in humus is another other of stability.

I still feel , besides the soil texture , organic matter content of the soil has the most cascading effect on the nitrification potential , since the nature  clay-humus complex is going to play a pivotal role , regardless of soil and other climatic conditions.