I remember a review by Kumar et al 2013 on dehydrogenase activity in soils. If I remember correctly they reviewed a number of methods for measuring dehydrogenase activity. You may be able to locate it.

check this article maybe useful, easy and simple but not sure about the cost! http://link.springer.com/article/10.1007/BF00335770

look here:

Schinner F, Öhlinger R, Kandeler E, Margasin R (1996)  Methods in Soil Biology, Berlin : Springer, 426 p, ISBN 3-540-59055-2

Dear Abdelgawad

You can read these papers.

Best Regards

Thanks to all colleagues