the passive pool is responsible for most of the cec of the som and,in addition to exchangeable cations, contains nutrients that are tightly locked into complex organic-mineral assemblages .Intermediate pools of som are also involved in nutrient cycling along the continuum from active to passive som fractions. Jenkinson developed a model that described there som pools with different turnover times to describe C and N dynamics. Paustian et al. distinguished two litter and three som pools to describe som dynamics and nutrient cycling . The chemical extraction of som of classic humic fractions can also describe the fate ofrecently added N to soil . Bird et al. showed the importance of soil humic fractions, ranging from labile light fraction to resistant humin, in controlling the availability of recently