Role of microbial biomass in soil nutrient dynamics along a jhum cycle gradient

The impact of shifting cultivation (jhum) on soil physicochemical properties and soil microbial biomass at lowland areas (< 260 in asl) of Arunachal Pradesh, north-east India was investigated by comparing fields under three-, five- and 10-year jhum cycle systems. The pH of the top,0 to 10 cm soil increased after the burn, but gradually decreased during cropping phase. The soil moisture content declined sharply after burning and slowly recovered with time to a level comparable to a mature forest of the locality. C and N concentrations decreased slightly during cropping but recovered with increasing periods of jhum cycling. Microbial biomass C and N were low in soils after burning:but : increased-thereafter. Contribution of microbial biomass to soil organic C was greater in 19-year jhum cycle system while their contribution to total Kjeldahl-N was greater in three-year cycle system. Longer fallow periods favoured greater improvement in humus and some nutrients. A jhum cycle of three to five years, now prevalent in the region, is considered short. Soil nutrient depletion was more in three-year jhum cycle systems.