MICROBIAL BIOMASS NITROGEN AND MINERALIZATION-IMMOBILIZATION PROCESSES OF NITROGEN IN SOILS INCUBATED WITH VARIOUS ORGANIC MATERIALS

The effects of the addition of organic materials on the change of the pattern of microbial biomass N in soils were assessed in relation to the mineralization-immobilization processes of N. Two types of field moist soils (Udifluvent and Melanudand) amended with chicken manure, clover leaves, farmyard manure, bark compost and rice straw were incubated for 16 weeks at 25-degrees-C, and the amounts of biomass N and inorganic N were measured periodically. The chloroform fumigation-extraction method was used for measuring the soil microbial biomass N. Chicken manure was rapidly mineralized from the onset of incubation and did not result in an appreciable increase of the amount of microbial biomass N (<4.0% of the added organic N). Clover leaves showed a lag period of 2 weeks before active N mineralization commenced and the amount of microbial biomass N increased during the lag period, reaching a value of 13.6% of the added organic N, while the amount of biomass N decreased rapidly. On the other hand, the addition of farmyard manure and rice straw resulted in N immobilization from the start of incubation, and N immobilization continued up to 4 weeks for the farmyard manure-treated soils and during the entire incubation period for the rice straw-treated soils. The amount of soil microbial biomass N increased within 2 weeks (10.5-13.5% of the added organic N) by the addition of farmyard manure or rice straw and the increase was maintained throughout the immobilization phase. Bark compost induced a slight N mineralization and immobilization, and the amount of microbial biomass N increased was low (<5.8% of the added organic N). Thus the addition of organic materials resulted in the increase in the amount of microbial biomass N regardless of the soil types. However, the changes in the pattern of microbial biomass N were related to the N mineralization and immobilization processes of each organic material.