DYNAMICS OF MICROBIAL BIOMASS NITROGEN AS INFLUENCED BY ORGANIC-MATTER APPLICATION IN PADDY FIELDS I - FATE OF FERTILIZER AND SOIL ORGANIC-N DETERMINED BY N-15 TRACER TECHNIQUE

The effects of annual application of rice straw or cow manure compost for 17-20 y on the dynamics of fertilizer N and soil organic N in Gley paddy fields were investigated by using the N-15 tracer technique dur ing the rice cropping season. The chloroform fumigation-extraction met hod was evaluated to determine the properties of soil microbial biomas s under submerged field conditions at the tillering stage before mid-s ummer drainage, with special reference to the fate of applied NH4+-N-1 5. The transfer ratios from applied NH4+-N-15 to immobilized N in soil and to uptake N by rice during given periods varied with the rice gro wth stages and were affected by organic matter application. The accumu lated amounts of net-mineralized soil organic N (net-M-j), immobilized N (I-j), and denitrified N (D-j) during the cropping season were esti mated to be 14.0-22.5, 6.3-11.2, and 3.4-5.3 g N m(-2), respectively. Values of net-M-j and I-j were larger in the following order: cow manu re compost plot > rice straw plot > plot without organic matter applic ation, and their larger increase by the application of cow manure comp ost contributed to a decrease of the D-j values, as compared with rice straw application. Values of E-N, extra extractable soil total N afte r fumigation, increased following organic matter application, ranging from 2.1 to 5.4 g N m(-2). Small residual ratios of applied N-15 in th e fraction E-N at the end of the given period indicated that re-minera lization of newly-assimilated N-15 through the easily decomposable fra ction of microbial biomass had almost ended. Thus, the applicability t o paddy field soils of the chloroform fumigation-extraction method was confirmed.