Dynamics of microbial biomass nitrogen as influenced by organic matter application in paddy fields

Abstract

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 15N tracer technique during the rice cropping season. The chloroform fumigation-extraction method was evaluated to determine the properties of soil microbial biomass under submerged field conditions at the tillering stage before mid-summer drainage, with special reference to the fate of applied NH4 +-15N. The transfer ratios from applied NH4 +-15N to immobilized N in soil and to uptake N by rice during given periods varied with the rice growth stages and were affected by organic matter application. The accumulated amounts of netmineralized soil organic N (net-Mj ), immobilized N (Ij ), and denitrified N (Dj ) during the cropping season were estimated to be 14.0–22.5, 6.3–11.2, and 3.4–5.3 g N m-2, respectively. Values of net-Mj and Ij were larger in the following order: cow manure compost plot > rice straw plot > plot without organic matter application, and their larger increase by the application of cow manure compost contributed to a decrease of the Dj values, as compared with rice straw application. Values of E N extra extractable soil total N after fumigation, increased following organic matter application, ranging from 2.1 to 5.4 g N m-2. Small residual ratios of applied 15N in the fraction E N at the end of the given period indicated that re-mineralization of newly-assimilated 15N through the easily decomposable fraction of microbial biomass had almost ended. Thus, the applicability to paddy field soils of the chloroform fumigation-extraction method was confirmed.