Soil NO3-N production and immobilization affected by NH4-N, glycine, and NO3-N addition in different forest types in Shikoku, southern Japan

Abstract

The characteristics of production and immobilization of NO3-N were evaluated for soils from four forest types in Kochi Prefecture, southern Japan. Net NO3-N production during the laboratory incubation differed among the soils from the four forest types, being high under Japanese cedar (Cryptomeria japonica D. Don) and deciduous hardwood, and negligible under Japanese red pine (Pinus densiflora Sieb. et Zucc.) and hinoki cypress (Chamaecyparis obtusa Endlicher). Nitrification under Japanese cedar and hardwood was mainly autotrophic based on the fact that nitrification was inhibited by acetylene or nitrapyrin, and was not affected by cycloheximide. Net NO3-N production in these soils increased by glycine addition, but did not increase appreciably by NH4Cl addition. However, net NO3-N production increased after the addition of CaCO3 with NH4Cl. These results indicate that the substrate of nitrification is NH3 rather than NH4 + and that the added NH4 + is not utilized by nitrifiers at low pH values. With NO3-N addition to soils under red pine and hinoki cypress, immobilization of NO3-N was observed followed by rapid production of NH4-N. These findings suggested that mobile NO3-N can be converted to less mobile NH4-N by the activities of soil microorganisms. This microbial process may play an important role in retaining nitrogen within forest ecosystems where the potential of N loss is high due to the high precipitation in the area.