Properties of Soil Nitrogen Mineralization and Nitrification in Mature Plantations of Japanese Cedar (Cryptomeria japonica) and Hinoki Cypress (Chamaecyparis obtusa).

Abstract

Properties of nitrogen mineralization and nitrification of brown forest soil were examined in mature plantations of Japanese cedar and hinoki cypress in the southern part of the Shikoku district. Net nitrogen mineralization rate of the surface soil was 8.4 mg N kg^-1d^-1 and 2.7 mg N kg^-1d^-1 beneath Japanese cedar and hinoki cypress, respectively. These values were high compared to the reported values. Nitrification rate beneath Japanese cedar was correlated negatively with soil pH and positively with soil CN ratio. Nitrification rate beneath Japanese cedar increased by the addition of NH_4-N, NO_3-N, glycine and urea. These results suggest that nitrification beneath Japanese cedar is active in acid conditions and nitrifiers rapidly utilize ammonium derived from other microorganisms. Nitrification beneath hinoki cypress was higher in soils of lower total nitrogen content. Nitrification rates beneath hinoki cypress decreased by the addition of cycloheximide and by the amendments of different forms of nitrogen. These results suggest that fungus beneath hinoki cypress may play an important role on nitrification and the nitrification rate was not affected by the external nitrogen sources.