Microbial Immobilization and Plant Uptake of Different N Forms in Three Forest Types in Shikoku District, Southern Japan

Abstract

Soil microbial immobilization and plant uptake of N were evaluated for three forest types in Kochi, Shikoku district. During 196-d laboratory incubation, soil NO3-N production in the Hinoki cypress forest was negligible for the initial 40 d and then rapidly increased, whereas NO3-N production was rapid from the beginning in Japanese cedar and deciduous hardwood forests. Microbial immobilization of the labeled 15N decreased in the order of NH4-N>glycine-N>NO3-N. The 15N immobilization was higher for soil in the Hinoki cypress forest than other two soils. The delayed NO3-N production in the Hinoki cypress forest was likely related with low availability of NH4-N due to NH4-N immobilization and substantial NO3-N immobilization. In the field experiment, 15N uptake by roots decreased in the order of NH4-N>NO3-N>glycine-N. The absorption of the labeled 13C suggested direct uptake of organic N. The preference of N forms by root uptake was not different among forest types. Trees in three forest types can absorb inorganic and organic forms of N, suggesting trees absorb the N form that is the most abundant in the soil.