The density of soil microorganism in volcanic ash soils was usually lower than that in non‐volcanic ash soils because of the effects of unavailable organic matters, toxicity of Al ions, etc. To evaluate the effects of long‐term fertilization on the activity and density of soil microorganism, we investigated the density and biomass C of microorganism, enzyme activities, and amount of phospholipid fatty acids (PLFA) at the citrus orchard where different fertilizer application methods had been managed for 10 years such as N‐F (No fertilization), N‐P‐K (28‐40‐28 kg 10a^-1), 3N‐3P‐3K (84‐120‐84 kg 10a^-1), compost (2 ton 10a^-1), and compost+N‐P‐K. In March 2003, Soil samples were collected from each treatment before fertilizer applied. There was no significant deference in the density of microorganisms and biomass C between treatments. However, the density of acid‐resistant bacteria (613.2 x10³ c.f.u g^-1 dry soil) in N‐F was about twice higher than that in 3N‐3P‐3K (324.8 x10³ c.f.u g^-1 dry soil) and the number of bacillus spp. in compost was highest among the treatments. There was a seasonal fluctuation in the number of microorganisms; fluorescent pseudomonas spp. was dominant in spring and gram‐negative bacteria were in fall. The activity of soil acidic‐phosphatase was high in compost and 3N‐3P‐3K; 311.5(N‐F), 478.4 (N‐P‐K), 586.3 (3N‐3P‐3K), 548.7 (compost), and 503.3 ㎍ PNP g^-1 h^-1 (compost+N‐P‐K). The activity of CMCase (Carboxy methyl cellulase) was high in compost and compost+N‐P‐K; 17.5 (N‐F), 68 (N‐P‐K), 20.5 (3N‐3P‐3K), 116.5 (compost), and 106 ㎍ GE g^-1 18h^-1 (compost+N‐P‐K). The amount of PLFA was significantly high in compost+N‐P‐K and compost. Principal components analysis with PLFA data showed that microorganisms in compost and compost+N‐P‐K formed different community from them in other treatments.

Key words : Citrus, Fertilization, PLFA, Soil enzyme