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Effect of organic-C on immobilization and nitrification patterns in acidic soil was examined during 20 weeks incubation period to verify if organic amendments such as composted material can increase soil retention of N by stimulating microbial immobilization of NH 4 + . Four treatments were laid out: control without fertilizer N and glucose (treatment code: S), ammonium sulfate (SN), ammonium sulfate with single glucose at the commencement (0 week) of incubation (SNG), and ammonium sulfate with double glucose at 0 and 4 weeks of incubation (SNGG). Glucose application (SNG) significantly increased microbial immobilization of NH 4 + within 1 week of incubation over SN. Immobilization was followed by re-mineralization thereafter; however, second-application of glucose (SNGG) restored NH 4 + immobilization. At the same time, nitrification was significantly inhibited by glucose application as indicated by consistently low NO 3 − concentration in SNG and SNGG soils, suggesting that microbial assimilation of NH 4 + is predominant compared to nitrification when available C-source is abundant. These results suggest application of chemical fertilizer-N with organic amendment would have beneficial effect on soil-N retention and environmental conservation by reducing production of NO 3 − which is likely to be lost through leaching or denitrification.