SOIL NITROGEN DYNAMICS IN THE EARLY ESTABLISHMENT PHASE OF A FOREST PLANTATION IN PENINSULAR MALAYSIA

A study was conducted on the dynamics of N in the soil as a consequence of clear-cutting and replanting of a forest area with Acacia mangium trees. Measurements were carried out within the first two years of tree growth. Soil N fluctuations at different soil depths under and between trees were monitored. Mineral N concentration fluctuated conspicuously within the measurement period. It varied between 1 and 7% of the total N. The nitrate-N (NO₃⁻-N) content for 0-45 cm depth soil continuously decreased with stand age. For example, for stand age of 3 to 6 months, the total decrease was 44 µg g⁻¹ (35% decrease) for the soil between the trees, and 74 µg g⁻¹ (51% decrease) for the soil under the trees. These values generally dropped with stand age. The concentration remained at the baseline from stand age of 18 months until the last measurement period. The ammonium-N (NH₄⁺-N) behaved rather differently. From the stand age of 3 to 6 months, the soil NH₄⁺-N concentration increased at both sampling locations, being most prominent between the trees. It then decreased and at 24 months, an increase was noted again in the 0-5 cm soil layer under the canopy. The amount of mineral-N decrease at the beginning of measurements was not in good synchrony with the amount of N increase in the Acacia mangium tree. The potential mineralisation measured showed a high rate during the first year, declined in the second year, and increased again after two years. This fluctuation coincided with the changes in mineral-N content. The increase of NH₄⁺-N at the beginning could be due to the mineralisation of root residues from the previous vegetation, while the apparent increase at the last measurement stage in the first 5 cm soil layer under the trees came from the mineralisation of litter. Apart from soil transformation processes, uptake by the plants was one of the reasons leading to the lower concentration of mineral-N under the trees compared to those between the trees. Satu kajian telah dijalankan berhubung dinamik nitrogen tanah kesan daripada penebangan menyeluruh kawasan hutan dan penanaman semula dengan anak benih Acacia mangium. Data-data diambil untuk dua tahun pertama pertumbuhan pokok. Turun naik nilai N tanah dicatatkan bagi beberapa kedalaman tanah di bawah silara, di bawah pokok dan di kawasan terdedah (antara silara pokok). Kepekatan mineral N menunjukkan perbezaan yang jelas bagi setiap masa pengukuran. la berubah di antara 1 dan 7% daripada jumlah N. Kandungan N-nitrat