Saturday, 15 July 2006

Net Soil Nitrogen Mineralization and Nitrification of a Lowland Subtropical Rain Forest in Southern Taiwan.

Chun-Chih Tsui, Dept Agricultural Chemistry, National Taiwan Univ, No. 1, Sec. 4, Roosevelt Road, 106-17, Taipei, Taiwan and Zueng-Sang Chen, Dept of Agricultural Chemistry, National Taiwan Univ, 1, Sect 4th, Roosevelt Road, Taipei, Taiwan.

The Nanjenshan Ecological Reserve Site is a major contributor for biodiversity because of its biographic value and abundance of endemic plant species with restricted distributions in the south of Taiwan. Our research was conducted in a one-ha transect along the ridge of Nanjenshan (12051'10''E, 2203'37''N). Previous studies indicated that there are significant differences of floristic compositions and soil properties along the altitudinal gradient, ranging from 300 m to 480 m. The objective of this study is to investigate the relations between soil nitrogen transformation and landscape positions in the study area by laboratory incubation. Mineral surface soils (0-15 cm) were collected from three replicate 10 m * 10 m plots of footslope and summit landscape position, respectively. Soils were sampled in Feb 2004 and Feb 2005 and incubated in the laboratory under field moisture condition at intervals of 1, 2, 3, 4, 6, 9, 12 and 15 weeks, then soils were extracted by 2M-KCl at the end of incubation. Soils sampled in Oct and Dec 2004 were incubated for short-term period (28 days) and extracted weekly by 2M-KCl. Inorganic N (ammonium N and nitrate N) of extracts were analyzed by the Kjeldahl appratus. Soil microbial biomass C and N of each sampling were also measured. Short-term incubations sampled in Oct 2004 indicated that net N mineralization rates of summit and footslope soils were 0.75 and 0.44 mgN/kg/d, meanwhile net nitrification rate of both soils was 0.57 mgN/kg/d. Net N mineralization rates of summit and footslope soils sampled in Dec 2004 were 0.75 and 0.63 mgN/kg/d, and net nitrification rates were 0.60 and 0.64 mgN/kg/d, respectively. This result coincided with a former study determined by buried bag method in the field of the study area. At the end of long-term incubation in 2004, soil inorganic N content at summit position was 74 mgN/kg and significant higher than that of footslope position (47 mgN/kg, p<0.05). High nitrate-N contents revealed that nitrification was the dominant N transformation process in this study site. In the result of incubation in 2005, nitrate-N was still the dominant inorganic N in the soil, but there was no significant difference between summit and footslope position. We propose that there are some other environmental factors except landscape position which can affect the soil N transformations. Significantly higher level of organic matter at summit can provide more mineralizable N, however, the lower pH may be unfavorable for soil nitrification. Differences of available P and exchangeable Ca between summit and footslope position may also affect the soil mineralization and nitrification rate. C/N ratios of leaf of dominant tree species are lower at footslope and N release rate from litter decomposition are significantly higher at footslope than at summit. Therefore, the substrate quality of footslope seems more favorable for N mineralization. Although the microbial biomass C/N ratio was significantly different between footslope and summit position, there was no distinction between their net mineralization rate and net nitrification rate. We suggested that further studies about the effects of microbial population and microbial activity on the N transformation in Nanjenshan site are necessary, and the influence of edaphic factors on soil N dynamics and their interactions also need to be confirmed.

Back to 3.0A Long-term Agronomic Experiments: Their Importance for Science and Society - Poster
Back to WCSS

Back to The 18th World Congress of Soil Science (July 9-15, 2006)