MONSOON EFFECTS ON THE SOIL NITROGEN DYNAMICS OF A SUBTROPICAL RAIN FOREST ECOSYSTEM IN TAIWAN

摘要

The Nanjenshan Natural Reserve Region, which is located in the south of Taiwan, is a typical subtropical to tropical rain forest ecosystem in Taiwan. There is strong northeastern monsoon from October to March in this region. A monsoon effect hypothesis was proposed that it probably affects the distribution of animal species, vegetation compositions and soil nutrient characteristics, but little data is available to verify this hypothesis. The objectives of our study are to estimate the effects of northeastern monsoon on soil available N, net soil N mineralization (SMN) rate, and soil microbial biomass (SMB) C and N. In-situ incubations of SMN are conducted and sampled from October 2003 to February 2004 during the monsoon season. Laboratory incubations of N mineralization are also conducted. The results of field incubation in the Nanjen Lake site indicated that the maximum soil ammonium-N is 8 ± 3 mg N·kg~(-1), soil nitrate-N is 2 mg N·kg~(-1), and maximum total inorganic N is 10 ± 2 mg N·kg~(-1). The SMB-C ranged from 500 to 1000 mg C kg~(-1) and SMB-N ranged from 30 to 60 mg N·kg~(-1). The estimated SMB-C and -N are not significantly affected by the soil water content, soil temperature, and different seasons in the study plots. The preliminary results indicate that monsoon does not significantly affect SMN under relatively low available N soil. It is possible that soils of the study area had suffered the monsoon for a long time, but the setting of treatment plots was relatively too short to display the effects. There are no significant differences for soil maximum inorganic N among different seasons and different incubation methods. We propose that SMN rate can be estimated and conducted in the laboratory. The SMN of the Valley transect site, ranged from 20 to 35 mg N·kg~(-1), are significant higher than that of Nanjen Lake site. This result indicated that SMN is also affected by landscape positions.