Spatial and Seasonal Changes of Soil Enzymatic Activities in Coastal Sand-Dune Forests at Houlong, Miaoli

Soil enzymes play a key function in litter decomposition and the recycling of essential elements in ecosystems. The present study was conducted at a coastal sand-dune forest at Houlong, Miaoli, west-central Taiwan. Sampling plots were set up at different topographic locations and with different vegetation. Soil samples were collected from the humic layer (sieved from the O horizon) and 2 mineral layers at depths of 0~10 and 10~20 cm in January and July 2002. The objectives of this study were to clarify the factors of topography, vegetation, and seasonal changes on the activities of such soil enzymes, as cellulase, xylanase, and urease. Results showed that the cellulase and xylenase activities in the humic layer were 6.08 and 14.61 mg g-1 d-1, and were higher at the upland site than that at the lowland site, while no significant differences were found in the mineral layers. Xylenase activity did not significantly differ between the upland and lowland sites in either the humic or mineral layers. Urease activities were 4.28 and 0.49 mmol g-1h-1 in the humic and surface layers, and were higher at the lowland site for both samples. In general, these enzyme activities were higher in the humic layer than in the mineral layers, and activities declined with soil depth. Cellulase and xylanase activities in the humic layer were 4.55 and 12.3 mg g-1 d-1 and were higher for the pure Casuarina forest than for the other sites, while a significant difference was found for different vegetative cover. Cellulase and xylanase activities in the surface layer were 0.74 and 1.65 mg g-1 d-1 were the highest in the pure Casuarina forest. Urease activity significantly differed among the different vegetative types. Urease activities were 28.45 and 0.79 mmol g-1h-1 and were the highest in the pure Hibiscus forest in both the humic and surface layers. Cellulase and xylanase activities in the humic layer were higher for the upland Casuarina forest than for the lowland site, while neither the cellulase nor xylanase activities in either the humic or mineral layers significantly differed between the Hibiscus and mixed forests. The dissolved organic nitrogen was 293.1 mg kg-1, which was highest in the humic layer of the mixed stands in July. Urease activity was lower in summer due to heavier rainfall. No seasonal changes in enzyme activities were found in the mineral layers.