Soil Nutrient Contents and Their Characteristics of Seasonal Changes Under Shorea chinensis Forest in Xishuangbanna

Abstract

Shorea chinensis forest of Xishuangbanna (21°29′ N, 101°34′ E) is a rainforest type of tropical rainforest in South_east Asia entering into Xishuangbanna, which is significant in connecting with the flora of tropical rainforests in South_east Asia and elucidating characteristics of tropical rainforest in Xishuangbanna. In southern Mengla county, Xishuangbanna, Shorea chinensis forest grows on the mountain in altitude 700_900 m, the soil is latosol, annual mean temperature is 21.1 ℃, and annual mean precipitation is 1 500_1 600 mm. In order to know the soil nutrient content, organic matter, total N, P, K, available N, P, K and their seasonal changes under Shorea chinensis forest in Xishuangbanna, we took soil samples in the top soil (0_10 cm) and under the top soil (10_25 cm) from 47 plots among 10 different patches of Shorea chinensis forest in May and November, 1999. 23 plots were located in valleys and 24 plots in lower mountainous areas. The results showed there are obvious difference in the soil nutrient content in different soil layers, in different seasons and in different areas: 1) in different soil layers, nutrients were rich in the top soil (0_10 cm) and poor under 10 cm depth. The content of nutrients under 10cm depth was about 56%_75% of the nutrients in top soil; available P content was only 0.039% of the nutrients in top soil. 2) Soil nutrient content changes with seasons; nutrients were higher before the rainy season than after the rainy season. The content of organic matter, total N, total P and total K in top soil after rainy season was 94%, 90%, 94% and 82% of the content before rainy season respectively, and the content of available N and available K in top soil after rainy season was 89%, 92% of the content before rainy season. Available P decreased sharply in top soil after rainy season; the content of available P in top soil after the rainy season was only 0.078% of the content before the rainy season. The change in nutrient content under top soil after rainy season was the same as the change in nutrient content in top soil. 3) Concerning soil nutrient content in different area, nutrients were also higher in valleys than in lower mountainous areas. The content of organic matter, total N, total P and total K in top soil in lower mountainous area were 79%, 80%, 84% and 83% of the content in valley respectively, while the content of available N, available P and available K were also lower in top soil in lower mountainous area than in valleys. From the above results, we can conclude that seasonal changes significantly affected total K and available P, but only slightly affected total P, available K and organic matter, and affected N content more in lower mountainous areas than in valleys. In the rainy season P content in soil is very poor. The environment and plant growth rhythm of Shorea chinensis forest have some effects on the soil nutrient content.