Analysis On Nitrogen Loss In Different Land Use Watersheds Of Menglun, Xishuangbanna, Southwest China

The land use/cover change(LUCC) is an important driver of global change, and its influence on water environment had caused more and more attention, but there's little report about it in Xishuangbanna, south of Yunnan. In the past decades, rain forest in Xishuangbanna has experienced serious deforestation, resulting in a great change in land use and land cover. In this study,31 typical watersheds in Menglun were chosen as the study area. We sampled nine times in different season and detected the concentration of total nitrogen, total dissolved nitrogen, NO3-N, NH4-N. in the lab. Then, we used four periods of land use classification maps and 1:50000 digital elevation model(DEM) to extract land use information in those watersheds, The study of LUCC and its impact on nitrogen loss had beed carried out. It had important scientific significances for understanding this impact. The main conclusions included:(1) Characteristics of LUCC in watersheds from 1976 to 2007.1) Forest, shrubland, rubber plantation and dry land are the main land use/cover types in the study area. From 1976 to 2007, the LUCC of this area was very obvious. The most significant changes were a decrease of forest and an expansion of rubber plantation at low elevation area. The change of shrubland and dry land was not consistent.2) In the study area, shrubland, rubber plantation and dry land tend to expand to high altitude. Shrubland and dry land increased more than 200 meters in 31 years, and rubber plantation increased nearly 100 meters in 19 years. The average slope of rubber plantation and dry land became increasingly steep.(2) Characteristics of nitrogen loss in the three types watersheds and their influencing factors1) Nitrogen concentration was relatively stable throughout the year in those watersheds covered by rain forest, the average slope had an impact on the nitrogen concentration seasonal variation curve. Nitrogen peaked in May in steep slop watersheds.2) In rubber plantation covered watersheds, the average slope played an important part in nitrogen concentration, in watersheds that slope values less than 15°, nitrogen concentration remained stable throughout the year, no significant peak appeared. Watersheds with an average slope greater than 20°, nitrogen concentration had an obvious seasonal variation, the concentration of Total Nitrogen and Total Dissolved Nitrogen peaked in the middle rainy season; area of watershed and average slope had an impact on the nitrogen loss.3) In watersheds covered by mixed land use, the coverage of rain forest and rubber trees affected nitrogen concentration a lot. For those watersheds have similar coverage of the two types, the average slope, in particular, the slope of rubber plantation was the most important influencing factors.(3) The relationship between nitrogen loss and land use1) Several high-slope watersheds compared, watersheds covered by rubber had the largest nitrogen concentration, watersheds covered by rain forest had the smallest concentration.2) In the rubber covered watershed with the average slope of 12.51°, the nitrogen concentration and seasonal variation trends were most close to tropical rain forest watersheds.3) Land-use structure had significant impact on the concentration of nitrogen. In those watersheds that similar in coverage of rain forest and rubber, but different in position, the concentration of nitrogen in the two watersheds would be different, the concentration of nitrogen in the watershed where the rubber trees located in the lower reaches (near the sampling points) was greater than the watershed where the tropical rain forests located in the lower reaches.4) The main loss component of the Total Nitrogen(TN) was Total Dissolved Nitrogen(TDN). The main loss component of Total Dissolved Nitrogen(TDN) was NO3-N, but it also had differences in the watersheds where covered with different types of vegetation. The content of total inorganic nitrogen (NH4-N+NO3-N) in DTN reached its peak in the middle and latter rainy season,.and the proportion was higher in watersheds covered by rain forestt than watersheds covered by rubber or mixed land use.