2007 Volume 23 Issue 2 Pages 153-164
To evaluate how human activities affect nitrogen (N) transported to the Changjiang Sanxia Dam, we constructed a database of county-level agricultural statistics collected every five years from 1980 to 2000 for calculating N budget of 350 counties in the upper Changjiang River basin. Using a riverine N transport model, we quantified the N transported to the main upper tributaries of the Changjiang River. The total amount of N transported to the surface drainage waters from the agro-ecosystem, which accounted for about 83% of the total riverine N transport, reached 1.61x10 6t in 2000, which was a 2.9-fold increase over 1980. If the in-river loss rate is considered to be constant at 37%, the total amount of N transported to the Changjiang Sanxia Dam from the agro-ecosystem of the Changjiang river upper basin was about 0.35, 0.47, 0.59, 0.64 and 1.01x10 6t annually in 1980, 1985, 1990, 1995, and 2000 respectively. The amount of riverine N transport of new anthropogenic reactive N approximately quadrupled, from 3.45x10 5 t in 1980 to 1.39x10 6t in 2000, while the amount of riverine N transport due to rural human waste varied between 2.14-2.67x10 5t during the period 1980-2000. Of the total N transported to surface drainage waters in 10 watersheds in 2000, the Jialingjiang watershed accounted for 35%, the Changjiang Sanxia area for 15%, and the Toujiang, Wujiang and Minjiang watersheds each for 11%. In 1980, N sources were mainly focused on the rural areas surrounding Chendu City and Chongqing City, but expanded widely to the whole Sichuan basin and even surrounding mountainous and hilly areas in the 1990s. The increase of synthetic fertilizer use combined with the decrease of fertilizer N-use efficiency is implicated as major causal factors in the large amount of riverine N transport. Furthermore, the calculated riverine N transport for the main tributaries agreed well with the measured data reported in the literature. This work indicates that the riverine N load in the upper reaches of the Changjiang River and Changjiang Sanxia Dam has been increased rapidly by human activities, which might further worsen the local water quality and cause further eutrophication.
