Changes in nitrogen budgets and nitrogen use efficiency in the agroecosystems of the Changjiang River basin between 1980 and 2000

Abstract

To investigate both the temporal and spatial changes in the nitrogen use efficiency (NUE) of agroecosystems in the different agricultural regions of the Changjiang (Yangtze) River basin, we constructed a nitrogen (N) budget by using a database of county-level agricultural statistics that was collected every 10 years from 1980 to 2000. Based on the mass balance model, we defined the NUE of agroecosystems as the proportion of all N inputs that are exported via the harvested crop biomass. According to our estimates, the mean total N inputs increased from 8.68 Tg N in 1980 to 13.4 Tg N in 1990 and to 19.8 Tg N in 2000 due to regional human activities. The proportion of anthropogenic new reactive N to the total inputs increased from 42% in 1980 to 68% in 2000 while the proportion of recycled N decreased. N from synthetic fertilizers was the largest contributor to the basin and dramatically increased to 12.23 Tg N in 2000, corresponding to a fivefold increase over that in 1980. While the amount of N from atmospheric deposition, biological N fixation, and recycled N varied slightly between 1980 and 2000, the proportion of N in harvested crops to the total N inputs decreased. Furthermore, the proportion of N lost by denitrification, volatilization, and riverine N transport, and that stored in soil increased between 1980 and 2000 as a result of intensified agricultural activities. It was found that the change pattern of the NUE differs both temporally and spatially. In the Sichuan basin and the plains in the middle and lower reaches that comprise the main agricultural regions of the Changjiang River basin, the NUE increased between 1980 and 1990; however, it dramatically decreased in almost the entire area between 1990 and 2000. On the other hand, in the mountainous and hilly regions of the lower Jinshajiang and Wujiang watersheds, the NUE decreased between 1980 and 1990 but increased between 1990 and 2000. As a result, the total amount of N transported to the surface waters from the agroecosystem reached 4.32 Tg N in 2000, showing a 2.4-fold increase over that in 1980. The export of riverine N increased, and the areas that exported large amounts of riverine N expanded widely from the Changjiang lower plain to the Changjiang middle plain and the surrounding areas between 1980 and 2000. It was apparent that the high rates of N fertilizer application were the most important factor that led to the dramatic decrease in NUE between 1990 and 2000.