Abstract
Using the net anthropogenic nitrogen input (NANI) approach we estimated the N budget for the Lake Victoria Basin in East Africa. The NANI of the basin ranged from 887 to 3008 kg N km−2 yr−1 (mean: 1827 kg N km−2 yr−1) for the period 1995–2000. The net nitrogen release at basin level is due primarily to livestock and human consumption of feed and foods, contributing between 69% and 85%. Atmospheric oxidized N deposition contributed approximately 14% to the NANI of the Lake Victoria Basin, while either synthetic N fertilizer imports or biological N fixations only contributed less than 6% to the regional NANI. Due to the low N imports of feed and food products (<20 kg N km−2 yr−1), nitrogen release to the watershed must be derived from the mining of soil N stocks. The fraction of riverine N export to Lake Victoria accounted for 16%, which is much lower than for watersheds located in Europe and USA (25%). A significant reduction of the uncertainty of our N budget estimate for Lake Victoria Basin would be possible if better data on livestock systems and riverine N export were available. Our study indicates that at present soil N mining is the main source of nitrogen in the Lake Victoria Basin. Thus, sustainable N management requires increasing agricultural N inputs to guarantee food security and rehabilitation and protection of soils to minimize environmental costs. Moreover, to reduce N pollution of the lake, improving management of human and animal wastes needs to be carefully considered in future.
Highlights
net anthropogenic nitrogen input (NANI) estimates in the Lake Victoria Basin
NANI calculations for the entire Lake Victoria Basin were made by area-weighting by country
225.0 79.1 137.5 1416.8 761.8 1445.9 631.1 153.6 31.1 1827.4 through the food chain to livestock and humans and back to the environment as organic and inorganic N in wastes, were the major inputs with average contributions ranging from 69.2% to 84.6% of total NANI in this basin
Summary
Reactive nitrogen (Nr), such as nitrate, nitrite and ammonium, is essential for the functions, processes and dynamics of ecosystems (Vitousek and Howarth 1991). Together with the advent of unlimited industrial nitrogen fixation at low costs by the Haber–Bosch process, anthropogenic activities have at least doubled annual global Nr inputs to ecosystems as compared. Increased Nr supports the food and fuel needs of a growing human population, but it causes numerous adverse impacts on human health and environmental sustainability, including eutrophication of aquatic ecosystems and increased N2O emissions—a potent greenhouse and ozonedepleting gas (Vitousek et al 1997, Galloway et al 2008, Sobota et al 2013). Compensating for the negative impacts associated with anthropogenic Nr inputs represents an important challenge faced by land and water managers worldwide
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