Nitrogen management challenges in major watersheds of South America
Nutrient management in South America must be an essential component of sustainable development and mitigation and adaptation to climate change. While there are good examples of strategies that contribute to nutrient management (e.g.deforestation control, agricultural practices, ,improvement of water access to urban and rural populations), efforts to control nutrient fluxes to inland and coastal waters are still restricted to some critical areas. Land use changes, urbanization and climate extremes in South America results on changes in the nitrogen cycle with critical consequences to the continent freshwater resources and coastal zones and human health, once access to clean water and water services still highly unequal in the region. We reviewed the status and drivers of land use change, in special agriculture and urbanization, linking to land-water interactions and nutrient loads in continental freshwater bodies in South America using three hydrological basins (Amazon, La Plata and Orinoco) as case studies.
A comparison of nitrogen load in the three selected basins indicates the importance of the organic fraction of the total dissolved nitrogen, in special in black water rivers (as Negro in the Amazon Basin) and in rivers with some level of anthropogenic loads, e.g., sewage (as in Jutaí, in the Amazon basin, and Corumbataí and Jaguari, in the Paraná/La Plata Basin). In general, for the white water rivers, draining forested watersheds in the Amazon, similar organic and inorganic fraction of dissolved nitrogen loads are found. In the Parana/La Plata Basin the importance of point source of nitrogen, through input of untreated sewage is higher. Nevertheless, less perturbed watersheds showed lower load of dissolved organic nitrogen, reflecting a smaller importance of pollution point sources. Despite of the huge difference in basin areas, TDN export of these small streams was similar to the nitrogen export of the main rivers of the Amazon basin.
Multi-sectorial and regional approaches and adequate policy frameworks are necessary to consider the land-water interactions as well transboundary fluxes and water resources (only South America has 69 out of 279 of the world’s transboundary river basins). However, the lack of spatial and temporal data on nutrient sources, mobilization, distribution, and monitoring of effects (social, economic and environmental) continues to represent a major barrier.