Implications of Land-Use and Land-Management Changes for Nitrogen Losses in South American Ecosystems

Abstract:

South America has experienced extensive land-use and land-management changes, which accelerated in the 19^th century and are projected to continue in the future. As a result, increased anthropogenic nitrogen (N) inputs via cultivation of nitrogen-fixing crops and application of fertilizer and manure have altered the terrestrial-aquatic N cycle. Anthropogenic perturbations to the N cycle propagate through the ecosystem, impacting downstream freshwater and coastal life, terrestrial and riverine N₂O emissions, and carbon cycling throughout the system. We used the Geophysical Fluid Dynamics Laboratory LM3-TAN (Terrestrial-Aquatic Nitrogen) model to simulate anthropogenic influences on nitrogen losses across terrestrial and riverine systems in South America from 1700 to 2050. The model’s representation of ecological, hydrological, and biogeochemical processes captures the transport and transformation of nitrogen throughout the vegetation-soil-river continuum, enabling a more comprehensive accounting of nitrogen losses than previous estimates. We find that land-use and land-management changes increase nitrogen losses and explore the difference in regional patterns between the Amazon and La Plata basins. We highlight the importance of biological fixation and demonstrate that historical changes in nitrogen losses have been more driven by the type of crop introduced than by the amount of fertilizer used.