Increased extreme precipitation challenges nitrogen load management to the Gulf of Mexico

Over the past 40 years, precipitation extremes have become more important for delivering N to the Gulf of Mexico, according to simulations with a hydro-ecological model. This is likely to diminish the effectiveness of alternative N use practices Although the hypoxia formation in the Gulf of Mexico is predominantly driven by increased riverine nitrogen (N) export from the Mississippi-Atchafalaya River basin, it remains unclear how hydroclimate extremes affect downstream N loads. Using a process-based hydro-ecological model, we reveal that over 60% of the land area of the Basin has experienced increasing extreme precipitation since 2000, and this area yields over 80% of N leaching loss across the region. Despite occurring in similar to 9 days year(-1), extreme precipitation events contribute similar to 1/3 of annual precipitation, and similar to 1/3 of total N yield on average. Both USGS monitoring and our modeling estimates demonstrate an approximately 30% higher annual N load in the years with extreme river flow than the long-term median. Our model suggests that N load could be reduced by up to 16% merely by modifying fertilizer application timing but increasing contribution of extreme precipitation is shown to diminish this potential.