Assessing the impacts of afforestation on nitrogen load into the northern gulf of America

Eutrophication of the northern Gulf of America (NGOA) (also known as northern Gulf of Mexico) due to excess nutrients has resulted in harmful algal blooms, the development of hypoxic zones, and negative impacts on seafood production, recreational activities, and marine transportation. With a growing recognition of afforestation to maximize timber production and improve water quality, there is a critical need to investigate impacts of afforestation on nitrogen (N) loads to the NGOA. Using the Pearl River Basin (PRB) located in Mississippi and Louisiana along with the HAWQS (Hydrologic and Water Quality System) model and the Kolmogorov-Smirnov (KS) test, we assessed the impacts of afforestation (by converting all corn and soybean lands in the PRB to mixed-forest lands) on total nitrogen (TN) and nitrate-N loads to the NGOA over a 31-year period from 1990 to 2020. Simulations showed that average annual TN and nitrate-N loads were, respectively, 26% and 28% higher in the base scenario than in the afforestation scenario, with statistically significant differences based on the KS test. The result indicates that afforestation contributed to a very significant reduction in annual N loading from the PRB to the NGOA, which could occur from enhancing N adsorption and immobilization within forest soils, reducing application of synthetic N fertilizers, and decreasing surface runoff after afforestation. Notably, the magnitude of N load reduction was not directly proportional to the area of cropland conversion, suggesting that other factors, including the specific location of afforestation (e.g., riparian zones), land slope, and the types of tree species planted, may also significantly influence N load. Two distinct daily TN loading phases were observed: 1) a slow-loading phase at daily streamflow ≤1200 m³ s^-1, and 2) a fast-loading phase at daily streamflow >1200 m³ s^-1. These findings have not been reported in the literature and underscore the value of strategically designed afforestation for optimizing N load reduction in the Gulf region. Additionally, very few studies have investigated the impacts of afforestation on daily N load to the NGOA, and this study would help fill the research gap.