Extensive Decline of Soil Nitrogen and Its Drivers in the Lake Victoria Basin of Tropical Africa (1996–2015)

Abstract

The Lake Victoria basin, which houses over 20% of East Africa's population, is facing challenges of soil degradation and lake eutrophication. Understanding the spatio-temporal dynamics of soil nitrogen and its reaction to both natural and anthropogenic factors is crucial for ensuring food security and environmental sustainability in the basin. This study utilized 3020 legacy soil samples and a Random Forest machine-learning model to generate high-resolution (250 m) soil nitrogen maps for the years 1996 and 2015 in this basin. The GeoDetector model was then used to analyze the factors influencing spatial and temporal variability of soil nitrogen. The results revealed an extensive decline in soil nitrogen density within the basin, from 503.02 g N m−2 in 1996 to 466.61 g N m−2 in 2015, indicating a decrease of approximately 7.24%, based on measurements at a 30 cm soil depth. The nitrogen stock decreased from 102.20 Tg N to 95.60 Tg N, resulting in a 6.60 Tg N decline, equivalent to an economic loss of 21.52 billion USD. Croplands accounted for 55.30% of this loss, or 3.65 Tg N. Forests exhibited the slowest decline in soil nitrogen density. However, a significant nitrogen decline occurred when forests were converted to other land uses. Deforestation has resulted in the loss of 2075.56 km2 forest area, leading to a decline in soil nitrogen stock by 1.44 Tg N. Soil nitrogen in the basin showed notable spatiotemporal heterogeneity, with high-value areas concentrated in the northeast and southwest, and low-value areas mainly in the south. The variation in soil nitrogen in croplands is influenced by a combination of factors, including GDP, fertilizer consumption, temperature, and soil texture. These findings provide critical data and theoretical support for nitrogen dynamics research and soil management in the basin. They serve as an important reference for understanding nitrogen biogeochemical cycling in the tropical regions.