![]() ![]() ![]() Human actions have altered this natural nitrogen cycle since the industrial revolution (Han et al. Finally, these nitrates are converted to N gas again by denitrification and released into the atmosphere, thus closing the N cycle (Zhang 2016). When plants and animals decompose, the incorporated N is released into the soil as ammonia, which is converted to nitrates via nitrifying bacteria. These reactive N ammonia and nitrates are taken up by plants and when animals consume the plants, they host these reactive N sources. The natural nitrogen cycle includes the conversion of N gas from the atmosphere to ammonia and nitrates by mainly lightning and biological fixation processes. Nitrogen (N) is one of the most common chemical elements and is essential for all life forms on our planet. This time lag between the N inputs to the watershed and the N loads in the river (about two decades) is a function of accumulation of N legacy. The N inputs have been decreasing since then, but N loads in the river did not document any decrease till the 1990s after which there was a decline. The results show that the increase in the amount of inorganic fertilizer applied was the main driver for the anthropogenic N loads in the watershed from 1950 until the beginning of the 1990s. ![]() Here, we have used the ELEMeNT (Exploration of Long-tErM Nutrient Trajectories) model to explore the buildup and depletion of N legacies over a 216-year period, across the Mondego River Basin, a 6645-km 2 watershed in Portugal, where human interventions have considerably changed the characteristics of the basin to prevent floods and improve farming conditions in recent decades. It is important to understand these legacies to quantify the relationship between N inputs and N concentrations in streams and implement best management practices for water quality improvement however, little is known about the magnitude of legacies in various landscape elements like soils and groundwater. Nitrogen (N) legacies have built up in anthropogenic landscapes over decades of agricultural intensification, and these legacies lead to time lags in water quality change measurable even beyond the moment of application of N. ![]()
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