Nitrogen legacies in anthropogenic landscapes: a case study in the Mondego Basin in Portugal

Abstract

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. 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. 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-km2 watershed in Portugal, where human interventions have considerably changed the characteristics of the basin to prevent floods and improve farming conditions in recent decades. 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. 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. 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.

Modification of exploration of long‐term nutrient trajectories for nitrogen (ELEMeNT-N) model to quantify legacy nitrogen dynamics in a typical watershed of eastern China

“…ELEMeNT-N has been successfully applied in America and Europe, e.g. Chesapeake Bay (Chang et al 2021), Mississippi watershed (Van Meter et al 2018), Susquehanna watershed (Van Meter et al 2017), Grand watershed (Liu et al 2021a), Weser basin (Sarrazin et al 2022) and Mondego Basin (Marques et al 2022). However, ELEMeNT-N has not been previously applied to Chinese watersheds, which have considerably different climate and agricultural cropping practices.…”