Long-Term Concentrations and Loads of Four Dissolved Macronutrients from Two Agroforestry Catchments in NW Spain

Understanding hydrological processes controlling stream chemistry and quantifying solute concentrations over time is crucial for estimating future alterations of water quality due to land use or climate change impacts, as well as for setting preventive or remedial actions. In the current study, soluble sodium (Na+), potassium (K+), calcium (Ca2+), and magnesium (Mg2+) concentrations and loads were assessed in waters from two small catchments located at different distances to the sea (9 km Valinas and 30 km Abelar) in NW Spain from 2003 to 2016. Solute concentrations were determined using spectrometric techniques, while streamflow data were employed for estimating loads. Moreover, concentration-discharge relations were calculated for each solute and catchment. The average concentrations of soluble Na+, K+, Ca2+ and Mg2+ were, respectively, 16.5, 2.6, 7.9 and 4.1 mg L−1 in Valinas, and 8.2, 0.9, 2.9 and 3.4 mg L−1 in Abelar, although variability among samplings was high. The four soluble ions showed a dilution pattern in Valinas, whereas in Abelar Na+ tended to a chemostatic behavior and K+ and Ca2+ were positively related to streamflow. In conclusion, the dominant processes controlling these relationships are local and depend on catchment characteristics such as land use (including slurry applications in Abelar), distance to the sea, and vegetation cover.