Assessment of the main factors affecting the dynamics of nutrients in two rainfed cereal watersheds

Nutrient dynamics and factors that control nutrient exports were observed in two watersheds, namely Latxaga and La Tejería, with similar climatic and management characteristics throughout 10 years (2007-2016). Similar patterns were observed in intra-annual and inter-annual dynamics with higher NO₃^- concentration and NO₃^--N yield during the humid seasons (i.e., winters and hydrological year 2013). Regarding concentration, Latxaga showed a higher decrease of nitrate due to a higher development of vegetated areas. High discharge events produced nitrate dilution due to the presence of tile-drainage at La Tejeria. At Latxaga, where tile-drainage was not observed, an increase in concentration occurred as a response to high discharge events. Comparing both watersheds, La Tejería presented ca. 73 ± 25 mg NO₃^- L^-1 while at Latxaga, the concentration observed was almost three times lower, with ca. 21 ± 15 mg NO₃^- L^-1 throughout the study period. Similar patterns were observed for the NO₃^--N yield, with 32 kg NO₃^--N ha^-1 year^-1 and 17 kg NO₃^--N ha^-1 year^-1 at La Tejería and Latxaga, respectively. Regarding phosphorous, the observed concentrations were 0.20 ± 0.72 mg PO₄^3- L^-1 and 0.06 ± 0.38 mg PO₄^3- L^-1 at La Tejería and Latxaga, respectively, with PO₄^3--P yields being 71 kg PO₄^3--P ha^-1 year^-1 and 33 kg PO₄^3--P ha^-1 year^-1. Annual phosphate-P yield distribution in both watersheds followed similar patterns to those observed for the nitrate-N yield, with higher yields in the humid season. Regarding concentration, highly erosive rainfall that occurred in summer, mobilizing sediments and probably generating desorption of phosphorous in the stream channel, increased phosphate concentration. This research adds to the knowledge base regarding the dynamics of nutrients and the controlling factors in complex agricultural systems with Mediterranean characteristics.