Effects of Changing Land Use from Agriculture to Forest on Stream Water Quality in a Small Basin in NW Spain

Monitoring was conducted in a basin with a history of agricultural pollution (mainly due to the spreading of livestock slurry) to evaluate the response of stream water ion concentrations to the change from agricultural land use to forest. We measured pH, electrical conductivity (EC), and major dissolved elements phosphate (PO₄) phosphorus (P), nitrite (NO₂) nitrogen (N), nitrate (NO₃) N, ammonium (NH₄) N, silicon (Si), sodium (Na), potassium (K), calcium (Ca), magnesium (Mg), sulfate (SO₄), and chloride (Cl)] in the stream water during the period 1997–2002 at irregular time intervals. Median values were significantly greater in the agricultural period than in the forest period (P < 0.05). Median pH values remained slightly acidic (5.8–6) for both land uses. NO₃-N was the dominant form of dissolved inorganic nitrogen (DIN) under both land uses (agricultural use: 80 percent; forest use: 98 percent. In the forest period, the median PO₄-P and NH₄-N concentrations were reduced by 95 percent compared to the agricultural period. The median NO₂-N and K concentrations decreased by 83 percent and 70 percent, respectively, whereas NO₃-N, Cl, Na, and Si showed minor differences (decreases of 15–20 percent). These findings are consistent with the poor agricultural management practices that have been developed in the basin, especially as regards the application of slurry.