Role of Rainfall Intensity and Hydrology in Nutrient Transport via Surface Runoff

ABSTRACT Loss of soil nutrients in runoff accelerates eutrophication of surface waters. This study evaluated P and N in surface runoff in relation to rainfall intensity and hydrology for two soils along a single hillslope. Experiments were initiated on 1‐ by 2‐m plots at foot‐slope (6%) and mid‐slope (30%) positions within an alfalfa (Medicago sativa L.)–orchardgrass (Dactylis glomerata L.) field. Rain simulations (2.9 and 7.0 cm h−1) were conducted under wet (spring) and dry (late‐summer) conditions. Elevated, antecedent soil moisture at the foot‐slope during the spring resulted in less rain required to generate runoff and greater runoff volumes, compared with runoff from the well‐drained mid‐slope in spring and at both landscape positions in late summer. Phosphorus in runoff was primarily in dissolved reactive form (DRP averaged 71% of total P), with DRP concentrations from the two soils corresponding with soil test P levels. Nitrogen in runoff was mainly nitrate (NO3–N averaged 77% of total N). Site hydrology, not chemistry, was primarily responsible for variations in mass N and P losses with landscape position. Larger runoff volumes from the foot‐slope produced higher losses of total P (0.08 kg ha−1) and N (1.35 kg ha−1) than did runoff from the mid‐slope (0.05 total P kg ha−1; 0.48 kg N ha−1), particularly under wet, spring‐time conditions. Nutrient losses were significantly greater under the high intensity rainfall due to larger runoff volumes. Results affirm the critical source area concept for both N and P: both nutrient availability and hydrology in combination control nutrient loss.