Controls on runoff components on a forested slope and implications for N transport

Transfer of atmospheric N deposition on shallow-soil forested basins on the Canadian Shield to receiving water bodies may be enhanced by rapid prefere ntial dow along the soil-bedrock interface (BR runoff) on basin slopes. Con trols on BR runoff, partitioning of event and pre-event water contributions to this how, and implications of this partitioning for N fluxes in BR runo ff were studied under natural and artificial inputs to an instrumented slop e. BR runoff as a fraction of water inputs to tile slope increased with ant ecedent soil wetness and input depth. Event water contributions to BR runof f initially increased with antecedent soil wetness, but then declined at la rge antecedent soil wetness, Export of applied NH4+ from the slope was maxi mized when event water contributions containing large NH4+ concentrations d ominated BR runoff; however, there was no relationship between the fraction of NO3- application transported in BR runoff and either application input or the event mater fraction of that runoff. The applicability of our result s to other shallow-soil areas of the Canadian Shield is limited by artifici al N inputs to the slope in excess of natural loads and by low rates of N m ineralization and negligible nitrification in the slope's soils, Neverthele ss, the study reinforces the need to consider how the hydrologic, geometric and pedologic properties of forest slopes interact with biotic and abiotic soil processes to control N transport and transformation. Copyright (C) 20 01 John Wiley & Sons, Ltd.