General soil-landscape distribution patterns in buffer zones of different order streams

Understanding landscape and soil distribution patterns in buffer zones along a stream network in a watershed can improve riparian zone management and the representation of soil-landscape parameters in watershed modeling. We analyzed landscape features and soil properties within a 300-m buffer zone of different order streams in a large agricultural watershed (the East Mahantango Creek Watershed) located in the Ridge and Valley Physiographic Province in Pennsylvania, USA. The mean elevation displays an obvious decreasing trend downstream, and increases gradually with the distance away from the streams. Within the buffer distance of approximately 75 m, buffer zone's mean slope shows a rapid increase regardless of stream order; however, it starts to decrease after 75 m in the 1st to 3rd order stream buffers, but continues to increase in a more gradual manner in the 4th and 5th order stream buffers. Agricultural land area percentage increases by 10-25% from near stream area to about 100 m buffer, and becomes nearly the same in 100-300 m buffer zone. The opposite trend holds for forested land area in all stream buffer zones. The 1st to 2nd order stream buffers have 12-28% greater agricultural land area percentage and 0.45-1 m shallower soil depth than the 3rd to 5th order stream buffers, suggesting that the headwater areas in the study watershed are important in preventing nonpoint source pollution due to the more intensive agricultural land use and potential erosion in headwater catchments. The distributions of soil properties in the buffer zones are consistent with the observed landscape patterns. The top two soil layers (approximately A and B horizons) in the 0-100 m buffer zones of the 1st and 2nd order streams generally display a greater clay content and a higher bulk density, but a lesser organic matter content and a lower available water capacity, than those in the similar buffer zones of the 3rd to 5th order streams. Beyond the 100 m buffer distance and in the third layer of the soil profiles (approximately C horizons), the soil properties examined in all stream buffer zones become less distinguishable. With a few exceptions, soil clay content and bulk density increase with increasing buffer distance (particularly within 0-100 m range), while organic matter content and available water capacity decrease with buffer distance. Such patterns reflect the impacts from the landscape features, fluvial processes, and land use in the Ridge and Valley Physiographic region. The results of this study, though maybe specific to the watershed studied, suggest that soil and landscape distribution patterns along stream networks are helpful to guide riparian zone management and nonpoint source pollution prevention in agricultural watersheds.