A Catchment-Scale Hydropedological Approach to Understanding Variations In Soil Genesis and Chemistry At Hubbard Brook, NH.

Soils, groundwater, and surface waters are not separate entities; they are inter-connected at many scales of observation. For example, pedon-scale soil morphology and chemistry can be influenced by hillslope-scale hydrologic processes. Hydropedology, the study of the inter-relationships between soils and water, shows that soils are formed at the hillslope as well as the pedon scale, since soluble materials can be transported from one pedon to another according to hydrologic flowpaths. In Watershed 3 (WS3) of the Hubbard Brook Experimental Forest, New Hampshire, six soil types have been identified (based on morphology, topography, and hydrology) and called hydropedologic units (HPUs). Over 80 pedons (including some from each HPU) have been described and sampled by horizon throughout WS3. Soil samples have been extracted with citrate-dithionite (d) and acid ammonium oxalate (o) in order to quantify pedogenically significant fractions of Al, Fe, Mn, and C. We used ratios of Fe_o to Fe_d and Mn_d to Fe_d to provide information about redox conditions and lateral translocation of spodic materials in the catchment. We compared all variables by HPU using analysis of variance, in order to determine potential of each soil type to contribute solutes to headwaters. Our results show redistribution of Al, Fe, and C generally downslope, with these elements accumulating most in the riparian zone. These results provide evidence that spodic materials are translocated at the hillslope scale, resulting in lateral as well as vertical podzolization in WS3. Soils in and around seep zones show the highest concentrations of Fe and Mn in the catchment; this is likely due to deep groundwater inputs. This research is part of a larger project examining the influence of hydropedologic processes on spatial and temporal variations in groundwater and stream water chemistry at Hubbard Brook.