The biogeochemistry of pristine,headwater Precambrian shield watersheds: an analysis of material transport within a heterogeneous landscape

The hydrology and elemental transport within five low order Precambrian shield catchments was investigated during 1988–90. Catchments were subdivided and instrumented to examine the vertical and horizontal fluxes of elements within and between two distinct landscape types: open, lichen-covered bedrock outcrops and patches of conifer forest. The dominant hydrologic pathways were Horton overland flow in the lichen-bedrock areas and shallow subsurface flow through organic rich LFH (forest floor) and Ah soil horizons in the forested areas. Annual runoff coefficients ranged from 0.3 to 0.7. Runoff chemistry was acidic (pH 4.01–4.72), with organic anion equivalents (RCOO^-), comprising 60 and 69% of the anion charge total for bedrock and forest runoff, respectively. Forested plots exported more H⁺ (2.6x), DOC (1.4x), Al (1.6x) and Fe (1.8x) and less N (0.40x), P (0.13x), particulate C (0.08x), Ca²⁺ (0.38x), Mg²⁺ (0.83x), Na⁺ (0.85x) and K⁺ (0.32x) per unit area than the bedrock-lichen plots. The catchments exhibited a net export of H⁺ (34), Mg²⁺ (24), Na⁺ (20), K⁺ (4) (units in eq ha^-1 yr^-1) and C (16), Si (5.6), Al (1.6) and Fe (0.47) (units kg ha^-1 yr^-1). The catchments retained N (5.66), P (0.08), Mn (0.03) (units kg ha^-1 yr^-1), and Ca²⁺ (37), and Cl^- (3) (units eq ha^-1 yr^-1). The strong retention of Ca²⁺ within the treed soil islands resulted in extremely low export rates of base cations (-15 to 38 eq ha^-1 yr^-1). The spatial distribution and hydrologic and biogeochemical linkages associated with each landscape unit interact to control element transport within the study catchments.