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 Precambria n shield catchments was investigated during 1988-90. Catchments were s ubdivided and instrumented to examine the vertical and horizontal flux es of elements within and between two distinct landscape types: open, lichen-covered bedrock outcrops and patches of conifer forest. The dom inant hydrologic pathways were Horton overland flow in the lichen-bedr ock areas and shallow subsurface flow through organic rich LFH (forest floor) and Ah soil horizons in the forested areas. Annual runoff coef ficients 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% o f 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+ (0.38 x), Mg2+ (0.83x), Na+ (0.85x) and K+ (0.32x) per unit area than the be drock-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 retain ed 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 hydrolo gic and biogeochemical linkages associated with each landscape unit in teract to control element transport within the study catchments.