Solute pathways in a forested ecosystem : a drainage basin approach
The project sought to identify direct links between calculate hillslope solute fluxes and input/output budgets in a forested drainage basin in
the United Kingdom. An intensive, short term monitoring and data
collection programme was devised in order to quantify the major components
for the hillslope hydrological cycle, the seasonal changes in overstorey
and understorey biomass, and the chemical composition of ecologically and
pedologically derived samples. Gross and net precipitation, infiltration,
and soil moisture fluxes, groundwater movement and streamflow were
monitored to identify the possible pathways which solutes might follow.
Regular sampling of the oak and bracken vegetation, litterfall and litter
enabled calculation of biomass accumulation and the rate of uptake and
release of plant nutrients. The spatial variability of soil properties
was quantified, but subsequent soil sampling permitted identification of
seasonal trends in exchangeable action and soil water solute concentrations.
All hydrological and ecological samples were analysed for nitrogen, phosphorus, potassium, calcium, magnesium, sodium and chloride. PH,
specific conductance and organic carbon were measured for selected samples.
The high infiltration capacity of the soil promoted rapid vertical movement
of water towards a perched water table. The presence of soil macropores and the high permeability of the soil precluded the generation of throughflow
on the hills-lope. Stream discharge was mainly controlled by groundwater
fluxes from the perched water table. Although seasonal trends in
the solute concentration of water samples and in the nutrient assimilation
of vegetation were identified, isolated events, such as frassfall and
storm-period litterfall, were shown to contribute significant quantities
of nutrients to the forest floor, The importance of solute movement via
macropores, especially phosphorus, was emphasised, with particular reference
to plant availability. The relative stability of the ecosystem was
reflected in the balance of the input/output budgets of solutes, with, the
exception of calcium and magnesium losses which were attributable to
weathering of the calcareous bedrock. Bivariate and multivariate statistical
analysis showed that no single biogeochemical process on the hillslope
could be identified as exerting a dominant controlling influence on stream
water chenistry, through isolated events may affect concentrations in the short term.