Determination of the Biomass Term in Watershed Geochemical Mass Balance Methods from Soil and Bedrock Bulk Chemistry.

In an ecosystem, biomass is capable of significantly influencing K, Mg, and Ca concentrations in surface waters.  During biomass aggradation macronutrient concentrations will decrease, whereas during biomass degradation macronutrient concentrations will increase.  A biomass in steady-state will have no influence on water chemistry which will reflect the purely inorganic hydrogeochemical processes operating within the ecosystem. 

Watershed mass balance studies may be used to quantify chemical fluxes between the different compartments of an ecosystem.  In order to accurately make such calculations the stoichiometry of biomass elemental uptake must be known for any nutrients included in the model.  Previously utilized biomass stoichiometries have been determined from published net primary production values.  However, the stoichiometry of macronutrients stored in biomass may not necessarily reflect the stoichiometic uptake of mineral-derived cations released during chemical weathering.  Based on the assumption that biomass can only consume those mineralogenic macronutrients available in soil, the bulk chemistry of a watershed’s soil may be used to calculate the biomass term stoichiometry. 

Zr-normalized total chemical concentrations from bedrock and soil may also be used to calculate molar ratios of macronutrients to a cation negligibly consumed by biomass.  Such a molar ratio reflects the abiotic cation loss from the watershed and allows for calculation of the macronutrient uptake flux by the biomass.  An assumption of determining the biomass macronutrient uptake rate in this manner is that the inorganic proportion of base cation release during chemical weathering is temporally invariant.

To investigate the application of the solid-phase bulk chemistry to watershed geochemical mass balance techniques, three Appalachian watersheds have been utilized.  These watersheds are Brubaker Run in Pennsylvania, and two watersheds located in Coweeta Hydrologic Laboratory, North Carolina.  The results demonstrate that the two proposed methods may be applied to watersheds with aggrading biomass, and with thoroughly characterized mineral weathering reactions.