『Abstract
Paired watersheds are used to develop a deciduous nutrient uptake
stoichiometry. The watersheds are those of the House Rock Run
and the Brubaker Run located in the Pennsylvania Appalachian Piedmont,
USA. These two watersheds are nearly identical with respect to
bedrock, regolith, climate, geomorphology, morphometry, baseflow
hydrology, and type and successional stage of forest vegetation.
They only differ by the percentage of deciduous forest cover,
with House Rock Run having 59% and Brubaker Run having 76%. From
differences in their stream chemistries the biomass nutrient uptake
stoichiometry of K1.0Mg1.0Ca1.4 was determined. This stoichiometry applies
to an aggrading deciduous biomass and differs from those previously
used which were derived from net primary production (NPP) data.
The difference may reflect that macronutrients in plant tissue
may also originate from atmospheric inputs and/or decomposing
biomass. Although this stoichiometry may not be applied to all
deciduous forest-covered watersheds, it is likely an improvement
over a stoichiometry determined from NPP data.
Mass-balance calculations of mineral weathering rates often suffer
from the number of unknowns exceeding the number of equations.
To add equations to the mass-balance matrices two methods are
introduced. The first method employs Zr-normalized bulk chemical
compositions of bedrock and soil to calculate a mass transfer
coefficient for chemical weathering. The second approach uses
chemical formulae and modal abundances of primary minerals undergoing
complete dissolution during weathering. Both methods allow for
calculation of weathering rate constraints without biomass and
cation exchange influences. These constraints serve as additional
equations in the mass-balance matrix. This study finds that the
watershed with the higher percentage of recently abandoned agricultural
fields previously used for growing row crops has the higher chemical
weathering rate. The higher chemical weathering rate reflects
greater runoff resulting from reduced evapotranspiration.』
1. Introduction
2. Background
2.1. Biomass
2.2. Adding matrix equations and use of solid-phase data in watershed
mass-balance
2.3. Site description
2.4. Conceptual framework
3. Methods
3.1. Stream water chemistry
3.2. Long term representativeness of data set
3.3. Influence of former agricultural land use on stream chemistry
3.3.1. Liming and agrichemical applications
3.3.2. Ground water flow paths
3.4. Determination of the biomass nutrient uptake stoichiometry
3.5. Biomass chemistry
3.6. Stream discharge measurements
3.7. Elemental input-output fluxes
3.8. Adding more mass-balance matrix equations using solid-phase
data
3.8.1. Zirconium-normalized bulk chemical concentrations of
bedrock and soil
3.8.2. Chemical formulae and modal abundances of primary minerals
3.9. Watershed mass-balance methods
4. Results
4.1. Stream water base cation fluxes
4.2. Biomass chemistry and nutrient uptake stoichiometry
4.3. Cation exchange
4.4. Watershed mass-balance matrices with more matrix equations
using solid-phase data
4.5. Mineral weathering rates
5. Discussion
6. Summary and conclusions
Acknowledgments
References