Geochemical mass balances and weathering rates in forested watersheds of the southern Blue Ridge II. Effects of botanical uptake terms

Abstract

Geochemical mass balance methods are commonly used in small-watershed studies to estimate rates of primary-mineral weathering and soil formation, and the contribution of these processes to cation budgets, nutrient cycling, and landscape susceptibility to acid deposition. Many researchers employ the “balance sheet” approach, a system of simultaneous linear equations with constant coefficients which represent the steady-state behavior of the modelled system. Most workers also assume that the steady-state assumption for the entire system means that botanical factors can be ignored; in other words, that there is no net elemental transfer between biomass and inorganic compartments of the system. However, the widely invoked assumption that biomass is at steady-state is mathematically a second, explicit assumption, not a built-in mathematical consequence of the assumption of overall steady-state. The common assumption that the biomass is necessarily at steady-state in a steady-state system ignores the possibility of uptake of elements into the forest biomass. To quantify the implications of the separate mathematical assumptions, weathering rates for seven forested watersheds in the southern Blue Ridge mountains are calculated twice, once allowing for net element exchange with biomass, and once assuming no net exchange with biomass. Our calculations show that misunderstanding the mathematical constraints which are built into the most widely used geochemical mass balance equations commonly causes errors (underestimates) of up to a factor of 4 in the calculated rates of mineral weathering and soil formation. This problem is most pronounced for minerals which contain major nutrient elements, and for rates calculated from mass balances of those nutrient elements.