Base cation and nitrogen budgets for seven forested catchments in central Ontario, 1983–1999

Abstract

There is widespread concern that the combination of acid (S and N) deposition and tree harvesting may deplete available base cation pools in soils to the extent that forest health and productivity may be impaired. Here we present mass balance budgets for Ca, Mg, K and N for seven forested catchments in Muskoka–Haliburton, Ontario over a 17-year period, during which time SO 4 deposition decreased by ∼40%, but N deposition was unchanged. Mass balance budgets were calculated as the difference between inputs from deposition and mineral weathering (base cations only) and export in stream water plus uptake in biomass. There were significant decreases in annual SO 4 export from all catchments, although temporal trends in base cation export were varied. Export of Ca, Mg and SO 4 exceeded inputs in deposition, but concentrations in stream water were highly variable from year-to-year. Temporal patterns in Ca, Mg and SO 4 concentration in stream water were synchronous among all seven catchments and appear related to climate variations over the 17-year period. Most of the N exported from each catchment was organic N, although export of NO 3 varied considerably (10-fold), both between catchments and between years. Differences in NO 3 export appear to be due to differences in catchment characteristics and also climate variations. Nevertheless, net N accumulation at the seven catchments was between 92.6 and 128.1 kg/ha N over the 17-year period. All catchments experienced a net loss of Ca, which varied between 33 and 287 kg/ha over the 17-year period. Net losses of Mg were lower, and in general, mass balance estimates indicate that most catchments are in balance with respect to K. Losses of Ca over this 17-year period, represent up to 60% of the current exchangeable Ca soil pool. If Ca losses, due to acid deposition and harvesting continue at the present rate, forest health and productivity may be impaired within just a few decades.