Calcium losses from a forested catchment in south-central Ontario, Canada

Mass balance studies at a number of calibrated watersheds in eastern North America suggest that large losses of Ca from soil have occurred due to acid deposition. However, there is considerable controversy over whether losses have occurred from the exchangeable pool or whether there are other sources of Ca that have not been considered. Mass balance calculations at a small, calibrated catchment (PC1) in south-central Ontario also indicate that large losses of Ca have occurred over the past 2 decades. If the exchangeable Ca pool has declined by approximately 40% between 1983/1984 and 1998/1999 but the exchangeable Mg pool has remained relatively stable, these changes should be evident in streamwater chemistry. The slope of the buffer curve relating Ca to (sigma)acid anions (F(Ca)) decreased significantly over the study period. We estimate that F(Ca) decreased by 12-24% between 1983 and 1999, although the level of decrease is affected by changes in hydrology. During the same period, there was no significant change in F(Mg), and the annual volume-weighted Ca:Mg (equiv/equiv) ratio in streamwater decreased from approximately 2.8 to approximately 2.1. Measured changes in streamwater chemistry translate into Ca losses from the exchangeable pool of approximately 250-570 mequiv/m2 over the study period, which is similar to estimated losses based on soil measurements (approximately 425 mequiv/m2) and mass balance calculations (approximately 380-430 mequiv/m2). The magnitude of Ca loss cannot be explained by decreases in Ca deposition or decreased Ca weathering. Large Ca losses from the soil exchangeable pool have serious implications for future forest health and the recovery of streamwater from acid deposition.